As the carboniferous period drew to a close, the tropics began drying up. The swampy equator-spanning rainforests of clubmosses, ferns and horsetails were gradually fragmented and replaced by communities of seed ferns and arid-adapted conifers. Amphibian diversity was on the decline and the amniotes – better equipped for dealing with the trials of a dry world – were well on their way to becoming the dominant vertebrate group on land. In this post, we shall consider the early evolution of an important clade of Amniotes that includes modern mammals as its only surviving members - the Synapsids.

Shifts in global climate and in the positions of the continents sounded the death knell for the great tropical rainforests of the Carboniferous

What are pelycosaurs?

Primitive or “basal” synapsids from the late carboniferous and Permian periods are often referred to informally as “pelycosaurs” – although the term has fallen into disfavor of late among paleontologists for various reasons, it rolls off the tongue easily and is convenient for our purposes, and so we shall use it. Crudely put, the Pelycosaurs represent the very first step on the road to mammalhood in the reptile-to-mammal transition.

Pelycosaurs are distinguished from non-synapsid amniotes by a number of features, the most important of which is the presence of one hole on either side of the skull behind the eye-orbits, bounded above by the post-orbital and squamosal bones. These holes are called temporal fenestra. A diagram will be instructive.

By contrast, other early amniote skulls bear either no holes (anapsids), two holes (diapsid) or a single highly placed hole (Euryapsid, similar to the synapsid condition, but differing in the location of the hole) behind each eye-orbit. The temporal fenestrae provide secure anchorage points for muscles associated with jaw function.

Pelycosaurs share a number of other general skull features. For example, the back of the skull slopes forward and the post-parietal bone is small and single rather than paired. These minor anatomical details need not concern us unduly here – and we shall take a cursory tour through the anatomy of the most famous Pelycosaur, Dimetrodon, in the closing section of this essay. The skeletons of pelycosaurs are structurally quite similar.

What did they look like? What did they eat?

Although Pelycosaurs share a closer phylogenetic affinity with mammals than with crocodiles or lizards, there is little externally mammalian about them. They were sprawling, scaly, ectothermic creatures and, if you discovered one ambling through your backyard, you’d probably class it, quite understandably, among the reptillia.

Depictions of two early pelycosaurs. Left: Archeothyris, Right: Eothyris

Archeothyris is among the earliest known pelycosaurs. It was a lizard-like creature, about 20 inches long with short limbs, a long snout and a number of slightly enlarged canine-like teeth towards the front of the mouth. Another early pelycosaur, Eothyris, had two pairs of very large teeth protruding from the upper jaw. These dental features suggest carnivory.  Here we see the beginnings of differentiation in the structure and function of the teeth among synapsids (more ‘primitive’ tetrapods have teeth that are essentially identical in terms of morphology) – an evolutionary process that would ultimately lead to the organization of the teeth, in modern mammals, into distinct morphological types: incisors, canines, premolars and molars.

An example of a caseid: Cotylorhynchus. Notice the unusually small head

The Caesids were herbivorous pelycosaurs. They had large nostrils and a shortened lower jaw. The body is almost comically large in comparison to the skull. This is a mark of herbivory – the enlarged, barrel-shaped ribcage housed a gut that was used as a chamber for the bacterial fermentation and digestion of large amounts of plant matter. The teeth are spatulate rather than pointed. A second set of teeth project from the palate and probably worked in concert with a muscular tongue to grind down food. And the low position of the joint between the lower jaw and the skull (below the tooth row) has been interpreted as a marker of increased bite force. These adaptations are all geared towards the processing of vegetation.

A herbivorous pelycosaur with a sail - Edaphosaurus

The most famous pelycosaurs, however, are the sailbacks – animals with elongated “neural spines” arising from the vertebrae of the neck and back. These spines did not project out individually, as some early researchers believed, but supported a large ‘sail’ – a thin enveloping sheath provided with ligaments, blood vessels and muscle. We shall discuss the probable function of these sails in the next section. Sails are observed in two pelycosaur groups: the sphenacodontia and the edaphosauridae. It is thought that sails evolved independently in the two groups. Edaphosaurus (a member of the Edaphosauridae) was a herbivore – armed with a battery of teeth designed to crop and process vegetation. Edaphosaurus had a suite of adaptations for a herbivorous diet that are similar to those found in Caesids: low jaw-joint, large body, palatal teeth (as well as teeth on the inner sides of the lower jaw) etc.

Tell me about Dimetrodon!

Dimetrodon grandis

Dimetrodon, the most celebrated and best studied of all the pelycosaurs, is a member of the Sphenacodontia. Dimetrodon was a large, apex predator that dined on everything from sharks to large land-going amphibians.

Dimetrodon made an appearance on Henry Levin’s Journey to the Center of the Earth (1959). Instead of the standard 50s claymation, the viewer is treated to some hilariously awful footage of iguanas tromping around with sails taped to their backs. Set to the right music, however, these absurd scenes acquire new and profound meaning. Check 1:08 of this video. Woah, trippy.

The largest species of Dimetrodon hit a length of about 4 meters from nose to tail – making it the biggest land predator of its time. We shall begin our brief overview of the animal’s skeletal anatomy with the skull. Dimetrodon was an animal with a long, high snout. We notice the large opening behind the orbit, common to all early synapsids – the temporal fenestra.

A sketch of the skull of Dimetrodon. The large arrow indicates the direction of force exerted by the jaw adductors while the double-headed arrow between the joint and the coronid eminence indicates the moment arm of the force.

The human jaw consists of a single bone, the dentary, which connects with the temporal bone of the skull. In Dimetrodon, however, the jaw is composed of multiple bones, of which the dentary is only one. Instead of the dentary-temporal bone connection seen in humans, we have a joint between the articular bone of the mandible and the quadrate bone of the skull. In an astonishing evolutionary transition, well-supported by the fossil record, the articular and quadrate bones would eventually be incorporated into the anatomy of the ear as ear ossicles (the malleus and incus; bones that help in the amplification of vibrations received by the ear-drum) in mammals. The dentary would ultimately come to be the sole bone in the mammalian mandible.

The position of the jaw-joint is noticeably lower than the tooth-row. The low placement of the joint lengthened the ‘moment arm’ of the force delivered by a set of muscles called adductors, which pull the jaw up and backwards, closing it. As a consequence, the torque experienced by the jaw as it snaps shut is greater. The back of the jaw is expanded into a “coronid eminence” to provide additional surface area for the attachment of muscles. These structural features of the jaw and its associated musculature, combined with the sturdy construction of the skull, enabled Dimetrodon to effectively snag and hold onto large, struggling prey with its mouth.

Dimetrodon walked with a sprawling gait, similar to that of many modern reptiles. The humerus and femur projected almost horizontally from the shoulder girdle and pelvis respectively. Rotation along the long axes of these bones (which moved the lower limbs through a broad arc) was important for locomotion.  Units called “intercentra” were present between the vertebrae – these intervening skeletal elements were lost over the course of amniote evolution in both synapsid and non-synapsid lines.

We now turn to the possible function of the animal’s spectacular dorsal sail:

Thermoregulation? Modern reptiles have evolved an interesting complement of biological adaptations to maximize the rate at which their bodies absorb heat and minimize the rate at which they lose it. This involves things like adjusting the distribution of blood flow in the body and controlling body color. Many reptiles also evolved large bodies capable of better storing thermal energy (think dinosaurs or certain modern crocodilians), a feature called “Gigantothermy”. The sail of Dimetrodon was, in all likelihood, well-supplied with blood vessels and may have acted as a powerful heat-exchanger. Dimetrodon could have basked with its sail positioned laterally to the sun’s rays, soaking up heat energy like a solar panel. The speed with which the animal could heat up, thanks to its sail, may have given it certain advantages over its poikilothermic prey. Dimetrodon might have achieved the minimization of unwanted heat-loss from the sail by methods similar to those employed by reptiles today – by varying the blood supply to the sail or by adjusting the coloration of the sail (by hormonal or nervous control). Later Dimetrodon species were certainly large enough to maintain a constant body temperature over extended periods of time by mere virtue of their body size.

Here we see a possible early trend towards the regulation of body temperature in the remote ancestors of modern mammals.

Sexual display? The sail may have been a colorful sexual display used to attract potential mates. Examples of such secondary sexual characteristics that determine mating success are common throughout the animal kingdom, from sail-like dorsal fins of certain mollies to the tail feathers of peacocks. It is unfortunate that we will never know what colors painted the strange sails of these pelycosaurs.

Other explanations involve the sail’s utility in swimming (where it really would function as a “sail” of the naval variety) or as a means of camouflage among the reeds, though these seem somewhat unlikely in my opinion.

The Sphenocodontia (the group to which Dimetrodon belongs) were ancestral to modern mammals (though the Sphenocodontid ancestor of mammals almost certainly did not bear a sail). The story of mammal origins is long and complex – but with our coverage of the pelycosaurs, we have set the stage for the next major leap – the origin of the Therapsids!

Steamy, water-logged swamps, primitive amphibians trawling the undergrowth for food and gigantic insects buzzing overhead – this was the image of the Carboniferous period (which lasted from about 360 to 300 million years ago) that every illustrated natural history book I ever read as a child drilled into my mind’s eye. Great rainforests of clubmosses, horse-tails and ferns ranged throughout the tropics. Oxygen levels were much higher than they are today – allowing invertebrates to grow to extraordinary sizes. Amphibians were the dominant land vertebrates – and ran the gamut from small newt-like creatures to large predators similar in appearance to modern crocodiles. It was in this lush, wet world that the very first Amniotes – the rather diminutive ancestors of modern reptiles, mammals and birds – made their debut. This section of Fins to Feet, will deal with the appearance of this animal group – a critical event in evolutionary history that completed the vertebrate conquest of land.

