04 December 2013

Fossil Vertebrate of the Month: Nimravus brachyops

As a paleoecologist, interactions between animals and their environments are my stock-in-trade.  For the most part, paleoecologists study these interactions by observing trends through time in the variables of interest in search of macroscopic patterns.  Every now and then, though, a fossil turns up that captures (or at least appears to capture) an interaction between an individual and some aspect of its environment (usually another animal).  The most famous examples of this are, unsurprisingly, saurian: I had the opportunity to view the amazing Mongolian "Fighting Dinosaurs" when they were on display in New York a few years back, and another pair has made headlines for all the wrong reasons in recent weeks.  However, no specimen provides as good an example of both the power and the pitfalls of "fighting fossils" as a skull in the University of Nebraska State Museum.  It is a specimen of Nimravus brachyops, a member of the eponymous Nimravidae.  Nimravids are a fascinating group of carnivores in their own right: they are mid-sized saber-toothed predators that were particularly abundant in the Oligocene and are nearly indistinguishable from cats but are probably closer relatives of civets.  This specimen, however, is of interest from more than just a scientific point of view.  It was found with a canine embedded in the humerus of another N. brachyops, showing that the two individuals had died fighting.  That, at least, was the conclusion of the field crew that first uncovered the specimen.  This crew included a young Loren Eiseley, who would go on to become one of the most prominent naturalists of the 20th Century.  Eiseley was so impressed by the specimen that it inspired him to write one of his most famous poems, 'The Innocent Assassins.'  The picture painted by the fossils and by the poem is certainly dramatic, but is it accurate?  Paleontology has long been plagued by studies that put good stories in front of the evidence of the fossil record and, unfortunately, this may be one of them.  First of all, there is no irrefutable evidence that more than one individual was present (in paleontological parlance, the Minimum Number of Individuals is 1, meaning that it is impossible to disprove that both bones came from the same animal).  Of course, no one would suggest that the nimravid bit through its own arm.  However, it is possible that the humerus and the canine were driven together after death, either during transport or, probably more likely, during burial.  As others have observed, this latter scenario is supported by the fact that the canine, while broken towards its tip, is largely intact.  One of the major paradoxes of saber-toothed predators is that elongated canines are, for the most part, exceptionally brittle and would have broken remarkably easily if too much stress were applied to them. It beggars belief that this Nimravus had canines robust enough to not only puncture bone but to remain mostly intact during the struggle that would have followed.  Perhaps the Nebraska specimen really does represent a death struggle, but the balance of probability is that taphonomy, not paleoecology, provides the explanation for the association between the two bones.  For those of you who find this depressingly banal, I hasten to add that this does not mean that nimravids never fought.  A well-known specimen from South Dakota seems to represent a Nimravus skull that has been punctured by the saber of the smaller nimravid Eusmilus, and a talk at this year's Geological Society of America Meeting suggested that such injuries might be more common than previously thought.  Nimravids may very well have been preternaturally pugnacious, but for all of Eiseley's eloquence, the true drama of the "Innocent Assassins" specimen lies not in the moment of death but in the evolutionary and ecological story into which the fossil fits.