Your Daily Fossil

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.

Cotylorhynchus

Fossil specimen on display at the American Museum of Natural History

Reconstruction by Hirokazu Tokugawa

When: Permian (~299 to 265 million years ago)

Where: North America 

What: Cotylorhynchus is a member of one of the most basal groups of synapsids, the  Caseidae.  Cotylorhynchus was a herbivore, and reached lengths of up to 20 feet (~6 meters) long, with a massive barrel chest, putting weight estimates at around 2 tons. This animal is very large for its time… well at least its body is. Cotylorhynchus has one of the most extreme cases of ‘tiny head’ I have ever seen. Even more so than the sail-backed Edaphaosaurus! Which is closer to modern mammals than Cotylorhynchus is. It is one of the most primitive animals known that unambiguously falls on the synapsid lineage. It is so basal that it does not even have any differentiation seen in its dentition, though there are less teeth than found in the non synapsid contemporaries of this wee-headed creature. 

Saurodon - a sword eel

Mounted reconstruction on display at the Rocky Mountain Dinosaur Resource Center,  Woodland Park, Colorado 

Reconstruction by Charles Bonner

When: Cretaceous (~ 89 - 83 million years ago)

Where: North America

What: Saurodon is one of the large fish which swam though the Cretaceous Seaway, the marine waters that covered much of North America during the late Mesozoic. This particular species was ‘only’ about 8.5 feet (~2.6 meters) long, with a relatively skinny body and large pointed lower jaw. These features are what gives the family Saurodontidae the nick-name ‘sword eels’. The Saurodontidae fall into the later group Ichthyodectidae, a completely extinct clade that contains some of the largest fish on record. Today the living relatives of these gigantic fishes are in the clade Osteoglossomorpha and are some of the largest bony fish that swim though today’s waters. 

This was not a very specious group - there are only three described species - but they have been known to science for almost two-hundred years. The first Saurodontidaewas named in 1824 by Richard Harlan (the discover of Harlan’s ground sloth) - but was misidentified as the jaw of an extinct marine reptile. This was corrected only six years later when the first Saurodon specimen was found, and it was clear that the fragmentary specimen which was previously named belonged to a large fish, not a marine reptile. The use of the long lower jaw in Saurodon and its kin is not well understood, but it has been hypothesized that perhaps these predatory fish dug prey out from the deep muds at the bottom of the seaway. 

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.  

Pachycrocuta - The Giant Hyena 

Mounted specimen from the Zhoukoudian Museum, Beijing. 

Reconstruction by Mauricio Antón

When: Pliocene to Pleistocene (~ 5 million to .5 million years ago)

Where: Europe, Asia, and Africa

What:Pachycrocutais a prehistoric member of the Hyaenidae. Today hyenas are restricted to Africa and western Asia, but their fossil record has revealed they were once much more wide spread. Pachycrocuta has been found in Africa and Asia, but most specimens have been found in Europe, with many localities in the Iberian Peninsula. The largest species was Pachycrocuta brevirostris, which stood over 3 and a half feet (~100 cm) at the shoulder and is estimated to have weighed over 400 lbs (190 kg). This makes it about the size of a modern lioness! Cave deposits in both Spain and China have revealed multiple almost complete skeletons, suggesting that these animals lived in packs and utilized these caves as their dens. 

As Pachycrocuta is even more heavyset and adapted for bone crunching than the living bone-crunching hyenas, it has been suggested that this fossil form was even more dependent on scavenging kills than living species. But there really is not much evidence to pack this up other than thought-experiments. As it appears that some large cat species were displaced when Pachycrocuta moved into their ranges, it is more likely it was a direct hunter that would take advantage of pre-killed remains when it could drive away other predators. Like 99% of carnivores today. The predator/scavenger divide is really not a fast or hard line at all. Even more evidence of hunting comes from the remains of interactions between Pachycrocuta and Homo errectus. These two species overlapped and bones of our poor relative have been found in Pachycrocuta dens in China! 

