Fins to Feet


Sauropods – Whale Lizards
May 13, 2012, 2:26 am
Filed under: Uncategorized

The force of gravity – together with certain physiological and ecological constraints – holds in check the evolution of ever larger body-sizes among mammals on land. By becoming secondarily adapted to life in water, however, whales have been able to circumvent at least some of these size restrictions.

Left – A Blue Whale, the largest living animal known to science, Right – The African Elephant, the largest extant land animal

The largest extant land mammal – the African Elephant – is considerably outweighed by sea-going baleen whales of even middling proportions. In all of the Cenozoic era (the 65 million year period following the extinction of the dinosaurs), no terrestrial mammal ever grew to match the modern Gray Whale, let alone the Blue whale, in body dimensions. The reduced weight constraints of an aquatic medium accounts for this apparent difference in maximum attainable size.

Outside of the mammals, however, there is one group of extinct land creatures that did approach, and in some cases surpass, the awe-inspiring lengths of the largest baleen whales* pushing the evolutionary envelope in terms of height, length, weight and girth in a way that no other terrestrial animal group ever did.

Together with whales, the sauropods are examples of animal gigantism par excellence.


What are Sauropods?

The term “Sauropoda” refers to group of quadrupedal, megaherbivorous dinosaurs that existed for a span of over 135 million years – from the close of the Triassic period to the very end of the reign of the dinosaurs. Their highly distinctive body plan was characterized by:

1)       An elongate neck. One that, in some genera, grew to double the length of the trunk.
2)       A small skull relative to body size, with enlarged eye-orbits and highly placed nasal openings
3)       A massive body with a long tail
4)       Stout, columnar limbs positioned directly below the body. The bones of the hands/forefeet were arranged into a roughly tubular configuration (vertical with respect to the ground), with the phalanges (finger-bones) reduced. Only the first digit bore a claw – and this too was lost in some of the later groups. The structure of the hind foot was notably different from that of the fore foot – the phalanges were larger and three of the digits were typically claw bearing. The bones of the hindfoot were not arranged vertically with respect to the ground, as was the case with the hand bones, but appear to have assumed a “flatter” posture (semi-plantigrade). A cushioning “pad” of tissue seems to have been present at the base of the hindfoot. Reconstructions of sauropod hands and feet as either elephant-like, with nail-like hooves, or lizard-like, with clawed fingers splayed out every which way, are equally incorrect.

There was limited deviation from this general body plan over the rather lengthy course of sauropod evolution. Paleontologists have puzzled for decades over the ecological, biomechanical and physiological implications of sauropod size and anatomy. How big did they get? What sort of diet fueled those enormous bodies? How did the sauropod heart pump blood across those serpentine necks, all the way to the brain? This article shall consider some of these questions.

 How big did Sauropods get?

I have seen books quantify the dimensions of sauropods in feet, meters, cars, double-decker buses, building stories, elephants and bulldozers. The longest of them (Diplodocus and Supersaurus) hit a length of about 33-35 meters (longer than a blue whale). Even conservative body mass estimates suggest that the heaviest sauropods (Argentinosaurus) weighed over 70 tonnes (10 times the weight of a male African elephant).

The sizes of various sauropod species compared. The depicted sizes of the red sauropod (Amphicoelias) and the grey sauropod (Bruhathkayosaurus) are controversial and I have opted to ignore them in this article.

There were, of course, examples of much smaller sauropods – Eoparasaurus, for example, was only 6 meters long from snout to tail.

The immense size of sauropods would have served as a deterrent against predators (and there was no shortage of large, powerful predators in the world they inhabited). The long neck would have given the animal a wide sphere of access to vegetation.

What did sauropods feed on? How did they process food?

Sauropod teeth – which ranged from pencil shaped to spatulate, depending on the species – were designed primarily for grabbing and tearing vegetation off shoots and branches (‘cropping’) rather than grinding down tough plant matter. There is nothing analogous to the chewing apparatus of modern mammalian herbivores in the oral anatomy of sauropods. No large, flattened, squarish teeth positioned at the back of the jaw to pulverize ingested food items. The head was small and the dentition weak.  We may infer that very limited mechanical breakdown of food took place in the oral cavity before it was swallowed.

