Fins to Feet

Therapsids Part III – Gorgon-headed terror

This post is the third installment in a multi-part series on the evolution of mammal-like reptiles.

Time has turned it to rock, but even across the distance of so many millions of years, the head of a gorgonopsid still retains an aura of unspeakable ferocity.

Gorgonopsia is a suborder of predaceous therapsids (“mammal-like reptiles”) that arose during the Middle Permian. For much of their early evolutionary history, gorgonopsians were rare, modest-sized carnivores, eclipsed in numbers and ecological relevance by thick-skulled, meat-eating Dinocephalians. The (rather unexplained) demise of the Dinocephalians at the end of the Middle Permian threw open new predatory niches for these “gorgon-faced” hunters to exploit.

Perhaps the most striking feature of the gorgonopsid skull is the presence of two enlarged canines – reminiscent of the dagger-like fangs of sabre-toothed cats from the far more recent Pleistocene epoch – that hang menacingly from the upper jaw. The lower canines were also massive – often only a third shorter than their upper counterparts. To sufficiently clear the canines before delivering a lethal bite, these animals must have been capable of drawing back their lower jaws far enough to produce gape angles in excess of 90o. Kemp’s (1969) functional analysis suggests that Gorgonopsid jaws were capable of performing two kinds of bites: (1) a killing bite in which the animal would use its fearsome canines as stabbing devices, piercing through hide and flesh to disable its prey. The front end of the lower jaw bears a large, massy “chin” that may have served as the mechanical equivalent of a war-club, packing an extra kinetic “punch” when the jaws were drawn shut. (2) a more surgical type of bite in which the incisors interlocked when the jaws were closed, slicing out a jagged edge in the meat. This would have permitted hunks of flesh to be more easily extracted and consumed. The transtion from biting mechanism (1) to (2) was made possible by the movement of the lower jaw to a more forwardly position so that the serrated incisors could intermesh. The large size of the snout indicates that smell was key to the hunting behavior of these ancient predators.

Compared to pelycosaurs like Dimetrodon, the Gorgonopsid post-cranial skeleton is rather gracile and the limbs are longer and more vertically oriented. There was probably less lateral swaying of the trunk during locomotion and the back was kept straighter. The length of the tail is reduced. The body was held well clear off the ground and some skeletal reconstructions even evoke a sense of wolf-like nimbleness. According to some workers, the nature of the articulation between the femur and the acetabular socket of the pelvis suggests that the gorgonopsid hindlimb was capable of switching between a primitive splayed posture and a more erect mammalian stance (similar to the “high walk” of a crocodile). The precise character of limb suspension in Gorgonopsids – which probably lay somewhere in the gray-zone between ‘sprawling’ and ‘fully erect’ – is still debated. The hind-limbs (which accounted for the bulk of the animal’s speed and propulsive power) made a closer approach to a condition of uprightness than the fore-limbs.

While body form did not vary considerably among different gorgonopsid genera, there was certainly a range in size . The smallest known gorgonopsids are comparable in body dimensions to modern day canids. The largest gorgonopsids (members of the genus Inostrancevia, for example) measured around 3.5 meters from nose-tip to tail-end, making them longer than any living land carnivore. These late Permian super-predators were well-adapted for hunting huge, stocky, slow-moving, plant-eating dicynodonts and paraeiasaurs. In some ways, this mode of life was replicated in a much later era by saber-toothed cats that preyed heavily on herbivorous megafauna.

It is worth listing some of the other important features gorgonopsids bear in relation to the pelycosaur-to-mammal transition (the ‘advanced’ traits are italicised and the ‘primitive’ pelycosaur-ish traits are indicated in normal font) – (1) a secondary palate is absent, (2) the dentary of the lower jaw is expanded, (3) the stapes is reduced, (4) cervical and lumbar ribs are present, (5) they display the same advanced bone histology seen in Dinocephalians, (6) no definite statements on the presence or absence of body hair in gorgonopsids can be made. There is, however, a limited amount of evidence indicating that their snouts may have borne vibrissae or whiskers, (7) the relative size and shape of the brain had not changed significantly from the pelycosaur condition.
The gorgonopsids were wiped out by the end Permian extinction event. After their disappearance, new non-therapsid animals would come to take on the mantle of terrestrial super-predator – among them, Raiusuchians and Theropod dinosaurs.

In the next post in this series, we’ll spend some time on two more Therapsid groups: the Dicynodonts and the Therocephalians.

Referenced: 

Kemp, Thomas Stainforth. “On the functional morphology of the gorgonopsid skull.” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (1969): 1-83.

Therapsids Part 2 – Terrible Heads and Food Webs

Generations of fossil hunters have visited the arid fields of the Karoo basin in South Africa in search of ancient life. The rock-layers of the Karoo constitute an invaluable storehouse of information on the evolutionary history of the Therapsids. They have, over the course of a century, yielded up many strange and forgotten Permian beasts – mineralized relics from a time when the Karoo was a much greener place.

