Your Daily Fossil

Toxodon

Mounted specimen on display at Harvard Museum of Natural History

Reconstruction by Roman Uchytel

When: Pleistocene (~2.6 million to 16,000 years ago) 

Where: South America

What: Toxodon is another one of the large herbivorous animals that roamed over South America. Charles Darwin purchased the skull of the first Toxodon known to the Old World during his journey on the Beagle. This skull was sent back to England were Sir Richard Own described it and named the animal Toxodon - ‘bow teeth’ based on the curving nature of its gigantic molars. Soon complete skeletons of this amazing animal were known. The first interpratations reconstructed Toxodon as a semi-aquatic animal, much like the modern hippo, but later studies of the limbs and teeth of speciemens show this was incorrect. Toxodon  was more the analogue of today’s rhinos than a hippo, a fully terrestrial animal with teeth well adapted for grinding tough plants in somewhat arid environments. Some Toxodon specimens have been found associated with arrowheads, showing that the first people to emigrate into South America had contact with these animals, and appear to have hunted them. 

Where does Toxodon fit into the tree of life? Like its contemporary Macrauchenia (which you can see in the background of the reconstruction), its relationship to living mammals is uncertain. It falls into the larger clade of Notoungulata, literally Southern Ungulates, but the placment of this group within placental mammals is highly uncertain. They maybe have a close relationship with animals in the group Afrotheria but research in mammalian systematics is only beginning to be able to evaluate that, and other hypotheses.  So what is Toxodon? We just don’t know. 

Plesiadapis

Mounted specimen on display at the American Museum of Natural History, NYC

Reconstruction by Jay Matternes

When: Late Paleocene to Early Eocene (~ 61 - 55 millon years ago)

Where: North America and Europe

What: Plesiadapis is a small tree-dwelling mammal that was fairly comment in the late Paleocene of North America and Europe. This ancient mammalian taxon was about the size of a house cat, and though it may look very reminiscent of a squirrel it is a member of the primate family, as part of the larger group Plesiadapiformes. The latest research has shown that Plesiadapis was actually atypical for its namesake clade; this genus tended to be much larger than the average plesiadapiform and was not as well adapted for climbing as its smaller relatives, lacking a hand specially adapted for grasping. Plesiadapis could climb trees, but it would have been an arboreal quadruped, like the living squirrels, rather than a grasping locmotion as seen in most primates today. Another features reminiscent of rodents in Plesiadapis (and this is found in most of its kin) is its enlarged front teeth and the reduction or loss of teeth between these massive incisors and the grinding cheek teeth. Plesiadapis has been reconstructed as a frugivore - meaning its diet was primarily comprised of fruit. As much of North America and Europe was covered with lush sub-tropical forests during its range, Plesiadapis would have had quite a large selection of fruits to feed on. 

The placement of Plesiadapiformes has been somewhat controversial in the past decade or so. There is uniform agreement that these animals fall somewhere near the group Euarchonta within placental mammals, but exactly where has been much debated. Euarchonta contains not only primates, but also the Scandentia (tree shrews) and Dermoptera (flying lemurs). Some early studies placed plesiadapiforms closer to the dermopterans than primates, but more recent studies tend to find this clade as either the first branches to spring off the primate lineage or just outside of Euarchonta itself, as stem taxa to all three orders. One last point to make things even more confusing! The group Plesiadapiformes? It is probably not a monophyletic (natural) group in reality. It is looking more and more like that some taxa previously grouped within Plesiadapiformes fall closer to living primates than to other taxa within  the group. 

To sum up that confusing mess, Plesiadapiformes are very important in understanding primate evolution, as at least some members of this assemblage of taxa are the first animals on the primate lineage. As this lineage includes me and you there is a lot of study focused on this group right now! Nice to see animals that are primarily paleocene taxa finally getting some attention.

Uintatherium 

Mounted specimen from American Museum of Natural History, and currently part of the traveling Extreme Mammals exhibit.

Reconstruction by Charles Knight

When: Eocene (~49 to 39 million years ago)

Where: North America

What: Uintatherium  is one of the first large mammalian herbivores. It stood about 6 feet (~1.8 meters) high at the shoulder and was roughly 13 feet (~4 meters) long. This isn’t that large for an animal today, but in the Eocene it was a giant! It lived in the lush sub-tropical forests of mid-Eocene North America, most likely eating a combination of terrestrial bushes and shrubs along with aquatic plants from lakes and marshes. Uintatherium has a nasty pair of upper canines, not what you would expect from a herbivore! It is thought that these teeth were involved in sexual display, as they appear to be much larger in males than females. Uintatherium vanishes from the fossil record in the late Eocene, at about the time the temperature of North America was falling and the vegetation was thinning out. 

