02 March 2011

Brontomerus, Hell Creek, and Mesozoic Ecology

Two dinosaur-related stories have been getting a lot of press this month.  The first is the naming of the new sauropod Brontomerus mcintoshi (Taylor et al. 2011), which is remarkable for its name (literally 'McIntosh's Thunder Thighs'), its oddly large legs, and its implications for Early Cretaceous sauropod diversity.  The story that is more intriguing to me, though, is the publication of the results of the Hell Creek Project dinosaur census (Horner et al. 2011).  For those who aren't familiar with it, the Hell Creek Formation of eastern Montana and adjacent states has produced one of the richest assemblages of Late Cretaceous vertebrates in the world.  The fauna has been extensively sampled and studied, thanks in large part to the efforts of Jack Horner at MSU's Museum of the Rockies.  The completeness of the Hell Creek fossil record makes it an appealing subject for paleoecological analysis, which is the focus of Horner's new paper.  The authors draw two major conclusions: that the bulk of the large-bodied dinosaurs from Hell Creek represent individuals of intermediate age, while juveniles and old adults are rare, and that Tyrannosaurus was so common that it must have been more ecologically analogous to scavenging, opportunistic hyenas rather than predatory big cats (which require huge amounts of food and are therefore almost always much less common than their prey).  The first point should perhaps not be surprising; as is observed in the paper, there are compelling ecological and taphonomic reasons why very young individuals should not be found at Hell Creek, and it is likewise to be expected that most dinosaurs probably did not survive to extreme old age.  The argument that Tyrannosaurus could not have been an active hunter - the part of the research, incidentally, that has attracted the most media attention - is somewhat more problematic.  Certainly, a modern mammalian predator would not be as abundant as Tyrannosaurus was in the Hell Creek fauna, but using mammals as analogs for dinosaurs has its drawbacks.  Dinosaurs were biologically distinct from mammals (no mammal, for example, could grow to the sizes of sauropods without outstripping their food supply) and the Mesozoic world was fundamentally different from that of today, and as such dinosaurs played by a different set of rules than does anything currently living (including the dinosaurs' descendants, the birds).  Because of this, patterns such as predator/prey ratios that can be very informative when discussing community structure in Cenozoic ecosystems may mean something very different in the Mesozoic, and the preponderance of Tyrannosaurus may be due to biological factors such as metabolism or social structure or to taphonomic biases.  This post may sound like a criticism of Horner et al., but that is not its intent.  In fact, I think the finding that Tyrannosaurus was aberrantly common in the Hell Creek fauna is extremely interesting and certainly the authors' interpretation may be correct.  Further, Hell Creek is one of the only Mesozoic ecosystems that lends itself to fairly robust ecological analysis, and it's excellent that work along those lines is being conducted.  However, at the end of the day, there's a reason dinosaurs are so popular: they are utterly foreign to modern eyes.  This is something of a double-edged sword, because it does make dinosaurs fascinating animals, but it also means they have no good modern analog and that any reconstruction of their ecology will always be cursed with a lower degree of confidence than studies of animals such as reptiles, birds, and, of course, mammals.