What are Dinocephalians and what do they have to do with Therapsid evolution?

The Dinocephalians represent the first significant wave of Therapsid diversification. They were a group of medium-to-large sized herbivores and carnivores that enjoyed global distribution during the Middle Permian. Paleontologists have identified a few features that seem to unite the group:

1) The upper and lower incisors interlocked when the jaws were drawn shut. The lower incisors had surfaces on the tongue-ward side called ‘heels’ that met with the upper incisors to form food-crushing platforms.
2) The nose projects ventrally in many forms, giving the face a markedly downturned appearance
3) The jaw is shortened and the jaw-joint is is more anteriorly situated
4) Significant thickening of the skull bones (“pachytosis”) is a commonplace feature – hence the name “Dinocephalia”, which means “terrible heads”.
5) The temporal fenestra is longer along the dorso-ventral axis than the antero-posterior axis.

Large herbivorous dinocephalians like Tapinocephalus and Moschops attained weights and lengths comparable to modern oxen and rhinoceri (I seem to recall one author referring to Moschops – an awkward-looking and ponderous creature, judging from standard skeletal reconstructions – as “the Fat Albert of the Permian”). The post-cranial skeleton of Moschops is heavy-set, with meany features that assisted in weight-bearing, including a massive shoulder girdle, arm bones with broad, flattened ends and short, stout vertebrae. The forelimbs probably assumed something of a sprawling posture, while the smaller hind-limbs were more erect. The frontal and parietal bones of the skull display an extraordinary degree of pachytosis, with thicknesses of upto 11.5 cm. The functional significance of the thickening of the skull roof in dinocephalians is unclear. The most popular theory is that the male members of these species used their skulls as rams and butted heads in dominance rituals.

The temperature physiology of these animals – as with many fossil taxa – is a point of uncertainty. Dinocephalians were apparently capable of living at high latitudes with highly seasonal weather regimes (Pelycosaurs were, as far as we know, restricted to equatorial climes) and their bone-tissues display patterns of vascularization and growth reminiscent of mammalian bones. These are the arguments usually presented in favor of Dinocephalian warm-bloodedness. However, it must also be noted that the Dinocephalians lacked a number of key skeletal features often associated with endothermy in mammals. For example, they did not possess a secondary palate – an anatomical structure that separates the nasal cavity from the oral cavity in mammals, allowing the latter to breathe continuously while feeding.

Plant-eating dinocephalians almost certainly found themselves, from time to time, in the predatory cross-hairs of their carnivorous cousins. The anteosaurs, for example, were a group of medium and large bodied meat-eating dinocephalians with high, narrow skulls, thickened skull roofs and large, robust canines. They were heirs to a set of ecological niches that, during the early Permian, would have been occupied by sphenacodontid pelycosaurs (including Dimetrodon). The largest anteosaurs were true giants, with skulls measuring over a meter in length.

Therapsids and Food-chains

Grasses carpet a hinterland plain –> The grasses are gnawed down by great itinerant masses of wildebeest –> The wildebeest are preyed upon by prides of lions. Food chains of this nature – in which energy flows from land-plant producers to a class of abundant herbivorous primary consumers to a much smaller population of carnivorous secondary consumers – are quite familiar to us. However, terrestrial ecosystems may not have always relied so heavily on a productive base of land-bound photosynthesizers. It has been suggested that, prior to the appearance of the Therapsids, land ecosystems were generally more dependent on aquatic productivity. This hypothesis is based on two observations: (1) the unusually high ratio of carnivorous pelycosaurs to herbivorous pelycosaurs in early Permian assemblages. An exclusively land-based food web implies a massive preponderance in numbers of herbivores over carnivores and (2) the abundance of fish-eating forms like Ophiacodon, that may have served as intermediate links in food chains that tied land to freshwater. The was an increase in the relative number of land-going vertebrate herbivores to carnivores after the rise of the Therapsids. The first large-bodied terrestrial plant-eaters were not Dinocephalians (both the Caseids and the Diadectids had played that part before during the early Permian; see my post on Pelycosaurs and on Amniotes), but Dinocephalian diversification (and Therapsid diversification in general) may have played a role in the creation of food-webs that were more plant-based and less fish-based.

The reign of the Dinocephalians was short lived – geologically speaking. They perished shortly after reaching the pinnacle of their taxonomic diversity around 263 million years ago, leaving no living descendants. But the Therapsid story was far from over. In the next few posts, we’ll discuss some of the Therapsid groups that ruled during the late Permian, starting with the Gorgonospids and Therocephalians.

Therapsids – Part I

Introduction to the Therapsids

2, 3, 3, 3, 3. That’s the number of bones in your thumb, index finger, middle finger, ring finger and little finger respectively. The same numbers apply to the phangeal bones in your toes, starting with your big toe. This basic structural plan (common to most land mammals) has a very ancient pedigree – the therocephalia, a group of carnivorous “mammal-like reptiles” that walked the earth over 250 million years ago during the Permian period, had hands and feet constructed in just this fashion. Four out of the five digits on each extremity contained 3 bones while the remaining digit (corresponding to the thumb or big toe) contained only 2.