It has been proposed that sauropods utilized large stones in the stomach (called gastroliths) to grind down food. This digestive adaptation is called a “gastic mill” and is observed in modern birds. But the small sizes of fossilized ‘gizzard stones’ relative to body dimensions as well as the possibility that they are simply a result of sedimentary processes, has led a number of researchers to dismiss the idea that this form of food reduction played major role in sauropod digestion. But, without a gastric mill or significant oral processing, how did sauropods physically reduce ingested plant matter into smaller, more digestible bits?
Perhaps such processing was not necessary. Like modern vertebrate herbivores, Sauropods almost certainly relied on a community of symbiotic microbes to break down the otherwise-indigestible cellulose present in the cell walls of ingested plant material. This microbe-mediated process, involving the enzymatic breakdown of cellulose (and other carbohydrates) into short chain fatty acids that can be absorbed by the host, is called fermentation. The tremendous sizes of sauropods might have permitted the retention of food in the digestive tract for long periods of time.  Prolonged food retention times and extensive exposure to microbial fermentation may have compensated for the limited mechanical reduction of food in the mouth and gut.

The lengthy necks of sauropods gave them an enormous foraging range. They fed on gymnosperms (conifers), sphenophytes (eg. Horsetails) and pteridophytes (ferns). As flowering plants diversified rapidly during the mid-cretaceous, they too were incorporated into the sauropod diet.

Bird lungs and long necks

The vertebrae and ribs of sauropods have well-developed air-spaces. These air spaces are similar in nature to those found in birds, their closest living relatives, suggesting that sauropods may have sported an avian-style respiratory system – with air-sacs distributed throughout the body. The presence of these air spaces lightened the enormous skeletons of these animals without compromising strength. In addition, the presence of air sacs may have permitted the evolution of one of the signature features of sauropods: an elongate neck. As the length of the pathway of air-conduction between the nostrils and the lungs increases, the amount of so-called anatomical “dead space” increases. Dead space refers to inhaled air, located in the conducting areas of the respiratory system, which does not participate in gas exchange. The large dead space present in the incredibly long tracheas of sauropods would, at first blush, appear to severely lowered breathing performance. Under an avian model of respiration, however, the additional air-storage capacity provided by the air sacs would allow the trachea to overcome this dead space and maintain respiratory efficiency.

The high rates of growth determined from histological analysis of sauropod bone tissues appear to indicate that, for at least part of their life span, sauropods had high basal metabolic rates comparable to large mammals. This high BMR may have slowed down later in the life of the animal. Adult sauropods would have retained heat energy and maintained a relatively stable body temperature by mere virtue of their size (gigantothermy).  Muscular activity, metabolic reactions and digestive processes, such as fermentation in the gut, produced heat internally. The air sacs described earlier served as surfaces for heat exchange.

How did the sauropod heart pump blood to the head?

The vertical distance between the heart and the head in sauropods is dependent on neck posture. If large sauropods did hold their necks upright, the vertical heart-brain distance in many cases would be over 8 meters. Scientists infer that huge blood pressures (over 700 mm Hg) – unheard of among modern animals – would be necessary to supply the head with oxygen and nutrients. The enlarged, highly muscular heart that would be necessary to produce this astonishing hydrostatic pressure would be grossly energy inefficient, take up an inordinate amount of space and suffer from a number of mechanical disadvantages. Various cardiovascular adaptations have been hypothesized to exist in sauropods to get around this issue.

Some workers suggested that the sauropod circulatory system featured multiple ‘hearts’ in series, each accessory heart capable of pumping blood to the next valved pump, making it possible to achieve effective blood flow between the primary heart and the brain. However, no such system has been observed to exist in modern vertebrates and it is unclear how the nervous co-ordination of this congo-line of secondary hearts would have operated. Perhaps sauropod blood had a higher viscosity and erythrocyte count, increasing its oxygen carrying capacity.
The neck posture of sauropods is still widely debated, but if the head were habitually positioned at low-to-medium heights, as appears to be the case in Diplodocus, then there is no need to invoke the presence of a grossly hypertrophied heart or outrageously high blood pressures. Browsing at high elevations for limited periods of time, though costly in terms of cardiac output – may have given sauropods access to critical food resources unavailable to other animals.