Uintatherium was also one of the fossils involved in the great ‘Bone Wars’ between Cope and Marsh. It was by far the largest of the fossils to come out of the Fort Bridger fossil localities in Wyoming (this fort gives its name to a land mammal age - The Bridgerian!), and thus highly prized. Cope and Marsh both applied multiple names to specimens from this region which would later prove to all belong to the same species. The name Uintatherium wasn’t even one applied by Cope OR Marsh. Joseph Leidy named this creature in 1872, just barely edging out Marsh’s names of Dinoceras and Tinoceras. So that particular battle in the bone wars was won by someone who didn’t even have much of an interesting in fighting! 

Uintatherium is not thought to have any living descendants, it is possible that the Eocene Uintatherium was the last of its kin. However, the position of Uintatherium and its brethren (grouped as the Dinocerata) within the mammal family tree is highly uncertain. They are well accepted as placental mammals, but beyond that? It is highly debated, and in my opinion, nobody has really done a rigorous enough study to support any one position over another.  

Palaeolagus

When: Late Eocene to Mid Oligocene (~38 to 27 million years ago)

Where: North America

What: Palaeolagus is a fossil lagomorph. Lagomorpha is an order of mammals, that contains rabbits, hares, and pikas. Within the bunny-order rabbits and hares are more closely related to either other than either is to the pikas. If you are not familiar with pikas go check out some pictures! They are really cute little guys that resemble guinea pigs more than they do rabbits, but they are most assuredly lagomorphs. Palaeolagus falls outside all living lagomorphs in their evolutionary lineage. It can be thought of as representative of the common ancestor of all living lagomorphs.  

Palaeolagus lived in North America in the late Eocene, after the dense forests had left and the grasslands of the plains started to expand. This 10 inch (~25 cm) long herbivore spread throughout the continent during the Oligocene as the grasslands grew. Palaeolagus could not hop, its hind legs show none of the features that make a hopping locomotion style possible in living rabbits.This ancient bunny is known from a large amount of fossil specimens, some of which are almost complete skeletons, but most are fragmentary pieces of bone or teeth. Most of these Palaeolagus specimens likely met their end as the lunch of one of the many predators roaming the grass lands of prehistoric North America. 

Hoplophoneus

Mounted specimen on display at the Zurich Natural History Museum. 

When: Late-Eocene to Early Oligocene (~38 to 30 million years ago)

Where: North America

What: Hoplophoneus is a saber-toothed carnivore which is well known from the several deposits in the Western states and provinces of the USA and Canada. This animal is not a felid, but a member of the group Nimravidae. One day I will probably be a true saber-toothed cat on here, but today is not that day! Hoplophoneus is a lot closer to true saber-toothed tigers than Thylacosmilus is, today’s fossil is a placental mammal, and has been closely linked to the order Carnivora, which is the order that includes cats. Exactly how closely linked has been a matter of debate for some time, pretty much since the first nimravid fossils were found in the late 1800s. At first nimravids, including Hoplophoneus, were thought to be felids. They were never thought to be sister taxa to the much later occurring saber-toothed forms however, so they have always represented another event of convergent acquisition of extremely enlarged canines. 

Hoplophoneus and kin were kept as felids for several decades, but their placement was then called into question when relationships within Carnivora were re-examined using cladistic methodology. Since the 1970s they have been, at various times, true felids, non felids in feliformia, in caniformia, or outside of Carnivora all together! The most recent studies, the first to look at all parts of the skeleton, have placed Hoplophoneus and the rest of the nimravids outside of Carnivora, as the sister taxon to the rest of the group. They are neither caniforms (dog like carnivorans) or feliforms (cat like carnivorans). 

This placement means that the leopard sized Hoplophoneus is an even more stunning example of convergent evolution than was first though. Not only are its enlarged canines convergent with some felids, but so are many other features of its anatomy. However, there are many differences as well, and these features are what place nimravids outside of Carnivora. Most of the differences are really specific anatomical features, most of which found in the ear region and the postcranial skeleton, but one easy one to see is the size of the brain. If you look at the reconstruction above the body looks very cat-like, but the head is quite different. According to one friend ‘Its not a cat, its a weasel-cat! sabertooth… thing.’ 