In fact, many of the most unique and widely conserved features of mammalian anatomical design and physiology – including a bony secondary palate, a three-ossicle ear, endothermy, an erect limb-posture, and quite probably hair and lactation – originated among ancestral groups of “mammal-like reptiles” during the Permian and early Triassic periods.

So how did the evolutionary process derive a class of highly active, fur-clad, milk-producing animals from ancestral forms that were so fundamentally reptile-like in appearance? This is the question that I shall attempt to answer in a series of posts over the course of this winter.

We discussed the biology and classification of some of the earliest “mammal like reptiles” in my post on Pelycosaurs. In this post, we shall begin to deal with their Permian heirs, the Therapsids. The first few installments of this series will deal with specific groups of mammal-like reptiles that lived during the Middle and Late Permian, while the remaining parts will deal with some of the long-term evolutionary ‘trends’ we can glean from the Therapsid fossil record.

What is a Therapsid?

Therapsida is a clade that owes its evolutionary origins to a pelycosaurian-grade ancestor and includes all of the most ‘advanced’ synapsids, including modern mammals. If that sentence is gibberish to you, perhaps reading the first few paragraphs of my post on pelycosaurs will help demystify things. To summarize:

- Birds, mammals and modern reptiles are amniotes by descent. The first amniotes were  terrestrially-adapted animals that laid water-proof eggs on dry land.

- Synapsida is a subgrouping of amniotes that includes modern mammals (and their various extinct relatives) but not birds or reptiles. Early synapsids can be distinguished from other groups of amniotes in the fossil record on the basis of a number of unique skeletal features, the most important of which is the presence of a single hole on each side of the skull behind the eye-orbit. Each hole is called a temporal fenestra. 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. Mammals are the only surviving synapsids.

- ‘Pelycosaur‘ is a somewhat informal term applied to a range of basal/’primitive’ non-mammalian synapsids. These were sprawling, cold-blooded creatures which, despite having a closer phylogenetic relationship with modern monkeys and cats than with lizards or crocodiles, displayed few external traits that could be characterized as mammalian.

What differentiates early Therapsids from pelycosaurs?

The most important differences lie in the anatomy of the skull.

Therapsids have larger temporal fenestrae than pelycosaurs – implying the addition of further mass to the jaw-closing musculature. An enlarged canine-like tooth is present on the lower and upper jaws of Therapsids, clearly demarcating the boundary between a set of incisor-like teeth in front and a set of post-canine teeth behind. Heterodonty – or the presence of morphologically distinct sets of teeth in the jaws (e.g. incisors, canines, molars, premolars) – is one of the signature features of mammalian design. In point of contrast, most bony fish, amphibians and modern reptiles have ‘homodont’ teeth that are essentially morphologically identical.

The back of the skull in Therapsids is also more vertical and the jaw-joint is more anteriorly placed than in pelycosaurs.

The mammalian jaw consists of a single bone – the dentary. Earlier Therapsids had jaws constructed from multiple bones in addition to the dentary, including the articular and angular bones. One diagnostic feature of the therapsid jaw is the presence of a wide leaf of bone called the “reflected lamina” that protrudes from the angular. The reflected lamina may have played some part in hearing. We will discuss the evolution of the mammalian auditory system in a future post in this series.

The post cranial skeletons of early Therapsids show signs of improved locomotory ability. They appear to have traded in the permanent sprawling posture of their pelycosaur ancestors for a more erect gait and developed shoulder and hip joints that permitted a greater range of limb movements.

Most of the traits listed above were present in some incipient form in one group of Pelycosaurs in particular- the Sphenocodontidae. This group includes the famous sail-backed Dimetrodon. The Sphenocodontids are nearly universally posited as the closest relatives of the Therapsids.

Up next, the Dinocephalia.

Reigning Reptiles – The Archosaurs

Many great dynasties of reptiles rose and fell during the early phases of the Mesozoic Era (a hefty block of geological time, sometimes called the “Age of Reptiles”, that lasted from 250 to 65 million years ago). The beaked rynchosaurs, the agile, erect-walking, predatory rauisuchians, the crocodile-like phytosaurs and armored aetosaurs all had their heydays within the first 50 million years of the Mesozoic.

The names of these lineages – successful though they were – are probably only familiar to professionals and die-hard enthusiasts. The same cannot be said of the ever-popular Dinosauria, whose first (relatively unassuming) representatives also appeared on the scene during this early-Mesozoic time interval, around 230 million years ago. The Dinosaur clade has attained tremendous public notoriety for its evolutionary experiments in large body-size, defensive weaponry and bipedal carnivory, as well as for its sudden fall from planetary dominance about 65 million years ago. Dinosaurs have penetrated the public perception of prehistoric life so completely that the casual museum-goer is prone to indiscriminately identify any large, extinct reptile (or, worse, any large, extinct animal) on exhibit as a dinosaur. The term “Dinosaur” refers to the members of a specific branch on the greater tree of reptile evolution – it is not a blanket descriptive term for anything extinct, gigantic and ostensibly reptilian. None of the early Mesozoic reptiles I mentioned in the opening paragraph are dinosaurs.