Louisella

Reconstruction by Marianne Collins

When: Cambrian (~505 million years ago)

Where: British Columbia, Canada 

What:  Louisella is a worm-type organism from the Burgess Shale formation in the Canadian Rockies of BC. This organism was about 12 inches (~30 centimeters) long, with a proboscis at its anterior end that could be inverted into the body or extruded. In the images above it is inverted into the body in the fossil specimen, but shown at its full  extended length in the reconstruction. This structure was ringed by a series of spines with shorter and more robust spikes on the end of the proboscis. These structures would rub past one another as the animal extended and retracted its proboscis, allowing it to ‘chew’ its food. The rows of short fringes on one surface of Louisella are thought to possible have been the animal’s gills. This worm has been reconstructed as a burrowing and carnivorous creature, and due to the grinding capability afforded to it by its proboscis, it likely ate animals of a relatively large size. 

Louisella is currently held as a stem fossil on the lineage leading to the Priapulida worms, also known as the ‘Penis worms’. I swear I am not making that up. These worms are very rare even today, with less than 20 living species known. They, like their ancient relative, are burrowing creatures which hunt other invertebrates. Fossils of priapulid worms are also rare, only a handful of Louisella specimens are known. What is far more common though are their distinctive shaped burrows, which are sort of an interlocking L-shape. The appearance of this type of burrows is one of the biostratigraphic markers of the start of the Cambrian period. 

Louisella on the ROM’s amazing Burgess Shale website

Kronosaurus

Mounted specimen on display at Harvard Museum of Natural History

Reconstruction by Jaime Chirinos

When: Cretaceous (~ 125 - 99 million years ago)

Where: Australia

What: Kronosaurus is an australian plesiosaur. Yes, it is a plesiosaur even though it lacks the long neck that many people associated with the group. Plesiosauria is roughly divided into two groups;  Plesiosauroidea - the long necked forms and Pliosauroidea - the short necked forms. Kronosaurus is an example of the latter clade, and shows many of the defining features of this group - such as an enormous head with massive jaws, a short neck, and a relatively short tail- the opposite in many ways of their cousins the plesiosauroids. This australian sea monster was one of the largest of its clade, with estimates of up to 33 feet long (~10 meters). Its teeth reach almost 5 inches (~12 cm ) long in crown length - the part above the gumline. The total tooth would have been over double in size. The large size of its teeth, combined with distinct shape and the lack of clear cutting surfaces also  for their easy identification if they are found as isolated material. 

The Kronosaurus specimen seen above was found in on private property in central Queensland, Australia in the 1920s. A crew from Harvard was shown where the specimen was weathering out, and set about excavating the fossil. After years of work, the specimen was boxed up into over 80 crates, weighing in at over 6 tons and shipped to the states, where it was mounted at the Harvard museum. Decades later the original discoverer of the material finally got the see the results of the preparation and mounting of what he termed ‘his dinosaur’ at the age of 93. In life Kronosaurus was a top predator; there are fossils of Elasmosauridae plesiosaurs that show bite wounds that could have come from Kronosaurus! No fish for this animal, it was after much bigger prey, leading to amazing plesiosaur vs plesiosaur encounters. Or so I like to imagine!

Obdurodon

Skull on display at the American Museum of Natural History, NYC

Reconstruction by Anne Musser

When: Late Oligocene to Miocene (~25 to 12 Million years ago)

Where: Australia

What: Obdurodon is a fossil platypus, as is fairly obvious from a look at its skull. Though upon closer inspection there are some very important differences; Obdurodon had a larger bill than the living platypus and retained teeth as an adult. Modern adult platypus are toothless, shedding all their teeth as juveniles. These teeth are important as they help us place monotremes (platypus and the echidna are the modern representatives) into the mammal family tree. It is now the consensus that marsupials and placentals are more closely related to one another than either is to the monotremes, but there are a great deal of extinct groups of mammals that may fall between therians and the monotremes - such as the multituberculates. Fossils such as Obdurodon which are most assuredly related to modern monotremes, but preserve more primitive features, are critically important for this phylogenetic issue. So then why is it still an issue? All we have of Obdurodon is a skull, despite the full body reconstruction above, and while there are fossils of even older monotremes they are even more scrappy - just isolated teeth or jaw fragments (that still enjoy full body reconstructions…).  