Left - Many arthropod species of gigantic aspect lived during the Carboniferous period, Right - Proterogyrinus, just one of a very diverse range of amphibians that dwelt in the rainforests of this period.

The amniote kinship of mammals and reptiles

In contrast to Amphibians, both Mammals and Reptiles are fully terrestrially adapted organisms. The exceptions to this rule include a relatively small number of reptile and mammal lineages that became secondarily adapted to living in the water – like, for example, whales and ichthyosaurs – though they still retain a number of features, common to all ‘amniotes’, that betray their terrestrial ancestry.

Both Mammals and Reptiles trace their origins to a common amniote ancestor – in other words, to creatures that laid waterproof eggs on dry land.

The first Amniotes were not the first backboned animals to walk on land – though they were more practiced land-lubbers than any vertebrate group that preceded them. Tetrapods, or four limbed vertebrates, made the first tentative forays onto land in the swamps of the late Devonian (374 – 359 million years ago). The epic vertebrate transition from water to land (the event referenced in the very title of this blog) will be the subject of a future post – and it can be regarded as having ended with the coming of the Amniotes. Early Amniotes appeared some 350 million years ago (in the first half of the Carboniferous period) in terrestrial environments already overrun with plant-life, insects and land-walking amphibians.

So what, then, are these “amniotes”? What distinguishes them from other tetrapods?

In younger years, I would have seen the anamniote-amniote transition as simply the transition from “amphibian” to “reptile” – and while this is not an altogether terrible way to think about it, some caveats are necessary. Modern amphibians, or Lissamphibians – that is, frogs, salamanders, newts, toads and the less familiar caecilians – are our only existing examples of non-amniote tetrapods. But we must not make the mistake of drawing a neat equivalence between modern Lissamphibia and the ‘amphibious’ tetrapod ancestors of amniotes. Lissamphibia lack ribs, have smooth, wet skins highly-adapted for cutaneous respiration and, for the most part, lead arboreal, aquatic or fossorial lifestyles. The ancient “amphibians” from which amniotes are derived were, by contrast, stout ribbed ground-dwelling creatures with bodies covered in thick dermal plates.

Hylonomus - the earliest amniote that can be incontrovertibly identified as such

The earliest uncovered examples of Amniotes are animals that resemble – at least superficially – insectivorous terrestrial lizards. These include fossil species like Hylonomus and Paleothyris. They have slender bodies that run a length of about 20 centimeters from nose to tail-tip. They shared their world with a wide range of small to giant primitive amphibians that together made up the bulk of vertebrate terrestrial life on earth, including – for example – temnospondylls and anthracosaurs. These animals lived under various degrees of dependence on bodies of water for reproduction and egg-development. The amniotes achieved emancipation from the aquatic realm by a number of important biological innovations. Foremost among these is the Amniote egg.

The Amniote egg is characterized by a hard semi-permeable shell membrane and a suite of “extra-embryonic” tissues that surround the developing embryo  – these include the amnion, the allantois and the chorion. These eggs could be laid on relatively dry land, away from the hydrating embrace of a pond or river. Terrestrial egg-laying probably reduced the risk of predation and also enabled eggs to weather seasonal dry periods better than aquatic amphibian eggs.

Later in evolutionary history, various amniote groups would make massive modifications to this basic design – mammals, for example, did away with the fibrous shell and allowed embryonic development to proceed entirely within the uterus – but for now, let us consider what makes the primitive amniote egg a “terrestrial” egg. To do this, we now turn to the amphibian egg and consider some of its limitations in comparison to the Amniote egg.

Most modern amphibians deposit their eggs in water. The eggs hatch and undergo an aquatic larval period – a “tadpole” phase during which the organism is equipped with gills and fins and lacks limbs. Eventually the tadpole metamorphoses into the adult form, growing limbs and discarding its gills. The amniote egg is distinguished by the fact that it gives rise to developed hatchlings (which are, essentially, miniature versions of the adult form) without an intervening larval phase. This lack of a free-swimming tadpole stage is also seen in certain amphibian species– largely occupants of wet and humid tropical rainforests – which lay their eggs away from free-standing water (instead, the embryo undergoes its tadpole stage within the confines of the egg). Such a “direct developing” amphibian egg might have served as the precursor to the amniote egg.

left: Amphibians often undergo a free-living aquatic larval stage after hatching, right: Amniote eggs give rise to young that are essentially miniature versions of the adult form, as the case with these baby alligators

The typical amphibian egg is bound by a vitelline membrane and one or more layers of egg jelly (together constituting the “egg capsule”). These structures protect the egg and provide support, but they do not help prevent water-loss – and thus, the egg must be laid in water or in a wet environment to avoid desiccation. Amniote eggs are more resistant to water-loss and can be laid in drier environments.

Left: A schematic diagram of the typical water-bound amphibian egg, right: A clutch of frog eggs

The egg capsule of amphibians is more of a barrier to the diffusion of respiratory gases (namely carbon dioxide and oxygen) than is the fibrous egg shell membrane of modern reptiles. This imposes stricter limits on the size and metabolic activity of the growing embryo in amphibians. To illustrate: As embryo size increases, the radius of the egg increases. As the radius of a spherical egg increases, the volume (4/3 pi r^3) increases much faster than does the surface area available for gas exchange (4 pi r^2 ). Given that the egg capsule is a poor agent of respiratory exchange to begin with, this relationship means that oxygen uptake through the capsule is insufficient to support embryos beyond a certain size.  Furthermore, an increase in size also means an exponential increase in the weight of the supporting tissues around it. The egg capsule would have to get thicker to support the mass, thereby reducing effective diffusion. A gelatinous egg-capsule simply does not provide enough physical support, beyond a certain egg-size and particularly on land, to prevent the deformation of the egg or the embryo by gravity or surface tension, without seriously compromising the rate of gaseous exchange.

Thus, the range of egg sizes that can be realized under the amphibian egg-plan is limited.

So how might early Amniotes have overcome this size/metabolic constraint? And how did they ultimately make the transition to egg-laying on dry land? 

1)      By enveloping the egg in a fibrous air-permeable shell membrane that [a] allowed for easier exchange of gases but kept fluids in and [b] provided enough mechanical support to resist the force of gravity on land. This proteinacious shell membrane is secreted onto the egg within the oviduct. This may have antecedents in certain amphibian species, where a layer of glycoproteins is secreted onto the egg-capsule in the oviduct before deposition.

2)      By moving water from the egg-jelly to the yolk – drying up the outer portions of the egg – effectively eliminating the egg-jelly as a barrier to gas diffusion. A process similar to this is observed during egg development in reptiles, when water is withdrawn from the albumen and transported to the yolk. Whether or not the albumen and the egg jelly are structures that are “homologous” is not known for certain.

3)      By the evolution of an extra-embryonic membrane specialized for respiratory exchange called the chorioallantois. We shall discuss this briefly in my section on extra-embryonic membranes.

All reptile eggs measured to date are larger than those observed in amphibians and generally appear to display a higher rate of metabolic activity and oxygen consumption. Amniotes on the whole display a vast range of egg sizes. Freedom from the mentioned size constraints allowed for bigger offspring and, by extension, bigger adults. This newly available range of body-sizes was crucial to the evolutionary success of the amniotes in the long run.

Some researchers suggest that elevated oxygen levels in the late Carboniferous may have eased the various size constraints (related to gas diffusion) discussed earlier and helped along with the evolution of large eggs.

Why are Amniote eggs “yolkier” than the typical amphibian egg?

Since development within the egg among Amniotes is geared towards the production of a fully-formed miniature adult rather than a free-living aquatic larva (as in most amphibians), it takes a large store of nutrients (in the form of “yolk”) to carry the embryo through the whole extent of development. In other words, amniote eggs are stocked up for the long haul – and have large yolk sacs richly provided with blood vessels to convey nutrients to the growing embryo. And so, the increased nutritional requirement imposed by “direct development” is the reason Amniotes needed bigger eggs, or larger amounts of yolk at any rate. Because more has to be invested in each egg, reptiles tend to lay fewer eggs than amphibians or fish.

What about those extra-embryonic membranes?

The position and function of the various extra-embryonic membranes has been a source of confusion for me throughout the course of my schooling, but hopefully a simple diagram will help us suss out the complicated attendant tissues and boundaries that compartmentalize the egg and regulate material exchange both between compartments and between the egg and the external world. Although there are amphibians that lay eggs on land, none of them show the complex arrangement of extra-embryonic tissues depicted below.

A sketch of the layers and compartments of the Chicken egg

Let us use the chicken egg, a familiar household food-item, as our model for understanding the essential structure of the amniote egg. The embryo and supporting tissues arise from a small disc of cells, called the blastoderm, settled on surface of the yolk of the egg. The movements of the various embryonic tissue layers need not concern us here, but sufficeth to say that the growth and rearrangement of the tissues of this disc gives rise to three extra-embryonic layers: [1] the chorion is the outer-most extra-embryonic layer and, apart from providing overall enclosure for the embryo, plays a role in respiration along with the allantois (mentioned below) [2] the yolk sac encloses the yolk and is supplied with blood vessels that convey food from the yolk to the embryo [3] during development, the chorion folds over the embryo (as diagrammed below) to enclose it in the amniotic cavity. The fluid in this cavity buffers shocks and acts as a protective mechanical barrier. Finally, a fourth extra-embryonic membrane, called the allantois, develops as an outpocketing of the hindgut. It functions to store nitrogenous wastes (i.e. uric acid) produced by the metabolic activity of the chick. It expands to make contact with the undersurface of the chorion (forming the chorioallantois mentioned earlier) and serves as the principal respiratory organ for the embryo. It is richly supplied with blood vessels for gaseous exchange. As development proceeds, the yolk sac diminishes in size while the allantois grows.