Could sauropods rear up?

Kinetic-dynamic modeling of the skeletons of sauropods indicates that at least some of them were capable of briefly rearing up on their hind legs and utilizing their tails as a “third leg” of sorts (a kind of tripodal stance) before dropping back down to a quadrapedal stance. This would have allowed for browsing at great heights. A rearing diplodocus would have been a sight to behold indeed.

One of my favorite television depictions of sauropods was in a BBC production called The Ballad of Big Al. This clip involves a pack of Allosaurus’ launching a concerted attack on a Diplodocus herd. Enjoy!

* These same whales do still have the sauropods safely beat in terms of sheer tonnage.

Sources:
1. Sander, P. Martin, et al. “Biology of the sauropod dinosaurs: the evolution of gigantism.” Biological Reviews 86.1 (2011): 117-155.
2. Farlow, James O. “Speculations about the diet and digestive physiology of herbivorous dinosaurs.” Paleobiology (1987): 60-72.
3. Wings, Oliver, and P. Martin Sander. “No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostriches.”Proceedings of the Royal Society B: Biological Sciences 274.1610 (2007): 635-640.
4. Seymour, Roger S. “Raising the sauropod neck: it costs more to get less.”Biology letters 5.3 (2009): 317-319.
5. Taylor, Michael P., Mathew J. Wedel, and Darren Naish. “Head and neck posture in sauropod dinosaurs inferred from extant animals.” Acta Palaeontologica Polonica 54.2 (2009): 213-220.
6. Apesteguía, S., V. TIDWELL, and K. CARPENTER. “Thunder-lizards: the Sauropodomorph dinosaurs.” The evolution of the hyposphene–hypantrum complex within Sauropoda (2005): 248-267.



Pelycosaurs
January 28, 2012, 11:07 pm
Filed under: Uncategorized

This post is a continuation of “Coming of the Amniotes”

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 each side of the skull behind the eye-orbit, bounded above by the post-orbital and squamosal bones. These holes are called temporal fenestrae. 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 shared a closer phylogenetic affinity with mammals than with crocodiles or lizards, there was 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 supported a large ‘sail’ – a thin enveloping sheath of skin, ligaments and blood vessels. 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, grinding teeth on the roof of the mouth 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. Some reptiles also have bodies that are large enough to effectively store thermal energy due to their low surface-area-to-body-volume ratios  (think dinosaurs or certain modern crocodilians), a trait 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. 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 the 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!

Sources:

1. Kemp, Thomas Stainforth. The origin and evolution of mammals. Oxford: Oxford University Press, 2005.
2. Kemp, Thomas Stainforth, and T. S. Kemp. Mammal-like reptiles and the origin of mammals. London: Academic Press, 1982.
3. Chinsamy-Turan, Anusuya, ed. Forerunners of Mammals: Radiation• Histology• Biology. Indiana University Press, 2011.
4. Van Valkenburgh, B. L. A. I. R. E., and I. Jenkins. “Evolutionary patterns in the history of Permo-Triassic and Cenozoic synapsid predators.” Paleontological Society Papers 8 (2002): 267-288.
5. Benton, Michael J. Vertebrate palaeontology. Wiley. com, 2009.



Coming of the Amniotes
January 5, 2012, 10:20 pm
Filed under: Uncategorized

Steamy, water-logged swamps. Primitive amphibians trawling the undergrowth for food. 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 during this time 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. Even these 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 bear a superficial resemblance to  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 amphibians lived under various degrees of dependence on bodies of water for reproduction and egg-development. The amniotes achieved emancipation from the aquatic realm through a number of important biological innovations. Foremost among these was the Amniote egg.

The Amniote egg is characterized by a hard semi-permeable shell membrane and various “extra-embryonic” tissues that surround the developing embryo, including 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.

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. For now, let us consider what makes the primitive amniote egg a “terrestrial” egg.

