Mounted specimen on display at the Harvard Museum of Natural History
Reconstruction by Roman Uchytel
When: Miocene and Pliocene (~17.5 - 4.5 million years ago, and maybe a couple million years more!)
Where: North America
What: Teleoceras was an aquatic rhinoceros. It was a very common beast in the North American Miocene. Yes, rhinos in North America! I have been eager to share with you all the amazing diversity of North American rhinos. The discovery of a tremendous amount of rhinos, not just in terms of numbers of species but their diversity, is one of the great surprises of North American paleontological expeditions in the 19th and early 20th centuries. This continent was home to rhinos the size of modern pigs, rhinos that could run quickly, and even aquatic rhinos! Teleoceras is one of these aquatic rhinos.
Teleoceras had very short legs for a rhino and a nubby horn. This horn is actually pretty large in the scheme of things. As much as the modern rhinos are famous for their horns the vast majority of fossil rhinos show no evidence of having a horn. We can tell this via the presence or lack of a rough surface on the nasal bones. In life Teleoceras would have probably occupied a niche very simular to the modern hippopotamus.
When: Late Miocene (~ 11 - 6 Million years ago)
What: Shansitherium is a fossil relative of the giraffe. Giraffes are a commonly used example of an easy to see evolutionary transformation, the neck getting longer and longer, and here is some of the evidence used to show that hypothesis. Shansitherium lived in the late Miocene of China, and falls closer to the giraffe line than any other extant clade of artiodactyls. It possesses horns like that of the modern giraffe and okapi, which probably would have been covered with skin in life (these are called ossicones), but over all looks much more like a moose than a giraffe.
Today the Giraffidaehas only two living species, the giraffe (duh) and the okapi, and is only found in sub saharan-Africa. In the Miocene however this group was far more diverse, with Shansitherium just one example of the over a dozen species that roamed all over not just Africa, but Asia and Europe as well. The late Miocene was a time of cooling and drying climates in much of the world, and this is probably what lead to the reduction of species in this clade.
Reconstruction by Willem van der Merwe.
Reconstruction by M. R. Long
When: Eocene (~55 to 35 million years ago)
Where: North America
What: Hyopsodus is a small herbivorous animal common in the fossil deposits of the American west. This animal was about a foot and a half (~45 cm) long at most, with a very long body with relatively short legs. It would have appeared rather ‘weasel like’, but it has no close relation to modern weasels. It is a condylarth - which means its in a giant waste basket group that nobody believes is real. You find this a lot with Paleocene and Eocene mammals. More and more animals formerly grouped under Condylarthra (for lack of any other place to put them) are being referred to as Archaic Ungulates of Uncertain Affinities. Modern ungulates are the hoofed animals (elephants, sheep, horses, cows, etc) and do not form a monophyletic group either, they just all have hooves and thus walk on the very tips of their digits. Hyopsodus itself has been variably placed with respect to living ungulates; sometimes it is found nearer to perissodactyls (horses, rhinos, and tapirs) and artiodactyls (hippos, cows, sheep, camels, etc) but other studies have found it to fall closer to the afrotherians - the clade that includes elephants, tenrecs, and elephant shrews. Personally I suspect a perissodactyl/artiodactyl link is more likely.
Whatever the true phylogenetic placement of Hyopsodus, it is one of the most common fossils found in the american west. Thousands of specimens are held in museum collections (triva: AMNH Fossil Mammal specimen #1? A Hyopsodus jawbone!) , and I myself have found dozens of Hyopsodus teeth while doing my own field work. The overwhelming vast majority of these specimens are just teeth, sometimes isolated teeth but commonly still held in the jawbone or maxilla. Why just teeth? Well, teeth preserve more frequently than bone in the first place, but on top of that, poor little Hyopsodus was the bottom of the food chain in the american jungles of the Paleocene and Eocene. The lack of any skeletons is partially due to the fact most of them probably when down a predators gullet. A few complete skeletons have been found, from sites that show evidence of extremely rapid burial. These skeletons show us that Hyopsodus was a generalized, not specialized for any one locomotion style, and most likely capable of some level of digging as well as climbing. The mounds of teeth clearly show this was a herbivorous animal, probably also a generalist, eating whatever it could find.
Despite the abundance of Hyopsodus material (albiet mostly dental) there is still a lot of work to be done on this little animal and its kin, not only for evolutionary relationships, but exactly how Hyopsodus lived as a common prey item in the jungles of Wyoming. I will also note that the reconstruction above is overall OK, but it does err in making the tail too long.
Megacerops - a titanothere
Note: The mount being assembled above is still on display today, at the American Museum of Natural History, in NYC.
When: Latest Eocene (~ 37 to 34 million years ago)
Where: North America, most fossils found in the west.
What: Megacerops is a brontothere. In the late Eocene it was one of the largest mammals in North America, standing over 8 feet (~2.5 meters) tall at the shoulder, with an estimated weight of over 2000 lbs (~1000kg). These beasts had very distinctive horns, comprised of an outgrowth from the frontal and nasal bones. It is now understood that these structures grew throughout life and were strongly sexual dimorphic; the oldest males had horns that were radically different from those of females. Some earlier paleontologists, not realizing this, basically erected a new species upon every skull that had different looking horns. The example images give an excellent example of the extreme differences present within the genus; the mounted skeleton has very minor horns, while the hanging skull and reconstructions have the very large characteristic Y-shaped ‘slingshot’ fully grown horn of an adult male.
The clade Brontotheriidae is a member of the order Perissodactyla, which also includes horses, rhinoceroses, and tapirs. Within Perissodactyla brontotheres are most closely related to horses. The last common ancestor of brontotheres and horses was a small hooved animal that browsed from bushes and lower branches. While later horses shifted to a more grass based diet, it is thought that brontotheres remained dependent on this softer vegetation. Thus, as North America started to become drier and cooler at the close of the Eocene, and forests gave way to grasslands, the brontotheres vanished.