The chorion folds over the embryo to form the amniotic cavity

The Albumen surrounds the yolk and provides additional support and nutrition.

Although mammals do not lay external eggs (with the exception of Platypi and echidnae), similar extra-embryonic membranes are seen around the developing fetus in the maternal womb.

So what really distinguishes the Amniote egg from the eggs of other tetrapods is the fibrous shell membrane and a host of extra-embryonic membranes. What else marked the first amniotes apart from their amphibious cousins?

The first Amniotes appeared to have smaller, narrower and deeper skulls than other tetrapods. A diagnostic feature that is often used to differentiate amphibians from amniotes is the absence in reptiles of an invagination called the “otic notch” which is present behind the eye-orbits in anamniote tetrapods. The anamniote-amniote transition also appears to have involved alterations in the musculature of the jaw, elaboration of the tongue, a reduction in the number of movable elements in the skull, strengthening of the ankles and the appearance of more slender bones. All in all, however, the skeletal differences between known basal amniotes and closely related tetrapods are rather minimal.

A large number of ancient amphibians, very probably including the carboniferous ancestors of amniotes, were armored in heavy dermal plates – the first amniotes appear to have traded these for horny keratinous epidermal scales that cover most of the body (with dermal gastralia on the underside). This loss of massy dermal bone lightened the body and made speedier locomotion on land possible.

What did the first amniotes eat?

Amniotes today exhibit a far wider range of diets than do amphibians – and this was key to their evolutionary success on land. Early fossil amniotes like Holonymus and Paleothyris have sharp teeth designed to pierce through the tough carapaces of invertebrates to reach the flesh within. It has been suggested that the radiation of amniotes may have to do with rising levels of insect diversity.

The development of terrestrial herbivory was a key event in the evolution of the amniotes and we shall spend a few moments trying to make sense of it.

Diadectes - a reptile-like amphibian that was herbivorous

Diadectes, a reptile-like amphibian closely related to basal amniotes, is one of the earliest terrestrial herbivores known to science (its diet is inferred from its dental and skeletal anatomy). The evolution of terrestrial herbivory (a trait unseen in modern amphibians, but common among amniotes) probably involved some sort of progression through the following stages: [a] an omnivorous stage where a diet of invertebrates was supplemented with low-fiber, high-nutrition, cellulose-poor plants or plant-parts (buds, shoots, young leaves etc.) [b] a stage in which the primary diet was high-quality, low-fiber plants and [c] finally, a stage of obligate herbivory where the diet consisted of abundant low-quality, high-fiber vegetation – [c] is an ecological role that has been continuously occupied by a succession of large land-walking animals, from Diadectes to Apatosaurus to modern grazing mammals, for the last 300 million years. High-fiber mature stems and leaves, though widely distributed, are not very digestible. They are rich in cellulose, a carbohydrate which cannot be broken down by the vertebrate digestive system without the aid of certain kinds of bacteria. It is likely that animals foraging in the leaf-litter picked up bacterial species that survived in the gut and, over evolutionary time, became members of the microbial gut flora of a species. What may have begun as incidental commensalism – where neither party benefited particularly from interaction – may have eventually evolved into symbiosis, where the bacteria broke down cellulose in ingested plant matter, making nutrients available to the host, while the host provided shelter and food to the bacteria. The disproportionately large, barrel shaped body in animals like Diadectes is designed to help bacterially ferment ingested plant material (bacterial fermentation is slow and a large space to store plant material for extended periods of time is supremely useful for an obligate herbivore). The evolution of terrestrial herbivory in Amniotes (and in closely related groups) allowed them to attain truly enormous sizes on land.

What divisions do we see among early amniotes?

We now turn, briefly, to consider some of the taxonomic divisions that can be established among early fossil amniotes by studying the structure of the skull. Temporal fenestrae are openings seen in the skull behind the eye-orbits. The establishment of these holes lightens the skull and provides additional edges for the attachment of muscles. The presence, number and position of these holes can be used to categorize amniotes. We observe four different conditions:

1)      Anapsid: No temporal fenestra is seen. Examples: Turtles and possibly Hylonomus and Paleothyris.

2)      Synapsid: A lower temporal fenestra is seen. Examples: Mammals and their extinct fossil relatives and ancestors

3)      Diapsid: Two temporal fenestrae are seen. Examples: Dinosaurs, birds, mosasaurs, lizards

4)      Euryapsid: A temporal fenestra is seen in the upper skull. This condition is probably derived from the Diapsid condition by the loss of the lower fenestra. Example: Icthyosaurs

The crucial bifurcation in the tree of Amniote life is between the Synapsida (mammals and their ancestors) and the Sauropsida (including animals with the Diapsid, Anapsid and Euryapsid conditions – that is, birds and reptiles).

The story of the Sauropsids and the Synapsids is matter for future posts. The sizes and shapes that amniotes would attain over the course of their evolution is truly astonishing. Once vertebrate life solidified its grip on land, sky really was the limit.

Birds are sauropsid amniotes that evolved powered flight

A prodigious mass of bone, scales and flesh lies parked on a submarine atoll in a Cretaceous sea. Swirling shoals of scavenging sharks gorge upon it without ceremony. From nose to tail, the dead creature runs a length of 15 meters – and none of these sharks would have dared approach it in life. He was the king of his world – an apex predator plying a great inland sea way that once ran through the heart of North America. Scientists call him Tylosaurus poriger – member of a group of marine lizards called Mosasaurs that represent the final chapter in the incredible history of giant reptiles at sea.

Tylosaurus poriger was the dominant sea predator in the Western interior seaway

What are mosasaurs, exactly?

One of the first things you ought to know about Mosasaurs is what they are not. That is, they are not Dinosaurs – nor are they particularly close relatives. They are squamates – and more closely related to modern day lizards (particularly monitor lizards) and snakes than to crocodilians, dinosaurs or birds (the so-called “archosaurs”).

Mosasaur Taxonomy (Grossly Simplified)

Mosasaurs were a highly successful and diverse group of sea-going predators that lived during the final age of the dinosaurs, the Cretaceous period. Mosasaur fossils have been found on every continent – and they dominated the world’s oceans for a space of 27 million years.

They appeared on the scene after the demise of one group of large marine reptiles, the Ichthyosaurs, and a drastic reduction in the diversity of another, the Plesiosaurs. Sea levels were higher during the Cretaceous period than at any other time in the Phanerozoic eon (“the age of multicellular life”) and vast competition-free spaces lay open for the Mosasaurs to radiate into. They ranged from 3 meters to 15 meters in length. While they did not quite attain the awe-inspiring dimensions of the largest modern baleen whales, the biggest Mosasaur is somewhat comparable in size to the Sperm Whale, the largest extant toothed whale. Modern toothed whales can, in many ways, be seen as the ecological analogues of these reptillian sea-beasts.

Reptiles play a relatively minor role in modern marine ecosystems, but in the Mesozoic they filled an impressive suite of predatory roles – from bivalve-munching placodonts to the large game hunting mosasaurs. The idea of oceans ruled by gigantic sea monsters excited the Victorian scientists and fossil hunters who first unearthed their remains. “In the mosasaurids”, wrote celebrated Paleontologist Edward Cope in 1869, “we almost realize the fictions of snake-like dragons and sea serpents, which men have have been ever prone to indulge”.

Whence cometh the mosasaurs?

It is clear that, like the Whales of today, Mosasaurs trace their evolutionary origins to a terrestrial ancestor. It is generally supposed that they are descended from a family of semi-aquatic lizards called Aigialosaurs – but gaps in the fossil record make drawing up a precise account of the transition from land-based lizard to aquatic reptile a little problematic. The occurence of Algiasaurs overlaps with the appearance of the earliest mosasaurs. Aigialosaurs display an anatomy that is “intermediate” between that of modern monitor lizards (the closest living relatives of mosasaurs) and primitive mosasaurs.  What changes did the shift to life at sea involve?

Left: Aigialosaurus dalmaticus, Right: Platecarpus tympaniticus

We observe a dramatic increase in the length of vertebral column and a reduction in the relative size of the limbs in the varanid-aigialosaur-mosasaur transition. These changes may represent an increasing commitment to an aquatic mode of life.

We also see a pronounced change in the structure of the tail – from the varanid (monitor lizards) condition where the tail shows little segmentation and all the tail vertebrae are morphologically uniform to the mosasaurine condition, where the tail can be segregated into separate functional units (each unit has a complement of vertebrae that are anatomically distinct from vertebrae in other units): a tail-base that provides the force for a propulsive stroke, an intermediate portion that sways and is displaced during the stroke, a “hinge” section that joins the main body of the tail to the fin and a set of downturned terminal vertebrae that supports a tail fin. This segmentation is diagrammed below.

Changes in tail structure in the varanid-aigialosaur-mosasaur transition

The ancestors of mosasaurs would have propelled themselves through the water by laterally undulating their entire bodies – as eels and sea-snakes do. In Mosasaurs, however, the front third of the body is stiffened while the rest of the body is flexible – it is the latter portion that undulates when the animal swims.

As far as the limbs go, Aigialosaurs are virtually indistinguishable from monitor lizards. As mosasaur evolution progressed, the five-fingered terrestrial lizard limb-plan had to be traded in for the paddle-like arms seen in later forms. This was likely achieved by changes in embryonic development. The development of the skeletal elements of the limb involves the formation of cartilage from dense connective tissue and the subsequent replacement of cartilage by bone (except at the joints). Genetic changes in the timing of the steps in this process or in the patterning of bone formation can result in major changes to the number and the morphology of bones in the limb – later forms show incomplete ossification (bone formation) and an increase in the number of finger elements in the limb. The evolution of webbed or paddle-like feet is related to the incomplete separation of the digits to form fingers during development (something that can be achieved in a laboratory via mutations to certain genes).

