This particular genus was very widespread for much of the Eocene, found across Europe, Asia, and North America, crossing back and forth between the continents via the North Atlantic land bridge.
The Jamaican Hyrachyus lived during the mid-Eocene, around 45 million years ago, and was very anatomically similar to the North American Hyrachyus affinis – with the known fossil material not being considered distinct enough to be assigned to a new species yet. It was also about 15-20% smaller than its mainland relative, standing only 25cm tall at the shoulder (10″), but it’s not yet clear if this was a case of insular dwarfism or not.
Its presence in ancient Jamaica suggests that there may have been some sort of land connection between the proto-island and Central America during the early Eocene, when a chunk of what would eventually become western Jamaica was located much closer to the coasts of Honduras and Nicaragua. It’s the only fossil ungulate known from the Caribbean, and one of only a few terrestrial mammals in the region with North American evolutionary roots (the others being the extinct rodents Caribeomys merzeraudi and Oryzomys antillarum, and modern solenodons).
Unfortunately these little rhinos didn’t get much time on their island home. Jamaica subsided fully underwater about 40 million years ago, drowning its unique Eocene ecosystem entirely, and wouldn’t re-emerge and be re-colonized until much later in the Cenozoic.
And back in the 1890s, a specimen of this species was discovered with soft tissue impressions showing a diamond-shaped tail fin.
But despite us knowing about plesiosaur tail flukes for such a long time, they’re surprisingly under-represented in reconstructions, never seeming to have become associated with the popular image of these animals in the same way that early pterosaur’s tail vanes did. It doesn’t help that no other direct impressions of plesiosaur tail fins have ever been found, or that the Seeleyosaurus specimen’s soft tissue got painted over at some point in the mid-1900s, making it incredibly difficult to study without causing further damage.
(Perhaps modern non-invasive scanning techniques could be able to see under the paintjob, but as far as I’m aware nobody’s tried that yet.)
These tail fins are usually assumed to have been vertically oriented like those of other aquatic reptiles, moving side-to-side and acting like a rudder. However, there’s also a hypothesis that their fins might have actually been horizontal more like those of modern cetaceans and sirenians, based on several anatomical quirks – such as their tail regions being very wide and flat at the base, and the vertebrae at the tip being unusually pygostyle-like, very different from the way the tail bones of vertically-finned reptiles look.
Living in Poland during the Late Triassic (~230 million years ago), it was a quadrupedal animal roughly the size of a large modern dog, about 50cm tall at the shoulder (1’8″) and 2m long (6’6″). The front of its lower jaw was toothless and covered with a keratinous beak, and there may have been a corresponding much smaller beak at the very tip of its upper jaw, too.