But what distinguished the earliest Dinosaurs from their non-Dinosaurian reptile contemporaries? Are modern-day crocodiles close relatives of dinosaurs? Answering these questions is not trivial and will require a fair amount of phylogenetic tree-climbing.

Step 1: Dinosaurs as Diapsids

Dinosaurs have two openings on each side of the skull behind the eye-orbits. These holes, called temporal fenestrae, have margins that provide secure anchorage points for muscles involved in jaw function. The presence of these paired holes in the skull roof places them among the Diapsida. All living reptiles –with the possible exception of turtles and tortoises – are Diapsids by ancestry.

The Diapsida may be sectioned up into two distinct groups of reptiles:

- A group of reptiles that includes lizards and snakes in addition to a number of now-vanished forms, such as the spectacular marine Mosasaurs. This group is formally called the “Lepidosauromorpha” and is of no further concern to us in this account of Dinosaur origins.

- A group that includes modern crocodillians, avian and non-avian dinosaurs, flying reptiles called pterosaurs and a bewildering assortment of lesser known extinct reptiles with varied body-plans, diets, native habitats and temporal ranges. These are the “Archosauromorpha“.

Step 2: Dinosaurs as Archosaurs

Archosaurs display a number of skeletal features that distinguish them from other reptiles:

- An opening in front of each eye-orbit called the “anorbital Fenestra”.  These holes may have served to accommodate additional muscle-mass or, alternatively, housed air sinuses or a salt gland.
- An opening on each side of the jaw called the “Mandibular fenestra”
- Teeth that are “laterally compressed” – that is, flattened from side to side – and serrated.
- Teeth set in sockets
- A large protrusion on the femur called the fourth trochanter that serves as an attachment point for a set of thigh retracting muscles.
- A high narrow skull
- Various anatomical characteristics that indicate an upright or semi-upright limb posture, as opposed to the sprawling limb posture observed in modern lizards.

One of the main thrusts of my post is this: the earth was already inhabited by a wide variety of impressive, large terrestrial reptiles – most of them archosaurs or close relatives of archosaurs – when the first dinosaurs made their appearance on the evolutionary stage.

Phytosaurs were superficially crocodile-like archosaurs of semi-aquatic habit from the early Mesozoic. They had elongate snouts and jaws rimmed with conical serrated teeth. Unlike modern-day crocodilians, they had nasal openings positioned high up on the skull – close to the eye-orbits in many cases – rather than at the tip of the snout. Only its highly-placed nostrils and eyes would be visible from above, were it wading just below the water-surface. It could have snagged unsuspecting prey on the edges of rivers and lakes with the same adeptness that modern crocodiles display.

Aetosaurs were herbivorous archosaurs armored in heavy, interlocking dermal plates – it is tempting to think of them as prehistoric armadillos. Many were armed with fearsome spikes and stout claws.  Rynchosaurs were strange plant-eating relatives of the archosaurs that had barrel-shaped bodies, stocky limbs and strong beaks that could cut through plant matter.

Before the ecological ascent of the dinosaur lineage that produced Tyrannosaurus rex and Velociraptor (the theropoda), the biggest land predators on the planet were the Rauisuchians. They are generally understood to have been active, toothy, largely quadrupedal* archosaurs related to modern crocodiles. They were erect-walking animals with limbs oriented vertically below the body, rather than jutting out sideways (as in lizards) – one of several traits they share in common with dinosaurs. The head of the femur fits into a downward-facing socket in the hip girdle called the acetabulum. This is known as a “pillar-erect” configuration as the femur bears the weight of the skeleton above in it in much the same way that a vertical column bears the weight of a roof-top. The largest raiusuchians hit a length of over 7 meters, far bigger than the carnivorous dinosaurs of their time, and hunted large game.

The first “true crocodiles” also appeared during the early Mesozoic. Modern-day crocodiles and alligators are built for a low-energy, semi-aquatic lifestyle. Crocodiles walk, for the most part, with a sprawling, lizard-like gait and spend hours basking on river-banks in a state of absolute intertia. Many early Crocodillians were, by contrast, slender-legged and fleet-footed terrestrial creatures – some of them were even herbivores! Crocodile evolution is a fascinating story in its own right and will be dealt with more fully in a future post.

Step 3: Dinosaurs as erect-walking animals

Crocodillians, Raiusuchians, Aetosaurs and Phytosaurs can be readily distinguished from dinosaurs on the basis of their ankle morphology. In the former groups, the functional ankle joint is situated between two foot bones of similar size: the astragalus (an ankle-bone; in these archosaurs it is sutured to the two bones of the lower leg, the fibula and tibia) and the calcaneum (heel bone). This configuration allows for considerable rotation of the foot during locomotion.