Aigialosaur forelimb vs forelimb of a Mosasaur

Primitive mosasaurs have five-fingered limbs that are not dissimilar to those of Aigialosaurs or monitor lizards. The close resemblance between the fore-limbs of certain later mosasaur species and the fore-limbs of whales is remarkable and is a splendid example of two very distant animal groups answering an environmental challenge with nearly identical anatomical solutions.

How did Mosasaurs move? Could they come ashore?

The transfer of locomotory function from the limbs to the tail seems integral to understanding how early Mosasaurs took to the open oceans. As noted earlier, it is the undulation of the tail that drives Mosasaur motion – this form of motion is also seen in Trout and is known as carangiform swimming. Mosasaur tails are deep and bear large tail fins, providing a large surface area to displace water and generate a propulsive thrust. The base of the tail is stiffened and well-muscled to help maximize the force generated.

The elongated bodies of mosasaurs are not optimized for reducing friction drag and they were probably not pursuit predators. It is more likely that they were ambush hunters – capable of quick targeted bursts of speed.

It is clear that Mosasaurs – so well adapted for life in the sea – could not haul themselves ashore like seals or walruses. The anatomy of the limbs and trunk simply does not permit it. This poses a problem: where, exactly, does a mama mosasaur deposit her eggs? Reptile eggs are not designed to survive or hatch underwater. Modern female sea-turtles solve the problem by clambering ashore to nest. Leatherback Turtle hatchlings are born in the beach sands and make their way towards the sea en masse (a rather dramatic natural event). But this could not have been the case with mosasaur hatchlings. The issue of mosasaur birth befuddled Paleontologists for several decades, until fossilized prenatal embryos were discovered amid the remains of the mosasaur Plioplatecarpus. 4 embryos were also discovered in the posterior trunk of an adult aigialosaur. It is now apparent that Mosasaurs gave birth to live young – a trait observed in a number of extant lizards and in other large marine reptiles (like Plesiosaurs and Ichthyosaurs). The orientation of the embryos in the aigialosaur specimen suggests that the tail came out first and the nostrils last – this minimizes the possibility of drowning.

What did Mosasaurs eat?

Probably just about anything that moved in the water. Mosasaur jaws bear a row of conical, pointed teeth (the complexity of which varies from species to species) designed to tear into large fleshy quarry. Another set of teeth called Pterygoidal teeth – also observed in modern snakes – rises out of the animal’s hard palate (the roof of the mouth) and serves to hold struggling prey in place. Mosasaurs swallowed their food whole without masticating it, so the identity of ingested prey can sometimes be determined from the the stomach contents of fossilized mosasaurs. This gives us a tantalizing window into their feeding habits. We know, for example, that Tylosaurus poriger fed on bony fishes, sharks, birds and even smaller Mosasaurs!

Some mosasaurs had bony “rams” on their snouts that projected out beyond the teeth that they could use to batter and stun prey – much as the prows of Greek triremes were once used to impact and sink enemy vessels.

Certain Mosasaur species had more rounded teeth – well-suited for crushing – and it is thought that they fed on hard-shelled animals like ammonites, which were ubiquitous in the Cretaceous seas.

Could Mosasaurs dive deep?

Deep diving mammals typically have bones of lower density than those that inhabit shallower waters. Animals with dense bones achieve neutral bouyancy (that is, they neither sink nor rise) at shallow depths, while animals with more porous and lighter bones (as well as compressible lungs) have access to wider range of depths. Thus, bone density can be used to determine the depths at which different Mosasaur species might have swam and hunted. Tylosaurus, for example, had a low bone density and was likely a deep diver.

Human divers ascending too quickly from high-pressure depths in the ocean are susceptible to a disease commonly known as the “Bends”. The rapid depressurization leads to inert gases dissolved in the blood, like nitrogen, coming out of solution as bubbles. These bubbles could potentially block blood vessels that supply the bones, leading to cell death or “necrosis” in portions of the bone. This sort of permanent bone injury has been observed in many Mosasaurs – and is a telltale sign of the bends or, if we are to be medically proper, decompression syndrome. This may imply that these Mosasaurs were members of a deep-water species and these decompression events occured when they ascended to shallow waters too quickly.

Scientists believe that, like other marine reptiles, Mosasaurs were at least partially endothermic (endothermy is the ability to generate body heat) – deep-divers like Tylosaurus would probably have to have been to deal with the cold depths of the continental seas they inhabited.

What became of the Mosasaurs?

The end of the reign of the Mosasaurs coincided with the demise of the Dinosaurs on land in the aftermath of KT event, 65 million years ago. The asteroid impact that put an end to the age of reptiles and sparked a worldwide nuclear winter, would have severely affected the productivity of phytoplankton in the world’s oceans. Such a productivity decline would result in the extermination of a very large number of animal groups, particularly those in the upper ranks of the food chain. It is unclear, however, why the extinction event was so selective: why did the Mosasaurs and non-avian dinosaurs perish, but crocodiles, sea-turtles and birds persist? Perhaps we will explore the issue in a future post.

Haeckel's (fairly disturbing) sketches of various bat faces. Microbats have evolved a rather grotesque complement of noses with "nose leaves" to help modulate sound emission

My principal experience with bats comes from summer nights at my Grandmother’s house in South India as a child. Swarms of fruit bats would circle our villa, swooping down on the surrounding vegetation to – we supposed – forage for fruits and nectar. We caught sight of one long-snouted bat drinking from the open lips of a banana flower. I wouldn’t hazard to guess what Taxon I was looking at, since India is home to over a hundred species of bats and I am far from being an expert. Several years have passed and the city I presently live in is home to the largest urban bat colony on the planet. I plan to make a visit to it in the near future – and host at least a few of the resultant photographs here!

Golden Crowned Fruit Bat

The bat holds a unique position among the mammals. They are the only mammals capable of powered flight. They comprise a whopping 20% of all known mammal species. They spend most of their lives in an upside down posture. They rank among the most widely distributed mammals on the planet. However, the most remarkable aspect of bat biology, for me at least, is echolocation – the biological sonar that bats use to navigate and hunt by nightfall. The idea of “Seeing” the world through rebounding sound waves is fantastically alien to our own sensory experience – and I reckon that it is well beyond the limits of human cognition to ever truly understand what it is “like” to experience the world as a bat. Nevertheless, understanding the physical and physiological underpinnings of bat sonar will help us appreciate what a beautiful evolutionary innovation it really is.

A large part of the business of echolocation rests on a simple fact: It is possible to determine the distance between two points by measuring the time it takes for a sound wave to travel from one point to the other and back. The calculation involved in figuring out the distance between two points from a given time delay (that is, the time that passes between sound production and echo) and a known value of sound velocity is trivial provided that neither point is moving.

But consider a bat’s situation, weaving through a cluttered environment at a considerable speed, sound waves bouncing off several objects of varying size, closeness and texture; think of the variety and complexity of the variables involved – and yet bats can accomplish incredible feats of aerial agility in pitch black conditions. Lazzaro Spallanzani, an 18^th century bishop and experimentalist, was surprised to discover that blinded bats could fly confidently around his study without disturbing the wires he had suspended from the ceiling as obstacles. He also discovered that blocking off their ear canals with closed brass tubes drastically diminished their ability to avoid the wires. Bats produce high frequency sounds that lie outside the range of audible frequencies for the human ear – and ultrasound was unknown to 18^th century science. So Spallanzani could only go so far as to say that object perception in bats (or echolocating bats anyway) was related to hearing. The true nature of bat echolocation was only uncovered in the 1940s.

A mental soundscape

The sound source for bat echolocation is expired air. The stroke of the wing and the contraction of the thoracic muscles produces a forceful exhalation. Air rushes past the larynx out through the open mouth or nostrils. Bats have evolved a fairly grotesque complement of noses with various flaps and folds to modify the emergent sound in various ways. Bat calls are ultrasonic and very loud . So loud, in fact, that many bat species find it necessary to close their ears at the moment of sound generation in order to avoid being deafened by their own calls. Some bats are known to produce vocalizations of around 130 db (louder that a rock concert), the very loudest sounds produced in all the animal kingdom. The sound energy is emitted as a directional cone. The call rate changes depending on how close to a target the bat is – from 10-15 pulses per second during normal flight to a continuous buzz just before snagging a winged insect.

Now bat calls are far more structurally complex than one might expect. Calls can sweep through a wide range of frequencies (FM or Frequency Modulated) or hold a single frequency over an extended period of time. CF and FM calls are used in different contexts on account of their different frequency-time profiles. They can also have multiple harmonics. Different species use CF and FM calls for different purposes.

FM call from Pipistrellus pipistrellus

Echo: The sound bounces off a target – say, a particularly unfortunate moth or a tree looming ahead. This reflected sound, or echo, can be orders of magnitude less intense than the emitted call – because of the dissipation of sound energy when it travels through air and when it strikes an object – and the bat ear has evolved to be appropriately sensitive to these quieter echoes. The bat receives the echo response and processes the information in its auditory cortex. Bats do not have an especially high brain-to-body ratio (they lie somewhere between primitive insectivores and other mammals on this scale). But they do have a series of specialized neural pathways and auditory nuclei that act to measure the time delay between echo and call. Bats process various pieces of time-delay and echo frequency information to help create an echo-image of the world. There is evidence to show that, apart from telling the distances to objects, bats can make amazing determinations of size, shape, movement and surface structure from the properties of the received echo.

