Taking a second look at the “Fire Beast”, Pyrotherium
A restoration of the head of Pyrotherium, from W.B. Scott's 1913 'A History of Land Mammals in the Western Hemisphere'
Until just a few years ago, I never paid that much attention to fossil mammals. Sure, I was impressed by the saber-toothed cat Smilodon and the American mastodon Mammut americanum - badass, extinct versions of living big cats and elephants - but beyond that they never really grabbed my interest. Although clearly different from living species, many fossil mammals seemed similar enough to extant forms that I didn't think twice about passing them by in museum fossil halls - get me to the dinosaurs, please.
How foolish I was! Since I never really looked, I never understood just how extraordinary fossil mammals were, and among the strangest was a hoofed mammal from the late Oligocene (~29-21 million years ago) of South America called Pyrotherium. This "fire beast" has been known for a long time - it was first described in 1889 - but it remains an especially enigmatic animal which has fallen through the cracks of academic literature. It is one of those poor fossil creatures which are rarely studied despite being well-known for their oddball status, but a recent redescription of the only known Pyrotherium skull by Guillaume Billet places this mysterious mammal in its proper evolutionary context.
A photo of the only known skull of Pyrotherium. From Billet 2010.
There are a number of features in the skull of Pyrotherium which stand out. Among the most prominent are its teeth. Jutting out from the upper jaw of Pyrotherium are two pairs of stout incisors, matched by a single pair in the mandible, and - along with a recessed nasal cavity which looks like it could have supported a trunk - the prominence of these tusks led some paleontologists to believe that this animal was an early type of South American elephant. That hypothesis was discarded long ago, but other scholars have not done much better - from marsupials to the knobbly-headed uintatheres, the problematic Pyrotherium has been shuffled around so much that some scientists have doubted whether it can be placed among any known group of mammals at all.
In his new description of the single Pyrotherium skull, however, Billet favors one hypothesis which has popped up over and over during the last century - that Pyrotherium was a notoungulate, or part of a diverse array of bizarre hoofed mammals which evolved in "splendid isolation" on the South American continent. Among the strongest evidence for this connection is provided by a small ridge of bone between the bulbous bony compartment of the ear bones and the side of the skull. It is quite similar to the ridge of bone which exists in the skulls of notoungulates, and along with other minute features (such as the placement of small holes, called foramina, around the ear region) it seems that the skull of Pyrotherium possesses distinctive landmarks only seen amongst this mammalian group.
A line drawing of the skull of Notostylops (left) and a photo of the only known skull of Pyrotherium (right). From Billet 2010.
Identifying Pyrotherium as a notoungulate is certainly progress, but Billet goes one better. Of the known notoungulates, Pyrotherium shares some unique dental characteristics with a rabbit-sized member of the group called Notostylops. Both, have large, forwardly-directed upper and lower incisors, and even though Notostylops has two pairs of lower incisors as opposed to the one in Pyrotherium, the difference in the development of the teeth suggest that the second incisors of Pyrotherium became so large that they crowded out the first set. In a sense, the front of the jaws in Notostylops looked like what one might expect for a forerunner of Pyrotherium. (Billet does not say this in the paper, but if his observations are correct I suspect that that anatomy of Notostylops may at least partially illustrate what the first pyrotheres were like.)
This close relationship between Notostylops and Pyrotherium (and, in fact, the whole swath of weird creatures called pyrotheres) was underscored by Billet's cladistic analysis of numerous notoungulate mammals. Through comparing each animal trait-for-trait with others in a computer program, it became clear that the shared traits Billet saw in both Notostylops and Pyrotherium really did indicate their close relationship and their membership among the notoungulates. Oddly enough, the previous inclusion of another pyrothere - Proticia - may have confused similar efforts to achieve resolution in the past. Its cheek teeth were similar to those of Pyrotherium, but otherwise so little is known of it that it often created uncertainty in this type of evolutionary analysis. Until it becomes better known, Proticia is best left out of the mix.
Yet, even though Billet has solved the mystery of Pyrotherium, much remains unknown about the mammals of South America and their relationships to each other. Numerous species are only known from single, fragmentary specimens, and many of the fossils which have been found have not been studied in years. For anyone looking to tackle interesting problems in mammalian paleontology, the questions surrounding the evolution of the notoungulates and other South American mammals present plenty of opportunities.
Billet, G. (2009). New Observations on the Skull of Pyrotherium (Pyrotheria, Mammalia) and New Phylogenetic Hypotheses on South American Ungulates Journal of Mammalian Evolution, 17 (1), 21-59 DOI: 10.1007/s10914-009-9123-0