Fins to Feet

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.