Constant Frequency calls are used in open spaces because they have a greater operational range. This is because bat ears are most sensitive to the frequencies in their CF calls. Bats also make use of the Doppler effect to detect motion with their CF calls. The Doppler Effect refers to the phenomenon where the frequency of a sound changes depending on the relative motion of the observer and the sound source. The classic example used to illustrate the Doppler Effect is this: a vehicle approaching you produces a sound with a higher pitch (i.e. frequency) than it does when it moves away from you or when it is stationary. The beats of a insect’s wings produces fluctuations in echo intensity that a bat can detect. Thanks to the Doppler effect, the sound returned from a moving target also has a broader range of frequencies than the original CF call. The bat brain can use this information to compute general direction and distance to moving prey. In cases where the frequency of an echo is actually raised above of the audible range for a bat by the doppler effect, they merely reduce the frequency of the call itself (Doppler shit compensation).

Frequency modulated calls can be used in more cluttered environments where it is necessary to clearly distinguish prey from background noise. The broad sweep of frequencies used in FM results in a complex echo structure (a higher resolution echo image) and allows for more precise timing of delay, but it has a smaller physical range. Changes in the spectrum of frequencies of this echo image could indicate a change in distance between the prey and the background. And thus, the bat is able to detect moving prey even in forested areas.

This description is intended to show what a remarkable affair echo-imaging really is. It has allowed bats become “independent of sunlight as a medium for perceiving their world” (“The Biology of Bats”, Gerhard Neuweiler, pg 141). But echolocation has its drawbacks: it involves a serious expenditure of energy and is limited in range.

Bat flight

Bat Anatomy

Bat wings are structurally very different from bird wings. For one, all the digits in the bird forelimb are fused. The 2^nd to 5^th digits of the bat forelimb are greatly elongated and unfused. The wing consists of a membrane of skin stretched out between the digits and between the fifth digit and the sides of the body.  The rigid wings of bird are a better suited for generating lift, but bat wings provide a greater degree of maneuverability on account of how adjustable and flexible they are. The adjustments are performed by the fine action of several separate muscle groups. Bats do however, as a general rule, have slower flight speeds – the fastest known bat clocks in at about 55.92 mph, while the fastest bird can manage level flight speeds of over 150 mph. Bats can brake very effectively by spreading out their hind limbs mid-flight, opening up the uropatagium – a membrane of skin that joins the legs and often encloses the tail – like a drag parachute.

The outspread digits of a bat are light-weight and highly bendable – this is the part of the wing that actually flaps during flight (rather than the entire forelimb, as in birds). Bats need about 8-15 wingbeats per second to stay airborne. The shape of the wing varies depending on the species – fast-flying bats have short, narrow wings, while large bats that eat fruit or pick prey off the ground have large, broad wings.

Bat anatomy and physiology is clearly adapted for life on the wing. The skeleton is light and fragile. The heart is large and muscular – accounting for more of the animal’s mass as a percentage than any other mammalian heart – to provide the rapid circulation required for powered flight. The delicate wing membrane can heal after sustaining tears and wounds. The wing is also provided with sensory receptors that can assess the flow of air over the membrane.

Bats are awkward animals on the ground, however. Their knees are bent backwards and outwards and they lack grasping hand claws. They crawl along surfaces like spiders. Bats have evolved a kind of locking mechanism where the muscles and ligaments of the leg are linked up in such a way that, in a relaxed posture, the sharp claws of the foot are clenched together. While it takes energy for us to close our hands, a bat needs to make an effort to open its foot. This allows bats to hang upside down from the ceiling of a cave without expending any energy!

Bat evolution

Unfortunately, flight-adapted bat bones are thin and do not fossilize easily – and thus the bat fossil record gives scant clues as to the early evolutionary history of the chiroptera. We face similar problems with understanding the early history of the Pterosaurs, a group of flying reptiles that are often mistaken for dinosaurs. Onychonycteris, the very oldest known Chiropteran genus, had longer hindlimbs, more clawed digits and shorter forelimbs than the modern bat. It is unclear whether or not Onychonycteris was capable of echolocation. The first bats appeared in the Eocene (as far as we can reliably tell, anyway), about 40-55 million years ago and, for the most part, appear to be fully differentiated, with a complex auditory apparatus and a nearly modern wing profile.

Icaronycteris, a bat from the Eocene - notice the similarity to modern bats

We observe two major taxonomical divisions in modern bats – the Megachiroptera and the Microchiroptera (megabats and microbats). Megabats have long snouts, big eyes, a claw on the second finger and are primarily found in the tropics and subtropics. Apart from bats of the genus Rousette (they generate ultrasonic calls by clicking their tongues), the megabats are incapable of echolocation. Microbats have small eyes, short snouts with strange noses and are capable of echolocation.

The most recent shared ancestor of microbats and megabats was most likely capable of echolocation. The evolution of flight (powered or otherwise) probably preceded the evolution of echolocation. There are no examples of echolocating ground-based insectivores. There are, however, examples of cave-dwelling birds that have developed a relatively crude form of echolocation. Bats may have developed the ability to echolocate to navigate through caves. Bats invaded a hitherto unoccupied nocturnal ecological niche when they took to the air and the associated selection pressures may well have driven the development of echolocation. Echolocation and flight may have evolved concurrently.

It is suggested that the ability to echolocate was secondarily lost in megabats The microbats retained it and, indeed, it is remarkable to see how closely the echolocative systems of different bat orders separated by many millions of years of evolution resemble one another – that is, they appear to display convergent evolution.

At any rate, bats are a highly successful and well-researched group of mammals – I only wish we had a more robust fossil record to seal the deal on their evolutionary history.

From left to right: Napoleon, Alexander I, tsar of Russia and the poster for Griffith's Birth of a Nation

An armed man on a rearing horse – this is the very image of martial valor. And it’s been done and redone by a great many artists over the centuries. The soft, glowing colors, stately battle gear and upraised armaments do set quite a scene, but the subject in these sorts of compositions – for me at least – has always been the horse itself, with muscles strung taut, nostrils flaring and hooves ready for the charge. Although the age of cavalry charges and horse-drawn plows has long since past, the horse remains a potent cultural and historical icon – more than 40,000 books have been written on the subject, from Xenophon to Michelangelo – and it might be worth investigating how this remarkable animal came to be. Happily, fossil horses are abundant and we can answer many questions about horse prehistory with some measure of certainty.

The horse fossil record is often seized upon by writers of elementary textbooks as a classic example of how paleontology can help inform our understanding of long-term evolution. Horse evolution, in these instances, is usually portrayed as a sort of evolutionary ‘procession’, with the humble Hyracotherium (invariably described as being “about the size of a fox terrier”) shunted off to the rear and the high-shouldered and gallant Equus leading the parade. Long term readers of this blog will know that evolution is a tremendously bushy affair, and that these sorts of neat, linear progressions are simplistic. Nonetheless, it does capture certain important trends we shall discuss in the succeeding paragraphs.

How do horses fit in, taxonomically speaking, with respect to other hoofed animals?

All hoofed mammals fit into the cladistic group, Ungulata. Hooves are, essentially, the modified tips of toes – and differences in the structure of the foot can be used to divvy up the ungulata into two broad categories: even toed ungulates and odd toed ungulates. Animals in the first category sport two major weight-bearing toes – the third and fourth toes of each leg. This group includes camels, goats, cattle, deer, pigs and a number of other hoofed animals. Odd toed ungulates, on the other hand, support themselves, for the most part, on one toe per foot – the third toe. This group includes horses, tapirs and rhinos.

There is evidence to show that, among the ungulata, horses share a more recent common ancestor with tapirs and rhinos. For one, the odd-toed ungulates all have an extended caecum (an outpocketing of the large intestine that is found in greatly reduced form in humans) that helps ferment and digest the cellulose in grass. They also share striking similarities in the anatomy of the teeth and the ankle bones.

Equus is the only surviving genus of the family equidae, and it includes 3 species of asses, 3 species of zebra and the horse.

Hyracotherium to Equus

Hyracotherium is the earliest known fossil horse. This unassuming animal was about 20 centimeters high at the shoulder and was probably a browser, seeking out its fill amidst the leaf-shrubbery rather than out on a grassy plain, as modern horses do. It lived around 50 million years ago, just 15 million years shy of the extinction of the dinosaurs (the Eocene). It spread throughout the Northern Hemisphere and was, by most counts, an evolutionary success story. But it differs from the modern horse in a number of key respects:

1) The modern horse dwarfs the dog-sized Hyracotherium.

2) Hyracotherium has four toes, whereas the horse sports a single sturdy toe/hoof on each foot.

3) the low crowned teeth of Hyracotherium imply a diet of soft leaves. shoots, nuts and fruits. Modern horses, however, are adapted to high-fibre grasses

4) the brain (specifically the frontal cortex) of the modern horse is considerably larger than that of Hyracotherium.

Why did horses get bigger?

Well, for starters, the fossil record does not tell us a story of uniform progression towards larger body size. For example, some of the distant descendants of Hyracotherium, like the Pliocene horse Nannipus, were even smaller than the earliest horses. The body size of Equids remained roughly constant for several million years before large horses appeared on the scene.

So what selective pressures might have led to an increase in body-size over time? The typical explanation has to do with large body size being a line of defense against predators on the open plains. It may also have to do with the shift in diet from high-quality forage to nutrient-poor high-fibre grass (which was roughly concurrent with the spread of open grasslands throughout the world).

