THE SIZE-STRUCTURE OF THE HERBIVORE GUILD THROUGH TIME

An age-old question in vertebrate paleontology is why dinosaur body size distributions looks so different from mammal body size distributions. While many have attempted to answer this question, most approaches have looked at the global size distribution, or distributions at coarse spatio-temporal scales, such as formations, which are biased by several factors. What is undeniable is that at the scale of well-sampled bone-beds and quarries, there are more co-existing species of large dinosaur than small dinosaur. It is yet unclear whether this is an aberration from the structure of local mammalian size distributions. Here, I present preliminary research that compares the size structure of the local mammalian herbivore guild in the Cenozoic of North America to the size structure of the dinosaurian herbivores from Morrison Formation quarries. Using the moments of the distribution, I show that, at least for mammalian herbivores weighing >1kg, the distribution becomes more negatively skewed, i.e., more large species than small species, in the Neogene and Quaternary. This structure is somewhat similar to that seen in Morrison quarries, but the average skew of the dinosaurian assemblages is statistically more negative than that seen at any time in the Cenozoic. This implies that adult dinosaur communities, even at the local scale, are structured in a substantially different way than North American mammal communities are. Lastly, I discuss potential reasons for these differences, including the absence of juveniles, and ontogenetic size shifts in dinosaurs.