Deinogalerix

Mounted specimen from the National Museum of Natural History in Leiden, the Netherlands. 

Reconstruction by Mauricio Anton with a common hedgehog for scale. 

When: Miocene (~11 - 5 million years ago)

Where: One island that now is part of Italy

What:Deinogalerix is a comparatively giant relative of the hedgehog. It lived on what is now the Gargano peninsula in Italy, but during the Miocene this region was a separate island. Much of Italy during this time period was a series of isolate islands, owing to the higher water level. Deinogalerix was about five times the size of a common hedgehog, more the size of a small fox. However, with a skull about 1/3rd the total length of its whole body, it was proportioned very differently. An eight inch (~20 cm) skull on a 24 inch (~60cm) body isn’t too out of proportion for many of the Lipotyphla (the order that includes hedgehogs, shrews, moles, and solenodons), and it appears Deinogalerix saw no reason to shrink down its head just because of its growth spurt.  I have called this animal a hedgehog, and it is in that grouping, but it did not look much at all like the little spiny animal shown above. Within the hedgehog family, its closet relatives are not true hedgehogs, but rather the gymnures or ‘moon-rats’. These animals have not developed spines as protection and are covered with a coat of long course hairs. 

Hesperocyon - The First Dog

When: Eocene to Oligocene (~39 - 33 Million years ago)

Where: North America

What: Hesperocyon is the oldest known and one of the most primitive members of the canids. It was a relatively small form, at 2ft 8 inches (~80cm) long on average, with slender limbs.  This ‘first dog’ is not the ancestor of all living canids, as it falls into a group of dogs that is now totally extinct; the Hesperocyoninae.  However, this small lithe form is thought to strongly resemble the true common ancestor of all canids. It was capable of climbing trees, but was more at home on the ground, and was showing more adaptations for the ability to run quickly on the ground than its non crown carnivoran ancestors. The ability to run fast for long distances is called cursoriality, and this is the primary locomotion type of most living canids. 

The world that Hesperocyon lived in was a changing world. The lush sub-tropical forests of western North America that characterized the early Eocene period were quickly disappearing, as the world became cooler and drier. The fossil record shows many groups of mammals becoming more adapted to open spaces, losing their arboreal adaptations as the trees disappeared and the great plains of the Oligocene spread.  Groups that did not adapt to this new more open world vanished, such as most North American primates, like Notharctus,and a group of very arboreally adapted stem carnivoramorphans, exemplified by Vulpavus. The canid-line adapted very well to this new open world, with three distinct subgroups quickly popping up; the group that today’s fossil belongs to, the Hesperocyoninae, the Borophaginae (see Epicyon), and the Canidae; the group all modern species of dogs fall into. 

Reconstruction by Mauricio Antón

Macrauchenia

Skeleton on display at the American Musuem of Natural History, NYC. 

Reconstruction is part of the traveling exhibit Extreme Mammals which started at the American Museum of Natural History. 

When: Late Miocene to Late Pleistocene (7 million to 20,000 years ago)

Where: South America 

What: Macrauchenia is a hoofed mammal from South America. This animal has been known to science for a very long time. The first fossils were found by none other than Charles Darwin when he was traveling on the Beagle. He gathered up the fossilized  vertebra and limb bones and brought them back to England, where they were studied by Richard Owen, who coined the name Macrauchenia (meaning ‘long neck’), and supposed the whole animal would have resembled a llama. Later fossil finds, including several almost complete specimens, confirmed that Macrauchenia  did somewhat resemble a llama, with its slender legs and long neck. However, it was very diffent in some critcal areas, such as having 3 hoofed toes per foot and a mobile trunk. How do we know this animal had a trunk from just the bones? In living mammals with long trunks (such as the elephant and the tapir) the skull is transformed for the musculature that allows such a structure to move, and the skull of Macrauchenia has many features which closely match that of these modern trunked species. 

So with this long llama-like neck and the tapir-like trunk, how does Macrauchenia fit into the mammal family tree? That is a subject of much debate, but it is certain that it is not especially closely related to either artiodactyls (llamas) or  perissodactyls (tapirs).  Macrauchenia is in the order Litopterna, a group of mammals which is only found in South America. Litopterna is assuredly an order of placental mammals, but its exact placement relative to the other major clades is uncertain at this time. It has been suggested they, and other South American ungulate groups, may fall somewhere close to Afrotheria. 

