Picked up on the recent Black Hills trip. Not light reading, even if you’re a paleontologist. The bulk of the book is extremely detailed anatomical descriptions of North American rhinoceros specimens, with tables of dimensions, tooth patterns, and so on. Author Donald Prothero notes that the paleontology of mammals mostly depends on teeth, and since rhinoceroses have complicated teeth, there are plenty of opportunities for creating new species based on single molars. Prothero cites a particularly egregious example where, based on a minor variation in one of the molars, two halves of a lower jaw were assigned to two different species. Thus a lot of early rhinoceros species descriptions – up until the 1960s or so – are invalid. (In cause you stumble on one, rhino teeth are fairly distinctive; the occlusal surface of the molars is shaped like the Greek letter pi (π). Sometimes it’s a rather sketchy π, as if the Greek involved had had way too much ouzo, but it’s still π).


Rhinos appear to have originated in Eurasia and made it to North America in the Eocene. The group quickly divided into the Amynodontidae, which look more or less like small, hornless, modern rhinos; the Hyracodontidae, which look like their main competitors in the Perissodactyla, horses – especially because horse at the time were also small and three-toed. Some older books, especially popular ones, refer to hyracodons as “running rhinoceroses”, which isn’t that bad a description. Both amynodonts and hyracodons go extinct in the middle Oligocene, but the third branch, the Rhinocerotidae, continue to the end of the Miocene (with a couple more infusions from Eurasia).


Interestingly, almost all the North American rhinos were hornless (the horn is keratin and isn’t usually preserved, but there are rough spots on the skull where it attached). The exception is the dicertherines, which had two horns – but they were side-by-side instead of fore-and aft. A lot of the rhinos had incisors that developed into short tusks and this may have provided some of the display and defense functions later assumed by horns. (These were just long enough to project out of the mouth a little, so are in no sense like boar or elephant tusks; but they still probably made an impressive display when the animal gaped).


By the end of the Miocene, the North American rhino groups were aceretherines, which were slightly smaller than a modern rhino and hornless, and the teleoceratines, which had a short horn but which were short-legged and more hippo-like than rhino-like. None of the giant indricotheres ever turned up in North American, but there are a couple of enigmatic fossil bones – three astragali and a humerus – that are much larger than any contemporary rhino and within the range of the smaller indricotheres. Unfortunately there’s no skull or teeth, so the bones can’t be assigned to anything more taxonomically detailed than “rhinoceros”.


All the North American rhinos were extinct by the end of the Miocene; there are a couple of dubious Pliocene fossils, but they could have washed in from older strata. Interestingly, none of the Pleistocene Eurasian coleodonts (wooly rhinos) ever made it to North America, unlike other Pleistocene megafauna.


Many of the known rhino fossils occur in dramatic assemblages. The Agate Springs National Monument in Nebraska had an assemblage where 4300 skulls and other bones were excavated from a 44-square-foot slab, and the total in the bone-bearing layer is estimates at 3 million bones from 17000 animals. The Agate Springs site is not a catastrophic assemblage - too many adults and too few juveniles - instead it is believed to be an attritional assemblage around a slowly drying water hole. At the other end of Nebraska, the Ashfall Fossil Beds State Park is a catastrophic death assemblage, with a herd of Teleoceras buried under volcanic ash. The skeletons are 78% female and include a number of calves – some still snuggled in nursing position – and fetal skeletons inside the mother’s pelvic cavity. Another interesting fossil is the Blue Lake rhino mold from Idaho, where the bloated body of a dead rhino floating in a lake was overrun by a basalt flow, creating a Pompeii-style body cast with enough detail to show the pads of the feet and the prehensile lip. Unfortunately the teeth and upper part of the head were not preserved so the rhino cannot be assigned to a species.


Not a coffee-table book; way too much technical detail and not enough pretty pictures. Lots of black-and-white photographs and drawings of rhino teeth, skulls, and other bones. Extremely well-referenced. Very pricy.
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