In Dinosaurs, however, the articulation between the ankle bones is modified to produce a simple hinge joint. The functional ankle-joint is between the upper bones of the ankle (the calcaneum and astragalus) and the bones immediately distal to it (refer to diagram). The calcaneum was very small compared to the astragalus. The important implication of this anatomical design is that the foot was restricted to motion in a vertical plane. In other words, it could move back-and-forth but could not be twisted to any considerable extent. This is a clear adaptation for upright, speedy running. A similarly structured ankle is also found in pterosaurs. Pterosaurs are aerially adapted reptiles and close relatives of dinosaurs.

Both the Raiusuchians and the Dinosaurs independently evolved an erect gait (where the limbs were positioned directly below the body rather than splayed outwards), but their anatomical approaches to this condition differed.

Three bones make up the pelvis: the pubis, the ischium and the illium. At the junction of these three bones, there is a socket into which the head of the femur fits called the acetabulum. In Dinosaurs, the head of the femur is bent – that is, offset at an angle to the main body of the bone. The head fits into the acetabulum. The dinosaur acetabulum, rather than being a solid bone-pocket, actually has an opening in it (an open acetabulum is one of the defining features of the dinosaur clade).  The dinosaur femoral head is rather more barrel-shaped that the bulbous head of the human femur, limiting thigh motion to a plane parallel to the body.

The oldest known dinosaurs from the fossil record are lithe, bipedal carnivores like Staurikosaurus and Herrerasaurus. One of the interesting implications of this ancient bipedal body plan is that all quadrupedal dinosaurs from succeeding ages – horned Triceratops and mighty Brachiosaurus among them – belong to lineages that, at some point, had to revert to walking on four legs from a state of two leged-ness.

On the road to upright-walking bipedalism, the dinosaurs evolved a host of anatomical features that, taken in summation, set them apart from the rest of reptiledom – including, among other traits, a distinct neck, a backward pointing shoulder joint, a hole in the center of the acetabulum, longer hind-limbs than forelimbs and long lower foot bones and digits.

Dinosaurs did not comprise a significant portion of terrestrial animal diversity for much of the early Mesozoic, but they eventually supplanted the raisuchians, aetosaurs, rynchosaurs and other reptile groups as the dominant vertebrates on land. Their spectacular success would last them through the middle and later Mesozoic – and beyond, given that birds are modern day descendants of dinosaurs.

But what made the ascent of the dinosaurs to ecological preeminence possible?

Some researchers have suggested that the upright-walking dinosaurs may have been more efficient terrestrial locomotors than their sprawling gaited (or semi-sprawling gaited) contemporaries – and that this was crucial to their long-term evolutionary success. Others have opined that the evolution of warm-blooded physiology in early dinosaurs may have given them a “competitive edge” over other reptile groups.

It seems likely, however, that the meteoric rise of this scrappy upstart clade had more to do with luck than any inherent superiority. After all, a number of other archosaurs evolved an erect gait but did not diversify to anywhere near the extent that the dinosaurs did, nor did they survive past the early Mesozoic. The fossil record suggests that two large-scale mass extinction events took place during the early Mesozoic, successively culling the landscape of various groups of large reptiles and transforming the vegetation. Early dinosaurs just happened to scrape through these tough times (mass extinctions are often more random in eliminating groups of species than ecologically selective) and inherited a world deserted of major competitors.

*There is evidence for at least facultative bipedalism in a number of raiusuchians.

The Paleocene: New Dawn, Old Mist

“And suddenly marble turns into animals, dead things live anew, and lost worlds are unfolded before us.” – Balzac, in La Peau de chagrin

Note: A few terms may need some clarification. The Mesozoic era can be thought of as the “Age of Reptiles” – a span of time running from 250 to 65 million years ago, during which reptiles were the dominant terrestrial vertebrates. The Cretaceous period, which lasted from 145 to 65 million years ago, represents the final subdivision of this era.

A million Hiroshimas would be a drop in the bucket in terms of gross energy yield compared to the asteroid impact that brought an end to the reign of the dinosaurs. The collision event ejected massive quantities of debris and dust into the atmosphere, blotting out the sun. Ecosystems worldwide were subject to wild-fires, acid-rain, reduced plant/phytoplanktonic productivity and plummeting temperatures. Most of the large reptilian faunal groups that dominated the land and seas of the Cretaceous – non-avian dinosaurs, mosasaurs, pterosaurs and plesiosaurs – were wiped out forever.

The K-T extinction event will be dealt with in more detail in a future post, but this section of finstofeet will focus on the brave, new Paleocene world that rose from the ashes of the Cretaceous. How did life on land recover after the dramatic demise of the dinosaurs?

Although the Paleocene forests were home to many strange and unfamiliar creatures (the remains of galloping crocodiles, shrews with trunks and kangaroo-like legs, hoofed predators and man-sized carnivorous birds have been unearthed from this period, along with other zoological oddities) they also bore the seeds of mammalian modernity: the Paleocene epoch saw the appearance of the earliest representatives of many present-day mammalian orders, including rodents, primates, ungulates and carnivorans.