South America  was isolated from all others for millions of years, in ‘splendid isolation’, during which time the mammals upon it radiated to fill all available niches, and this resulted in dozens of cases of convergent or parallel evolution. There are ungulate fossil froms known from South America that closely resemble not only llamas but also horses, rabbits, and even elephants! Carnivorous forms got in on the act too, such as Thylacosmilus, which looked very simular to the Saber-toothed ‘tigers’ of North America. Many South American natives went extinct during the great faunal interchange, but Macrauchenia survived until the end of the last glacial period. There is hope that one day we might recover some ancient DNA of this animal, which would be very helpful in determining where it falls in the great family tree of placental mammals. 

Livyatan melvillei 

When: Miocene (~12 - 13 million years ago)

Where: Peru

What: Livyatan is a gigantic toothed whale. It is fairly closely related to the living sperm whale, and is thought to have been about the same size, at 45 feet (~14 meters) long. This is an estimate as the whole body was not found, but its head was fairly well preserved, and its skull alone is 10 feet (~3 meters long) Unlike the modern sperm whales, it had a full set of teeth in both its upper and lower jaws, and its lower jaw was not reduced compared to its skull. Inside these giant jaws were giant teeth, the largest of which are 1.2 feet (~36 cm) long. What did they eat with these massive jaws and gigantic teeth? Well, living sperm whales eat very large prey, such as giant squids and megamouth sharks with their comparatively small jaws and teeth. It has been suggested that Livyatan was feeding upon other whales at the time! Such as the reconstruction above where a Livyatan  dramatically ruins the day of a Cetotherium (an extinct baleen whale). 

The name ‘Livyatan melvillei’ is meant to bring to mind Melville and his famous white sperm whale Moby Dick. Originally the name published was Leviathan melvillei, but it had to be changed, as the genus name of Leviathan was already taken! It belongs to a poorly known species of mastodon named by a researcher in the mid 1800s. Thus, the spelling of this giant whale’s name had to be altered, as once a name is applied to something it is there forever! Let this be a lesson to carefully check your species names before you publish them, as there are a few cases of something like this happening. Mostly it seems species of theropod dinosaurs are accidentally given names that have already been applied to beetles. Whoops! 

The area of Peru where Livyatan  was found is today a harsh desert, but geologists think that during the Miocene this area was an ocean paradise; a warm shallow lagoon. Dozens of marine species have been found in this desert, not only a variety of toothed and baleen whales, but also sharks and pinnipeds. 

Reconstruction by C. Letenneur, Muséum National d’Histoire Naturelle, Paris, France

Doedicurus 

Skeleton on display at the Museo Histórico Nacional in Buenos Aires, Argentina. 

When: Pleistocene (2 million to 11,000 years ago)

Where: South America

What: Doedicurus is a glyptodont. The Glyptodontidae were a subclade of armadillos that ranged throughout South America (and North after the land bridge reappeared). Doedicurus was one of the largest glyptodons, coming in at about 12 feet (~3.6 meters) long. Its shell was gigantic, a grown person can crawl inside one of these structures, and there has been some theories that ancient peoples could have used these shells for shelter.  The shell of Doedicurus, like all glyptodonts, was different from that of the living armadillos. The carapaces were thicker and in one solid piece, unlike the several segments present in armadillos that allow them to curl into a ball. Doedicurus had a highly domed shell, that connected to its pelvis posteriorly, but was separated from its shoulder girdle. It has been speculated that glyptodons with this type of shell stored fat in this space above the shoulders, such as a modern camel stores fat in its hump. Doedicurus also had an armored skull cap, which you can see in the fossil image but sadly has been omitted from the reconstruction. 

One of the most distinctive features of Doedicurus is its spiked tail club. In most of these entries when I present something cool that you can imagine being used in intraspecific competition I have to say ‘but it was just for display or protection’. NOT THIS TIME. There is a great amount of evidence for the hypothesis that these spiked tail clubs were used in battles between males. Not all specimens of Doedicurus have a well developed pedestal for the spikes, leading researchers to conclude this was only present in males. It is very unlikely this would have been anymore of a deterrent for predators than the large shell in the first place, and just as unlikely that Doedicurus would have been agile enough to defend itself from a swift carnivorous attacker with this club. Most compelling of all, several Doedicurus specimens have been found with healed wounds in their carapaces that match the predicted impact from a rival’s tail club. 

Doedicurus, like all of the remaining glyptodonts, went extinct about 11-10,000 years ago, at the end of the last major glaciation.