In terms of energy derived per unit bulk, horses cannot process food as efficiently as even-toed ungulates that chew their cud and have a modified stomach with four chambers. The fermentation process that digests cellulose (with the aid of symbiotic bacteria) in the caecum of the horse is almost exactly mirrored in the proverbial “four stomachs” of cows and goats. While they cannot match even-toed ungulates for energy efficiency per unit mass, they can push a greater amount of material through their digestive system in a given amount of time. They are also specially adapted to subsisting on low-quality grasses which their even-toed counterparts could not survive on for long.

Larger animals are able to conserve energy better (on account of their greater ability to retain heat compared to smaller animasl) and this might have been driving force towards greater size. Body size may have also contributed to an increase in running speed.

Hooves and legs

As horses became more adapted for life on seas of rolling grass, they underwent a number of crucial anatomical changes. The length of the bones of the foot increased (a trend we saw in earlier posts on cats and theropod dinosaurs) and the number of hooves decreased, with the third digit becoming more pronounced: these are both adaptations for a cursorial lifestyle. The arrangement of tendons in the lower leg and the connections between the leg bones work to store elastic energy and reapply it with each stride (the so-called “springing step”). Horses have even evolved a way to expend less energy standing up than sitting down.

Grasses are hardy plants and have evolved various means of protecting themselves from plant predators. They inflict heavy wear and damage upon the teeth of herbivores. And, as a consequence, herbivores like the horse, have evolved high crowned teeth, covered in cementum with folds of enamel (hypsodonty) to deal with the tough food. Many of premolars changed to molars. There is also a long gap between the incisors and the premolars that is absent in the very earliest horses, like Hyracotherium. This adds distance between the nose and the eyes, allowing the horse to keep an eye out for predators whilst grazing. Rather fortitiously, it also provides space for the insertion of a bit, an important part of horse riding.

The Horse brain also increased in relative size over the course of the last 50 million years, although the precise reasons for this change are uncertain. It may reflect a major increase in intelligence (a notoriously difficult concept to define in animals to begin with) or may be related to the increasing complexity of the sensory apparatus of the horse.

Frankly, I haven’t been good about posting here with anything even approaching regularity, so it’s no wonder that the growth of my “readership” (if it can be called that) has proceeded in fits and starts - jumps punctuated by long periods of declining traffic. I’ve gotten comfortable seeing this history as a long-term project that I can return to whenever the circumstances suit me. But blogging doesn’t work that way and the lengthy pauses in my progress have prevented me from really building up a regular audience. I’m going to to try and reduce the content-to-post ratio to manageable levels, so I can post more often.

At any rate, I really am quite appreciative of those who have taken the time to read my material or even link to it in the past! These include David Orr at Love in the time of Chasmosaurs (I’ve been a long-time reader of his blog!) and David Tana at Superoceras, both fine science writers well-worth checking out.

Also, there appears to be an industrious soul by the name of Henrik Osterman who is about to release a paleontology-centric podcast over the course of the next few days – something I’ve rather been looking forward to! So head to over to palaeocast to get a hold of the program when it gets out.

A whale dies.

Her great teal-skinned carcass plummets to the depths of the ocean.

She strikes ground three thousand meters below the water surface. No sunlight ventures here, but life continues to hum along (albeit with somewhat reduced vigor) in this pitch-black benthic universe, where water pressures can run up to an astonishing 300 atmospheres (that is, 300 times the pressure exerted by the overlying column of air at sea level) and photosynthesis is impossible. Adult whales attain weights that range between 30 to 160 tonnes and, upon their death and fall, represent a significant input of biomass for the carbon-starved lower reaches of the ocean. In fact, the dead and decaying bodies of whales can sustain a mini-ecosystem of opportunistic deep-sea scavengers over a period of decades. Within a month, the body will be swarmed over by hundreds of slimy, superficially eel-like scavengers called hagfish. The hagfish has no jaws and is instead equipped with a protrusible “tongue-like” structure that bears “teeth” (or, really, two serrated tooth-plates that are designed to grasp and draw meat into the mouth). The animal forces its head against the flesh, ties itself into a knot against the skin of the dead whale and proceeds to rasp away at the muscle tissue. Often times it only eats away as much muscle as is necessary to bore a hole into the body cavity – where it can then feast on the soft internal organs. Some hagfish seek the easy route to the viscera by entering the mouth or anus.

One of the most chilling scenes in Attenborough’s masterful documentary on the Earth’s Oceans, the Blue Planet, is deep-sea footage of wormy hagfish, “thick as an arm”, chowing down on the pale remains of a whale.

Well, that’s a pleasant image – but what do hagfish have to do with vertebrate evolution?

Hagfish are unusual creatures. For starters, they aren’t really vertebrates, let alone fish – they aren’t endowed with a vertebral column. They lack jaws. They have multiple hearts. They can absorb organic matter through their skins, not unlike several lower invertebrates. They bear two simple eye spots which can detect differences in light levels but cannot, as far as we know, construct detailed images. A lot of the vertebrate innovations we take for granted are nowhere to be found in this curious organism.They do, however, posses a skull made of cartilage – and this property grants them a place within the clade Craniata, which we shall define and discuss shortly. Hagfish are one of two possible remnants of a very ancient group of so-called “jawless fish” (the latter word being used loosely in this context) that first made its appearance some 400 million years ago. Jawless fish were probably the first animal group to evolve cellular bone, paired limbs and complex sensory systems to detect sound waves and monitor pitch, yaw and roll. They therefore represent a crucial evolutionary stage in the history of vertebrate life.

Vertebrates? Jawless fish? Craniata? What’s all this about?

It can be difficult keeping the various taxonomic hurdles that separate tunicates (sessile filter-feeding animals we discussed in a previous post) from vertebrates in mind as we move forward. But there is a logic to the madness. Let’s play God and try to construct a vertebrate from scratch.

So we start with a small eel-shaped organism with a series of paired muscle blocks that runs down the length of the body.

We need a bundle of nerve cells (and their attendant supporting cells) to run along the back to carry electrochemical signals to and from each of these muscle blocks – this is the nerve cord. We also need a rigid supporting rod to run along the back for muscle attachment and to prevent the body from telescoping or shortening when the muscle blocks contract. However, it should not be so rigid as to prevent the body from twisting or curving at all (throwing the body into a series of lateral undulations or curves is precisely how the muscle blocks achieve propulsion). Cartilage performs this task adequately. We have just constructed the Notochord – a rod of cartilaginous tissue that runs below the nerve cord.

Our organism will naturally possess a mouth through which it can draw in food. But the absence of jaws in the earliest vertebrates (as we mentioned earlier) means that they must rely on filter-feeding to acquire food. The food and water will have to be drawn into a sac, which we shall call the pharynx. The water can be expelled through slits along the sides of the pharynx – the so-called gill slits – while the food is conveyed to the gut. In higher forms, the gill slits are richly provided with blood vessels which can extract oxygen from the water being expelled. The gills are supported by a series of catilaginous gill arches.

Pretty much any organism that displays these characteristics – nerve cord, notochord, pharynx, gill slits- at some point in its life cycle (humans display these during embryonic development, many tunicates in the larval stage and lanclets in the adult stage) may be identified as a chordate. So we’re past the first hurdle. Pikaia (530 mya) is the among the earliest examples of a primitive chordate.

Pikaia, a possible primitive chordate from the Cambrian.

So we’re done with chordates, but what are craniates?

No invertebrate head displays the degree of functional and structural complexity seen in the vertebrate head. Although a “front end” can certainly be identified in lancelets and tunicate larvae, there is no “head”, so to speak. The notochord extends right up to where we’d expect the head to be. So the next step is the creation of the head. The anterior (or front) end of the nerve cord must be expanded into a complex brain which can wield centralized command over the nervous system. New sensory apparatus and cranial nerves must be built to detect oncoming sources of food or possible predators. The earliest traces of this process can be seen in Pikaia. Furthermore, a cartilaginous brain case became neccessary to house and protect this new equipment. We have now constructed a “craniate” – a chordate with a highly specialized head. This group contains both true vertebrates and hagfishes.

This transition was made possible by the duplication of clusters of “Hox genes” over evolutionary time by mutations (gene duplications are a widely observed and studied phenomenon). The end result of this was the creation of a special zone of tissue above the nerve cord in the embryo, the neural crest, from which cells migrate away during development to create various features important to the form and function of the brain. The origins of the neural crest – which is also associated with other “typically vertebrate” tissues like bone – is an area of active research.

And what are vertebrates?

To provide for further muscle attachment and protection of the nerve cord, a series of cartilagnous arches called neural arches running along the length of the back became neccessary at some point. A complementary series of inverted arches called centra was added to this design. A neural arch plus a centrum constitutes a vertebra – and the series of vertebra may be referred to as the vertebral column. In vertebrates, this segmented backbone replaces the notochord during embryonic development. Congratulations, we have now constructed a primitive vertebrate!

We’ve been speaking of cartilage thus far, but what of bone?

The next great evolutionary leap in this story is the appearance of a hard mineralized tissue we commonly call bone. Many explanations for the emergence of this hugely important verebrate characteristic have been formulated, but it must be remembered that many invertebrate groups have evolved hard mineralized tissues – as seen in the armor of crustaceans or the calcium carbonate shells of mollusks. The formation of bone involves the secretion of a matrix of long chain sugars and fibrous protein by certain cells (called Osteoblasts) and the consequent deposition and crystallization of calcium phosphate in this matrix, hardening it. This process is roughly mirrored in invertebrates that build exoskeletons and shells, but with different cells, matrix components and minerals. In a sense, vertebrates have two skeletons: an outer dermal (exo)skeleton and an endoskeleton which can be constructed from either cartilage or bone. Of the former, little remains in humans outside of the skull. Dermal bones are formed within the skin and are derived, as you might have guessed, from the dermis, a layer of the skin.