What was the Paleocene? Did any modern mammal groups exist before the Paleocene?

The ‘Paleocene’ refers to the geological epoch that immediately followed the mass-extinction of the dinosaurs at the end of Cretaceous period. It lasted from 65.5 million years to 56 million years ago.

The first mammals of roughly modern aspect arose well before the end-Cretaceous extinction event, perhaps around 200 million years ago. Mammals in the age of reptiles were, as a rule, diminutive creatures – most of them were rodent-sized insectivores (probably of nocturnal habit). A great majority were exceeded in size by even the smallest non-avian dinosaurs in their environment. A scant few attained dimensions comparable to the modern beaver or house cat. Repenomamus, the largest known mammal from this time period, was about a meter in length and is known to have preyed on small juvenile dinosaurs. There are examples of Mesozoic mammalian forms specialized for a semi-aquatic lifestyle, for ant-eating and even for gliding flight, but these are exceptional cases – most mammals of the age displayed rather unspecialized skulls, dentitions and skeletons.

For much of the Mesozoic, the mammals appear to have languished in a kind of evolutionary purgatory – restricted to a relatively small number of morphological types and ecological niches over an immense stretch of geological time. It is generally thought that the dinosaurs competitively excluded mammals from medium-to-large predator and herbivore niches.

The Mesozoic Triumvirate

Three major groups of mammals carried over into the Paleocene from the Cretaceous period.

The placenta is a complicated mammalian tissue that serves as the interface between the maternal uterine wall and the developing fetus. It anchors the fetus to the uterus, supplies the growing embryo with oxygen and nutrients and eliminates the metabolic waste produced by it. It also acts as an important endocrine organ. Placenta-bearing mammals probably emerged around the middle of the Mesozoic and are represented today by over 5000 species, from elephants to humans to bats. These animals display a long gestation period and give birth to well- developed live young. Early placental mammals in the fossil record are recognized on the basis of certain shared features of the teeth, jaws, leg bones, foot bones and ankle joints (the presence or absence of a soft organ like the placenta cannot, after all, be used as a diagnostic tool when working with fossils).

The marsupials also arose in the Mesozoic and persisted into the modern world – though their contribution to the present-day range of mammalian diversity is meager compared to that of placental mammals (there are a total of only 343 known marsupial species, mostly distributed in Australia and South America). Marsupials are popularly thought of as ‘pouched animals’ – in fact, the very name comes from the latin word for pouch, marsupium – but only 50% of living marsupial species actually possess a permanent pouch. Marsupials can more properly be distinguished from their placental counterparts on the basis of their reproductive cycle: Marsupials possess only a rudimentary placenta, with limited nutrient and oxygen exchanging capabilities. They have short gestation periods and give birth to tiny, incompletely developed young. The younglings are nursed on breast milk for an extended period of time (the lactation period far exceeds the gestation period). Subtle features of the upper and lower molars, in addition to the total number of molars in each jaw, also distinguish marsupials from placental mammals. Marsupials generally have lower metabolic rates, slower rates of postnatal growth and smaller brain dimensions than placentals of comparable size.

Adding to the mammalian diversity of the late cretaceous was a group of primitive, essentially rodent-like mammals called the Multituberculates. The clinical-sounding name refers to the fact that each cheek tooth in the jaws of these animals bore multiple rows of tiny cusps (bumps) or “tubercules” that operated against similar counter-rows in the opposite jaw. Like modern rodents, they bore a pair of enlarged shearing incisors at the front of each jaw. In terms of geological longevity, it could be argued that they were the most successful mammalian order of all time, lasting for a span of over 120 million years. Marsupials and Placental mammals are much more closely related to one another than either is to the Multituberculates. Judging from the structure of the pelvis, it seems very likely that Multituberculates gave birth to immature, live young rather than laying eggs. Unlike the marsupials and placental mammals of the Mesozoic, the Multituberculates left behind no living descendants.

It is worth mentioning here that a group of primitive egg-laying mammals, the monotremes, also made it into the Paleocene. They are represented today by just one species of Platypus and four species of Echidna.

The sudden disappearance of the dinosaurs opened up a plethora of new niches for the mammals to radiate into. The world was warmer and wetter in the Paleocene than it is today, with rainforests ranging over most of the continents. The continents themselves, while not entirely alien in shape and extent, occupied markedly different longitudinal and latitudinal positions in the Paleocene than they do today. A map of the world at that time is included below.

So we’ve set the stage. What sorts of mammals could we paint into a Paleocene landscape?

The marsupials appear to have undergone a significant reduction in diversity at the Cretaceous-Paleocene boundary – only one genus, Peradectes, is known to have made it across successfully. The Placentals and Multituberculates sustained fewer casualties by comparison.