How did Obdurodon live compared to the modern platypus? Well, the living form uses its bill in the water to help it sense prey, and as Obdurodon had an even larger bill, it seem likely it also was aquatic, though without a postcranial skeleton it is unknown if it had the same swimming and digging adaptions seen in its extant relatives. 

Europasaurus

Reconstruction by Gerhard Boeggemann

When: Jurassic (~ 156 - 151 million years ago)

Where: Germany

What: Europasaurus is the smallest sauropod known. Now remember I said smallest SAUROPOD known, so we are still looking at an animal that was about 20 feet (~6 meters) long. Keep in mind though, this includes the long neck and tail - taking those out of the picture and the animal’s body length was about 6.5 feet (~ 2 meters).  This length, coupled with the shoulder height of Europasaurus ( about 5 feet [~1.5 meters]), gives us a sauropod with a body that was about the size of a modern elephant.  There are a number of proposed tiny sauropods out there, but I have selected Europasaurus as it is known from several almost complete skeletons, from juveniles to adults, so we can be sure this is not just a baby sauropod that would have grown much larger. Above I have included a diagram from the original description of Europasaurus rather than a reconstructed mounted skeleton to show you just how much of the skeleton is known. Why is Europasaurus so small? Its our good friend Island Dwarfism back again! The specimens have been found in Germany, on what would have been one of many islands in the area during the late Jurassic, owing to the higher sea levels. 

This smallest of all sauropods is in the clade  Macranaria within the sauropod family tree. This group actually contains some of the biggest sauropods (and thus dinosuars) ever known! Talk about some size diversity within a group! An adult Europasaurus could have easily walked between the legs of some of its larger cousins. The great range of ages in the recovered specimens of Europasaurus have allowed for detailed study of the microstructure of the bones, revealing how these animals grew. From this study it has been proposed that Europasaurus  managed to stay small on its restricted island habitat by not stopping its growth exceptionally earlier than its relatives, but instead by dramatically slowing down its growth rate. 

I hope this cute little sauropod makes up for ruining all of those childhoods with the reality (or should I say unreality) of Brontosaurus a few posts ago. ;) 

Saurosuchus 

Mounted specimen from Ischigualasto Provincial Park, San Juan, Argentina

Reconstructed model from the Titanes de Ischigualasto (Titans of Ischigualasto) exhibit while at the Museo Argentino de Ciencias Naturales in Buenos Aires, Argentina. 

When: Late Triassic (~231 to 226 million years ago)

Where: Argentina

What: Saurosuchus is a very large basal member of the crocodile lineage within the Archosauria. It lived in what is now Argentina during the late Triassic, where it was a dominante apex predator, reaching lengths of up to 23 feet (~7 meters).  A good deal of this length was its massive skull filled with pointy recurving teeth. Saurosuchus is a member of the Rauisuchia, one of the first branches to come off of the Crurotarsi (crocodile line archosaurs) lineage.  Like other rauisuchians Saurosuchus had an erect leg posture, you can see this in the model above, it looks very much like a large crocodile (albiet with more dinosaur looking like head on), but its legs are directly underneath its body, not sprawling out like in modern crocodiles. This posture is the same as in mammals and dinosaurs, but was accomplished in a different way. Mammals and dinosaurs modify the femur itself for erect posture, but in rauisuchians it was the pelvis that was transformed. Saurosuchus was a fully terrestrial creature capable of fast speeds as it hunted its prey (including the much smaller dinosaurs) in the ancient flood plane. 

Saurosuchus fossils are from the Ischigualasto formation (and national park!) in Argentina. This area is open to the public and is more popularly called the Valle de la Luna (Valley of the Moon).  Not only do basal crocodile line archosaurs come from this formation, but some of the oldest dinosaurs as well. These animals, which would go on to dominate later in the Mesozoic, were a minor part of the fauna. The Triassic here was dominated by rhynchosaurs and cynodonts, these groups would both suffer great losses during the end Triassic extinction event.