The earliest bones were dermal bones. A cartilaginous endoskeleton was probably already in place by the time bone made a real appearance in craniate anatomy. Cartilage does not usually make it into the fossil record unless it is particularly dense and thus our knowledge of the skeleton that supported the tail and trunk of the earliest fishes is wanting. The dermal bones we will deal with in the next section, on the other hand, are far better preserved and shaped out some of the most extraordinary heads in all of vertebrate history.

In later jawless fish we observe the beginnings of the cartilage-derived bones that constitute the endoskeleton of most vertebrates.

Even in primitive Jawless fish, bone comes in two flavors: cellular and acellular. These types are essentially the same in terms of matrix composition, but differ in the fact that cellular bone has cell-spaces for bone forming cells. The dermal bones of these early fishes consists of a lower compact and closely layered base of acellular bone, a middle layer of spongy cellular bone and an overlying later of dentine projections.

So why did fish evolve bones?

1) Bone might have initially served as a storehouse for phosphate and calcium. Calcium plays a massive role in cell signalling and in maintaining osmotic pressure in the body. Phosphate plays a similarly major role in cell metabolism and the generation of energy. By maintaining a ready supply of these, marine jawless fish might have been able to venture into brackish or fresh waters with lower calcium or phosphate levels without experiencing any distress – they could merely make withdrawals from or deposits in their calcium phosphate “banks” as necessary.

2) Bones can serve as a protective casing for delicate sensory apparatus or viscera. A rather picturesque suggestion from some workers in the field has it that bone evolved in early fishes in response to predation from giant sea-scorpions called Eurypterids in the Ordovician seas. Indeed, early fish are often described as “armored” – and with good reason.

Other suggestions point to bones as being beneficial for swimming in some way and for the functioning/insulation of a set of electrosense receptors.

When and where did the first fish evolve?

The earliest fossil fish which can incontrovertibly be identified as such – dates to about 430 million years ago – a period in the earth’s geologic history known as the Ordovician (which, as I have recently learnt from a book on Roman History, is actually named after a Welsh tribe that took on the advancing legions of the empire). As might be expected, these creatures lacked the biting mechanism so common among higher vertebrates. It is a little strange to think of a vertebrate with no movable bony mouth parts to speak of – jaws are, after all, central for the feeding behavior of most backboned animals, from sharks to elephants.

Astraspis is a jawless fish with two far-set eyes and a bony “head shield” made up of a number of plates. The trunk of the body is covered in small overlapping scales. A large number of gill openings may be found on the sides of the organism. The head shield bears a few other holes – two for the eyes, one for the nostril and one for a third eye or pineal organ. The pineal organ is sensitive to light levels but uses a mechanism of photoreception that is different from that of the other two eyes. The pineal organ is actually visible in some modern animals, including certain amphibians. The animal is roughly torpedo shaped, a design that limits drag.

It must be noted here that Hagfish lack bony parts and therefore represent a condition that is even more primitive than this.

These early fish – ostracoderms to be precise – appear to have dwelt in warm, shallow seas on the margins of the continents (the supercontinent of Gondawanaland is sometimes posited as the cradle of ostracoderm evolution). Deeper sediments yield numerous invertebrate fossils, but no examples of ostracoderms – suggesting that these animals were incapable of swimming across stretches of open ocean. In terms of ecological niche, many ostracoderms were somewhat close to the lowly tribolite. The position of the mouth on the underside of the body (as well as various details of the skull anatomy) seems to indicate that they were bottom-feeders, sucking up detritus and other organic matter and burrowing into the sediment. The true patricians of this era were eurypterid scorpions and giant squid-like nautiloids. BBC’s Walking with Monsters had a decent scene depicting an encounter between a school of Cephelaspis and a mighty eurypterid.

How diverse were Ostracoderms?

As in other vertebrates, the structure of the skull can be used to divvy up Jawless Fish (ostracoderms) into a number of taxonomic categories.

1 - Astraspis, an early jawless fish, 2 - Hemicyclaspsis, 3 = An Anaspsid, 4 - 2 Heterostracans

One group is the Heterostracans. Their head bones include one bony plate on the top, the bottom and on each set of gill slits. The water isn’t expelled through individual slits, but through two vents at the back of the skull. These were the first fishes to invade freshwater.

During the Ordovician, the earth experienced a massive glacial episode that locked up large amounts of water in ice-sheets, thereby reducing sea levels. New lanes of shallow water opened up between the continents – and the ostracoderms may have taken advantage of that to distribute themselves across the seas.
Among the heterostracans, which were at their height in the following Silurian period, we see numerous experiments with horn-like structures which may have helped stabilize the organism, anchor it to the sea bed or stir up food particles. They did not have paired fins behind their heads, making them fairly clumsy swimmers.

While the tail and posterior half of the body were covered in small scales, allowing the body some degree of flexibility and the tail the opportunity for propulsion, the weight and dimensions of the head shield probably made stable directional swimming a difficult affair (think of how difficult it is to push a heavy cart from behind and keep it on course!). This would not have been a serious impediment anyway, given that these creatures probably spent most of their lives wriggling about in the tidal sediments.

Some jawless-fish, like Hemicyclaspsis, a member of the Osteostraci (yet another group of jawless fishes), were optimized for bottom feeding in terms of body shape.  It had two closely-set eyes and a large roof of solid bone  covering the head (with a fairly low slope from the rim of the head shield to the eyes). The flatter the organism, the lower the pressure differential between the top and bottom. And, therefore, the lift is lower – meaning that the organism has to expend less energy trying to stay close to the bed. Interesting case studies have been done where researchers demonstrate that the evolutionary process converges on what essentially amounts to the solution to a fluid dynamics problem.

Hemicyclapsis also sports two paired fins and many complex bits of sensory hardware – a proto-nose, a proto-ear and the rudiments of a lateral-line system for detecting movement and vibration in the surrounding water (seen in modern fish). A line of sense receptors (probably chemosensory or electrosensory) is located on the head shield. Water was forced into the pharynx by the act of swimming – this allows for water to pass through the gills. This sort of respiration, called “Ram respiration”, means that the fish will asphyxiate if it ceases to move for a sufficient time period.

The structure of the brains of some jawless fish can be inferred from studying the thin outlines of cartilage-derived bone that enveloped many of the soft organs of the head within the cartilage parts of the skull. Reconstructions reveal a primitive brain similar to that of lampreys and hagfish. It can be partitioned into a fore, mid and hindbrain.

This is a simplified sketch of an Ostracoderm brain cast. I have omitted lots of extraneous detail.

Not all jawless fish were bottom dwellers. Anaspsids, for example, were scaly streamlined jawless fish with paired fins that could feed on suspended food particles – possibly algae. The skull is not weighty – there is no head-shield – and anaspids may have been active swimmers. They may be ancestral to the modern lamprey – which is not covered in scales, but in skin. There are traces of a circular cartilage with surrounds the mouth – a situation similar to that seen in lampreys. The lamprey and the hagfish are the only two surviving jawless fish.

Illustrations of some jawless fish resemble surrealist works of arts. Some of these fish have heads shaped like flying saucers, screwdrivers and vacuum cleaners. Many of these adaptations have no modern analogue and their precise functional significance may remain unknown forever. Some of them had rod like bony processes that could be used to suck up food particles. Others had huge dorsal crests or eyes set apart like headlights. The diversity of forms is massive and one gets the sense that we’re trying to reconstruct a lost opera from just a few notes. What is clear is that the jawless fish were successful in terms of both distribution and longevity – after all, they lasted into the geological epoch during which the first fish were making that giant leap onto land.

Tusk met tusk on the arid fields of Rafiah, Palestine in 217 BC. Alexander’s great empire had fallen into the hands of a number of feuding successor dynasties that ruled all the known world between Macedonia and the Punjab. Syria was a disputed border-land between the imperial domains of two princes in particular, Antiochus III and Ptolemy V. Antiochus, who would later challenge the rising Roman superpower and acquired the epithet “the Great”, approached from the east with 62,000 infantrymen, a few thousand horsemen and over a hundred war elephants. Ptolemy fielded similar numbers of men, horses and elephants. The signal was given and the elephant contingents charged at one another. Antiochus’ Elephants were of Indian origin, while Ptolemy’s were North African. The ground rumbled and dust clouds leapt into the air as they thundered across the sand.

Things turned sour quickly. The North African elephants, spooked by the strange smell of their subcontinental adversaries – from whom they were separated, evolutionarily speaking, by a space of 7.6 million years – suddenly began to panic and retreat, throwing Ptolemy’s right wing into a dreadful rout. Greek troops fought in a closely-knit military formation called a phalanx, where the mobility of a single soldier was severely limited. It’s difficult to imagine the sheer horror of being stuck in a box of shields and spears while a 4 ton animal rampages towards your position, tearing through ranks of armed men with ease. The Battle of Raphia, as the engagement came to be known, is generally regarded as one of the largest elephant battles in the classical world.

Elephants were used to break formations and wreak havoc by many generals in the ancient world: most famously by Hannibal, who hurled them at the Roman legions – with varying degrees of success- during the second Punic War. The Romans themselves would later make use of elephants, albeit in far reduced numbers, against Celtic armies in Iberia and Britain.

Apart from their use as instruments of war, Elephants have been – in various times and places – objects of reverence, beasts of burden and symbols of imperial might. The caliph Harun-al-Rashid  gifted an albino elephant named Abu-Abbas to Charlemagne as a token of friendship. Legend has it that Charlemagne later called upon Abbas in a tremendous battle against the Viking Danes. That’s epic.