The explosive diversification of Mammalian morphotypes to fill ecosystems effectively emptied of large vertebrates did not begin immediately after the fall of the dinosaurs. For example, it was only towards the end of the Paleocene epoch that the first truly large-bodied herbivores and carnivores began to arrive on the scene. Agusti and Anton’ (2002) go so far as to describe much of the Paleocene as being “an impoverished extension of the late Cretaceous world”. Any overview description of the mammals of this period is destined to devolve into a tiresome catalogue of strange names and anatomical characters. I have tried my level best to supplement my writings with pictures to help you visualize the animals I describe below.

The Multituberculates (hereafter shortened to ‘multis’) reached the peak of their evolutionary fortunes during the Paleocene. The Ptilodonts can be regarded as typical Multis – they had large, rodent-like incisors perched at the front of each jaw, separated from the cheek-teeth by a toothless space (the so-called diastema). They had an elongated blade-like lower premolar equipped for cracking nuts and hard seeds. Like most Multis, the Ptilodonts appear to have been largely herbivorous – possibly supplementing their food-intake with the occasional invertebrate. Ptilodonts like Ptilodus (after which the group is named) had grasping claws, a prehensile tail, large toes that could be extended out or retracted and feet with a wide range of motion, indicating a heavily arboreal lifestyle. In summary, the Ptilodonts were squirrel-like animals, both ecologically and morphologically.

The largest known multi approximated the size of a beaver – the short-snouted, heavily built Taenolabis. It had large grinding molars and was clearly a ground-dwelling herbivore. Resilient though they were, the Multis ultimately bought the farm about 30 million years ago – succumbing, perhaps, to stiff competition from true rodents, primates and herbivorous ungulates.

The transition into the Paleocene was turbulent for the Marsupials and they never truly recovered their former level of diversity in the Northern Hemisphere. The southern continents, however, were a different story. Marsupials formed a sizeable chunk of the mammalian fauna in Paleocene South America (up to 50%). A number of these Marsupials can be characterized as belonging to the same taxonomical order as modern opossums. Some of these were adapted for burrowing, others for scaling trees. Interestingly, marsupial equivalents of rodents and carnivorans also evolved in South America during the later phases of the Paleocene. The weasel-sized arboreally-proficient marsupial Mayulestes probably sought out frogs and small mammals as prey; It was similar, in many ways, to the marten.

Placental mammals, piddling contributors to the range of mammal diversity for most of the Mesozoic, managed to outshine both the Multituberculates and the Marsupials during the Paleocene. We shall consider several unique and interesting placental animal groups that lived in the Paleocene in the following passages.

The Lepictids – It is tempting to observe the tiny sizes, small brain-cases, pointed snouts and insectivorous diet of hedgehogs, shrews, golden moles, elephant shrews, treeshrews, tenrecs and moles and conclude that they all belong to a single taxonomic category of mammals. This is not really the case, and molecular analyses – as well as studies in comparative anatomy – demonstrate that these 7 animals represent up to five different mammalian orders: Erinaceomorpha (hedgehogs), Soricomorpha (shrews and moles), Macroscelidea (elephant shrews), Scandentia (treeshrews) and Afrosoricida (tenrecs and golden moles). The relationships between these orders are as yet uncertain.

The tree-tops and underbrush of the Paleocene were home to a great many such small-to-medium sized insectivorous creatures, representing different genera, families, orders, superorders, infraclasses and subclasses – a number of these (in South America, at least) were marsupials, others were Multituberculates and some were early members of the currently existing placental orders listed in the previous paragraph. Others, however, belonged to placental families and orders that kicked the bucket by the end of the Paleocene: the long-legged lepictids, the shrew-like paleoryctids or the semi-aquatic, fish-eating, otter-like pantolestids, for example. One of the most charismatic fossil species recovered from the Paleocene is lepictidum – a sort of strange cross between an elephant-shrew and a kangaroo. It was about 60 to 90 centimeters long, had lengthy hind-limbs, shortened fore-limbs and a slender snout that sported a short trunk. Like many other mammalian insectivores, the skull was quite unspecialized. It is unclear whether this animal ran on all fours or hopped like a wallaby. And yes, as fossil genera go, Lepictidum is unbelievably cute. The video below represents one animator’s impressive attempt at bringing this animal back to life.

Plesiadapiforms - We can tease out the beginnings of our own order, Primata, from amidst this Paleocene profusion of tree-climbing and insect-munching forms. Modern Primates share a number of features: among them, a short muzzle, forwardly directed eyes with stereoscopic vision, hands with nails rather than claws and cheek teeth with rounded cusps. Most Plesiadapiformes display none of these characters, possessing a long snout, strong, curved claws and side-facing orbits for the eyes. They can, however, be related to primitive tarsier-like primates mostly on the basis of shared features of the teeth and the auditory bulla, a bony structure that encloses the bones of the middle ear. It is likely that they were close relatives of true primates, if not directly ancestral to them. They were extremely abundant in the Paleocene and are interpreted as being lemur-like in appearance. They had long digits and flexible limbs for maneuvering through the forest canopy. Over 25 genera and 75 species of Plesiadapiformes have been discovered from this period – leaping about the tree cover as far north of the tropics as northern Wyoming, which was warmer and less arid during the Paleocene.

