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.
Reconstruction by Dmitry Bogdanov.
When: Late Jurassic (167-155 million years ago)
What: Metriorhynchus is an extinct marine crocodile-relative. It is a member of the most aquatically adapted group of crocodile-line archosaurs ever known, the Metriorhynchidae. Like the rest of this group, it was more streamlined than most crocs, having lost the heavy dermal armor (osteoderms) that characterize the vast majority of crocodylomorphs. Its tail was finned and its feet were transformed into paddles, making it an excellent swimmer. Its snout was very narrow, which is correlated with eating fish in living crocodiles. The group Metriorhynchus belongs to is the only clade in the crocodile lineage to have ever developed paddles, even though for much of the history of crocodiles they have been heavily linked with aquatic habitats.
It is unknown how much Metriorhynchus and its relatives came onto land. As its feet are paddles, it is tempting to think that it never left the water. It would have assuredly been very slow and awkward on land, dragging its bulk with its relatively small paddles. However, there is no evidence of live birth in Metriorhynchus or its relatives. In fact, no archosaur (dinosaurs + crocodiles) has ever been shown to assuredly have developed live birth. Thus, it is likely that Metriorhynchus did drag its 10 feet (~3 meter) long body out of the water to lay its eggs, much like the modern sea turtles.