Fins to Feet


The Paleocene: New Dawn, Old Mist
May 23, 2012, 8:28 am
Filed under: Uncategorized

“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.

In terms of gross energy yield, the simultaneous detonation of every atomic weapon on earth would be a puny sputter 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: An asteroid collision brought an end to the age of the dinosaurs, 65 million years ago. Image credit to Don Davis, NASA

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. 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.

Repenomamus, the largest known mammal from the Mesozoic, sating itself on a dinosaur hatchling. Art by Nobi Tamura.

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 the 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 (naturally, the presence or absence of a soft organ like the placenta cannot be used as a diagnostic tool when working with fossils).

The Marsupial clade also arose in the Mesozoic and managed to persist into the modern world – though its 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 like their reptillian forebears. 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.

A Paleocene World Map

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 mammal 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. They had elongated blade-like lower premolars, designed for cracking nuts and hard seeds. Like most Multis, the Ptilodonts appear to have been largely herbivorous, perhaps supplementing their diet with the occasional invertebrate. Ptilodonts had grasping claws, a prehensile tail, large toes and feet with a wide range of motion. These traits indicate a heavily arboreal lifestyle. In summary, the Ptilodonts were squirrel-like animals, both ecologically and morphologically.

Left: The rodent-like skull of a Ptilodont (by Nobi Tamura), Right: The beaver-sized Taenolabis (by Bruce Horsfall)

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. 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.

Mayulestes: A Marsupial weasel

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 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. Still others 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 placental mammals 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. Many of them 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. Its 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 had digits that terminated in hoof-like structures. Many different groups of condylarths have been recognized in the fossil record.

Primitive Ungulates – left: Arctocyon, by Dmitry Bogdanov; right: Phenacodon, by Charles M Knight

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

Titanoides, a late Paleocene herbivore. By Smokeybjb

By the end of the Paleocene, various large bulky herbivores appeared on the scene – some of them 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.

Gastornis, the ‘terror bird’

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 and small, non-functional wings. They had large, powerful beaks for capturing and tearing prey apart. It is often depicted as feeding on carrion and mammals. They are related to modern waterfowls.

Update/Correction (January 2013):  Some recent evidence seems to suggest that the Gastornithidae were actually herbivores!

Pristichampsus, a cursorial crocodile. Illustration by Robert Bakker

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

Sources:

1. Agustí, Jordi, and Mauricio Antón. Mammoths, sabertooths, and hominids: 65 million years of mammalian evolution in Europe. Columbia University Press, 2010.
2. Prothero, Donald R. After the dinosaurs: the age of mammals. Indiana University Press, 2006.
3. http://www.paleocene-mammals.de/ Accessed: 23 May 2013


7 Comments so far
Leave a comment

Very informative article! Just to nitpick though, to my understanding true moles are indeed close to shrews; it’s the golden moles (chrysochlorids) that are closer to tenrecs. Also, there are three living monotreme genera; the echidnas contain two genera.

Comment by albertonykus

Oops, thanks for catching those mistakes, Albertonykus – much appreciated!

Comment by arvindpillai

great blog, keep up the good work!

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