Condylarths - Primitive ungulates, called Condylarths, also existed in the Paleocene – but took on forms that, in many cases, bear little semblance to modern hoofed animals like horses or deer. For example, Arctocyon was a wolf-sized condylarth with the limb proportions of a bear and a diet that included meat. It had a pair of impressive lower canines and a long, robust skull. The cheek teeth indicate that it was primarily a plant-eater. It is identified as a primitive ungulate by subtle features of its limb joints and dentition. Dissacus was another example of a condylarth that consumed meat (it was probably a fish eater). It had digits that terminated in hoof-like structures. Many different groups of condylarths have been recognized in the fossil record.

Some of the Condylarths were browsers like Ectoconus or Phenacodus, and possessed clear signs of tapir-like hooves and, in the latter case, adaptations for running. The condylarths are generally regarded as being the basal ancestral stock from which the odd and even toed ungulates arose.

By the end of the Paleocene, various large bulky herbivores appeared on the scene – some of them ultimately Condylarthian in origin and others not. Titanoides was a strange non-condylarthian herbivore that approached the size of a Rhinoceros and had giant saber-like canines and long forelimbs. The earliest civet-like ancestors of dogs, cats, hyenas and bears also evolved in the Late Paleocene – though their story will be told in another section of finstofeet. It is astonishing how many wildly different lineages – ptilodonts, marsupials and plesiadapiformes, among others – evolved chisel-like incisors and rodent-like skulls during this epoch. True rodents arrived onto the scene at the end of it all, competing with and ultimately eclipsing the various pseudo-rodents of the Paleocene.

Were there any large non-mammalian predators during the Paleocene?

One of the most unusual aspects of the Paleocene was the presence of large predatory birds and reptiles that occupied the topmost rungs of the food-chain in terrestrial ecosystems across the world.

The K/T extinction event did not put the dinosaurs out of business entirely; they were survived in the succeeding age by birds (some aspects of the dinosaur-to-bird transition have already been dealt with in the ‘Taking Wing’ series). Fossil evidence suggests that birds weathered the Cretaceous-Paleocene transition poorly, with only a few taxa escaping extinction. These surviving forms eventually gave rise to all the different families of birds that we currently observe.

There is molecular data that contradicts this picture, pointing to a pre-K/T event divergence for many of today’s major bird lineages.

In any case, by the late Paleocene, one group of birds had lost the ability to fly and assumed the form – in likeness of some of their extinct theropod relatives – of large, land-bound, predaceous bipeds: the Gastornithidae. These “terror birds” were up to 2 metres tall, had powerful, sharp beaks and small, non-functional wings. They had thick legs with impressive talons for capturing and tearing prey apart. It probably fed on carrion and a variety of mammal species. They are related to modern waterfowls.

Crocodillians and crocodile-like Champsosaurs flourished during the Paleocene. Pristichampsus was a land-walking 3-meter long crocodile from this period that had hoof-like toes and long legs. It was heavily armored and capable of ‘galloping’ after fleeing prey. Archaic marine crocodiles roamed the seas, while a variety of crocodiles, alligators and now-extinct champsosaurs inhabited swamps and marshes the world over – relicts of a by-gone age of reptiles. Just recently, specimens of a monstrous 40 to 50 foot snake, appropriately named Titanoboa, were uncovered from this period. It was, by far, the longest and heaviest snake known to science.  I’ll leave you with this rather entertaining teaser for the Smithsonian Channel’s special on Titanoboa

NOTE: Title card art by Nobi Tamura

Sauropods – Whale Lizards

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.

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).

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 many of the times and places in which sauropods existed). 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 built into 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 (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) to 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 actually 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 primarily on gymnosperms (conifers), sphenophytes (eg. Horsetails) and pteridophytes (ferns). As flowering plants diversified rapidly during the mid-cretaceous, they too were no doubt incorporated into the sauropod diet.

Bird lungs and long necks

The vertebrae and ribs of sauropods have well-developed air-spaces, which have been interpreted as emanating from the respiratory system. 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 one increases the length of the pathway of air-conduction from the nostrils to the lungs, 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 (also seen in the much smaller necked giraffe) would have severely lowered breathing performance were it hooked up to a mammalian-type lung. 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 far more comparable to large mammals than to modern ectothermic reptiles. 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, can be assumed to have produced heat internally. The air sacs described earlier would have 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 of 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, cause space problems and suffer from a number of mechanical disadvantages. And so, various cardiovascular adaptations have been hypothesized to exist in sauropods to get around the hypertension issue.

Some workers have suggested that the sauropod circulatory system featured multiple ‘hearts’ in series, each accessory heart capable of pumping blood to the next valved pump in the circulatory pathway (thereby allowing effective blood-flow from the primary heart to 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.