Depictions of Elephants from medieval Europe

Elephants were, apparently, quite mysterious to the writers of medieval bestiaries. They have this to say about the subject:

“They possess the quality of mercy. If by chance they see a man wandering in the desert, they offer to lead him to familiar paths. Or if they encounter herds of cattle huddled together, they make their way carefully and peaceably lest their tusks kill any animal in their way.” – Aberdeen Bestiary, 1200 CE.

 ”There is an animal, which is called “elephant,” which possesses no desire for sexual intercourse … They live 300 years.” – Harley MS 3244, 1255 – 1265 CE

This one is trippy:

“Between elephants and dragons is everlasting fighting, for the dragon with his tail bindeth and spanneth the elephant, and the elephant with his foot and with his nose throweth down the dragon, and the dragon bindeth and spanneth the elephant’s legs, and maketh him fall, but the dragon buyeth it full sore: for while he slayeth the elephant, the elephant falleth upon him and slayeth him.” – Batholomaeus Anglicus, 13th century CE.

In fact, male Elephants have an incredible libido; have been seen killing livestock, live for about 60 years and only occasionally engage dragons in mortal combat.

Elephants … and Hyraxes?

Unlikely relatives: From left to right, a Hyrax, a Dugong and an Elephant

Bizarrely, the closest living relatives of modern Elephants are Dugongs, Manatees and Hyraxes. The evidence for this curious relationship comes from DNA/protein sequence data and shared characteristics like the late eruption of permanent teeth and undescended male gonads (yes, I went there). They collectively belong to the Mammalian clade Afrotheria – so named because of the African origins of the group in the middle Cretaceous. Elephants belong to the subgroup Proboscidea, an order that, until a few thousand years ago, survived in every type of terrestrial environment on the planet – from icy tundra to tropical rainforest.

The tree of Elephant evolution

Where and when did Elephant evolution begin?

The Proboscidea seemed to have begun their divergence from the Sirenia (the group containing manatees and dugongs) in the Eocene along the swampy shores of the Tethys sea (now the Mediterranean rim) at sites like modern-day Fayoum, Egypt, where fossil specimens of the two groups can be found in close proximity to one another.  Global temperatures were then much higher than they are today and there were no ice-caps at the poles at that time.

Among the earliest members of the order Proboscidea was a hippopotamus-like creature named Moeritherium that lived about 35 million years ago. It had short,  stout  legs,  a long body and a short tail. The animal probably spent a great deal of time wading through swamps and riverine habitats, consuming fresh water-vegetation. It had two pairs of short tusks which, like the tusks of modern Elephants, are simply modifications of the second incisor. Tusks are the longest teeth in the animal kingdom!

They have trunks?

Moetherium had a flexible upper lip for grasping food, a sort of “proto-trunk” similar to what we see in modern tapirs. The trunk or “Proboscis” of an Elephant is a combination of the nose and the upper lip. The existence of a trunk in a fossil species can be inferred from the size and position of the nasal opening and the structure of a bone canal below the eye socket (called the infraorbital canal) which conveys nerves and blood vessels to parts of the face or, in the case of elephants, to the trunk. The trunk is a phenomenal multi-purpose tool that can be used for everything from the delicate handling of branches to siphoning up water to drink.

Is Moetherium ancestal to the modern elephant? 

Semi-aquatic habits are seen in a number of early Proboscideans and many of them bear some resemblance to the tapir. But Moeritherium is probably not a direct ancestor of the Elephantidae (which includes all living Elephants), but an offshoot on the family tree that has left no living descendants.

Paleomastodon, a possible ancestor of both mastodons and elephants and a close relative of Moeritherium, also had two pairs of the tusks. The lower pair was shovel-shaped and possibly used to scoop up freshwater plants. The trunk is more obvious in this species than in the roughly contemporaneous Moeritherium.

Paleomastodon

Mastodons? Are those like Mammoths?

Mammutidae is a major Proboscid family that (probably) owes its ancestry to Palaeomastodon.  It includes the iconic woolly Mastodon – which occupies a somewhat august position in the history of Paleontology as one of the earliest large fossil species to have its anatomy fully reconstructed and exhibited to the public. Thomas Jefferson used the past existence of the mighty Mastodon on American soil as an argument against the (primarily French) notion of American Degeneracy, the goofy idea that atmospheric conditions in the New World weakened both men and animals – making them smaller and less intelligent. It’s easy to confuse Mastodons with Mammoths, but the two are actually only distantly related and the superficial similarities between them are more the result of convergent evolution than any phylogenetic affinity. Mastodons were shorter than mammoths and had stockier legs. They browsed on shrubs and the crowns of their molars had pointed cusps for clipping leaves. This stands in contrast to the high crowned molars of mammoths that were better suited for grinding down grass. They had longer and less curved tusks. They both survived to the end of the Ice Age and faced predation from human beings.

So, we’re done with proto-elephants – what sorts of Proboscideans have appeared since?

The order Proboscidea is notorious for its incredible experiments in tusk shape and length. A rapid proliferation of forms took place in the Miocene, producing shovel-tusks, downward curving tusks and a number of other strange parodies of the conventional spiral curved elephant tusk. We will deal with three major branches that evolved in the midst of this diversification, the Gomphotheres, Stegodons and Deinotheres.

From left to right: Deinotherium, Platybelodon (a shovel-tusked Gomphothere) and Stegodon

Deinotheres thrived during the Miocene. Some of them attained heights and lengths that dwarf those of modern elephants. The best known of them, Deinotherium, stood at a shoulder-height of about 3.5-4.5 meters, making it about as tall as a double-decker bus. It is the third largest land mammal known to science. The build of its skull and its dentition was so unusual that it led one anatomist to suggest that it was an aquatic beast that anchored itself to the riverbed with the aid of its tusks. Deinotheres lack any upper tusks, but they sport a pair of dramatically downward-curving tusks that may have been used to dig up food or as a sexual display. The lower jaw is itself bent downwards and lacks any canines. The retracted facial and nasal bones also seem to indicate that these animals had trunks. Deinotheres were never very diverse and the only evolutionary trend they seem to display within the family is a general increase in size.

The Gomphotheres represent a diverse collection of elephant-like animals that appeared on the scene in the Miocene. They originated in Africa and radiated out throughout Eurasia and the New World. They bear four substantial tusks, two upper and two lower. Some specialized forms developed shovel-shaped lower tusks that might have been used to scrape up plants for consumption.

Modern elephants posses a gland called the “musth” gland on the side of the face that produces chemicals during heightened sexual activity. These chemicals are associated with agitation and violent outbursts – and elephants in musth on an elephant farm are usually kept under lock and chain because they have an increased likelihood of going on a stampede. The musth gland first appears in the Miocene and can be inferred in fossil species from the shape of the sides of the skull.

One common feature among shovel-tusked Gomphotheres is a long lower jaw – something that contrasts strongly with the greatly reduced lower jaw of modern elephants. A short, strong neck was necessary to hold the head and dental apparatus up.

Both Stegodonts and modern Elephants are derived from Gomphotheres. Stegodonts differ from Elephants in the structure of their teeth, but generally resemble modern Elephantidae. A pair of vestigial tusks remained in the lower jaw. The upper tusks were long and nearly touched the ground. In an interesting case of island dwarfism, one species of Stegodon seems to have undergone a dramatic reduction in size on the Indonesian island of Flores. It is comparable in size to a small water buffalo. Interestingly, they lived contemporaneously with a species of dwarf Hominid named Homo florensis (often dubbed “Hobbits”) on the same island. And, while Homo sapiens took on Mammoths in Eurasia and North America, hobbits may well have speared and killed pygmy elephants in the tropical mists of Flores (evidence to this effect has been uncovered).

A number of general trends can be observed throughout the course of elephant evolution:

1) A general increase in size

2) The loss of teeth. The typical mode of replacement of teeth seen in most mammals (including ourselves) was abandoned for a system where older worn out teeth in the front of the jaw are replaced by newer teeth from the back of the jaw – rather like a conveyor belt. The old tooth drops out or is swallowed. Each half-jaw of an elephant bears either a single tooth, or two teeth – one on its way out and one on its way in. An elephant will run through 6 or, if it’s very lucky, 7 molars in its life time. The destruction of the final tooth means certain death.

3) An increase in the complexity of teeth and the length and diameter of tusks.

4) A transition from browsing to grazing that roughly coincides with the spectacular rise of grasses across the planet as CO2 levels and global temperatures dipped in the Miocene.

By 11,000 BC, Gomphotheres still roamed the forests of South America, Mastodon herds still rolled across the frozen wastelands of North America and Stegodons could still be found in island forests in Indonesia. In fact, there may have been well over a dozen Proboscidea species on the planet when Humans first started migrating out of the African continent. Unfortunately, there are barely 3 species left today. In the next post, we’ll deal with the Elephantidae: the Wooly Mammoth, the Asian elephant and the African elephant.

Truth be told, I’d rather sit through a poorly-executed podcast than a badly written blog entry. I’ve tried rummaging through the wastelands of the science podcastosphere in search of a Paleontology podcast of some sort. So far, I’ve only located this one: This Week in Dinosaurs. It lasted for a single episode. So R.I.P TWID, I guess.

This is my plea: If you’re a Paleo-person (and I include professionals and enthusiasts alike under this umbrella) of any kind and have the time, equipment and expertise to host a Paleontology podcast, please do consider doing so. I’m willing to hazard that there’s a large community of hobbyists and amateurs out there just waiting to get their hands on such a show. I’d be glad to help/promote in any way possible.

EDIT: Turns out that there is a non-defunct Dinosaur podcast show out there: http://www.dinorama.net/ Awesome!