Most modern amphibians deposit their eggs in water. Once the eggs hatch, an aquatic larval period commences – 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. Amniote eggs are laid on land and give rise to hatchlings which are essentially miniature versions of the adult form. There is no 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. In these species, 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, these components constitute 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, this relationship means that oxygen uptake through the capsule is insufficient to support embryos beyond a certain size.

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 transferring water from the egg-jelly to the yolk over evolutionary time, leaving the outer portions of the egg dry and effectively eliminating the egg-jelly as a barrier to gas diffusion. A process like this is observed during egg development in reptiles, when water is withdrawn from the albumen and transported to the yolk.

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.

Reptile eggs are larger than amphibian eggs and generally display a higher rates 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 miniature adult rather than a free-living, immature aquatic larva (as in most amphibians), a large store of nutrients (in the form of “yolk”) is needed 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. The increased nutritional requirement imposed by “direct development” is the reason Amniotes needed bigger eggs and larger amounts of yolk. Because more has to be invested in each egg, reptiles also 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 [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 had 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 non-amniote tetrapods. The ancestral-tetrapod-to-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. It has been suggested that the radiation of early amniotes may have  been related 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

Sources:

1. Benton, Michael J. Vertebrate palaeontology. Wiley. com, 2009.
2. Sumida, Stuart, and Karen LM Martin, eds. Amniote Origins: Completing the Transition to Land. Access Online via Elsevier, 1997.
3. Gilbert, Scott F. Developmental biology. Sunderland (Mass.): Sinauer associates, 1994.



Last of the Sea Dragons
December 17, 2011, 9:52 am
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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. 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 imaginations of 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?

Like modern whales, mosasaurs trace their evolutionary origins to a terrestrial ancestor. It is generally thought that they are descended from a family of semi-aquatic lizards called aigialosaurs. Gaps in the fossil record make drawing up a precise account of the transition from land-based lizard to aquatic reptile a little problematic. One complication is that the occurence of Algiasaurs in the geologic record overlaps with the appearance of the earliest mosasaurs. Nonetheless, aigialosaurs display an anatomy that is “intermediate” between that of modern monitor lizards (the closest living relatives of mosasaurs) and primitive mosasaurs.

Left: Aigialosaurus dalmaticus, Right: Platecarpus tympaniticus

What changes did the shift to life at sea involve?

There was a dramatic increase in the length of the vertebral column and a reduction in the relative size of the limbs durng 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 (i.e. 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 divided into anatomically distinct functional units: a tail-base that provides the force for a propulsive stroke, an intermediate portion that sways 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 propelled themselves through the water by laterally undulating their entire bodies  like eels or sea-snakes. 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 was replaced by paddle-like arms better suited for an underwater lifestyle. 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 mosasaur species 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 similar 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 distant vertebrate 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 powerful propulsive thrust. The base of the tail was 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 would have made this impossible. This poses a problem: where, exactly, does a mother mosasaur deposit her eggs? Reptile eggs cannot survive and hatch underwater. Modern female sea-turtles solve the problem by clambering onto land 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, which are also observed in modern snakes, emerged from  animal’s hard palate (the roof of the mouth) and served 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.

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 (the condition under which they neither sink nor rise) at shallow depths, while animals with lighter, more porous bones can maintain stable and efficient swimming at wider range of depths. Bone density can be used as a rough guide for determining the depths at which different Mosasaur species might have 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” or decompression syndrome. 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”. This sort of permanent bone injury has been observed in many Mosasaurs – and is a telltale sign of the bends. 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. Deep-divers like Tylosaurus would probably have to have been to navigate 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.



Suggestions
December 3, 2011, 6:57 pm
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So, I’m thinking of dealing with Mesozoic Sea Reptiles and then the Great American Biotic Interchange next. I’d be glad to take any requests for vertebrate groups (extinct or extant) or topics you think I should cover in the comments section.



Chiroptera: The Bats
November 26, 2011, 4:31 pm
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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, with 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 18th 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 18th 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 digits of the bat forelimb are unfused and the 2nd to 5th digits are greatly elongated. 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.

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.



Evolution of the horse
November 5, 2011, 2:38 am
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NOTE: This piece is under construction!

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.