New Species Suggests Small Dinosaurs Were More Numerous Than We Thought

Size matters in dinosaur celebrity---hefty T. Rex is a household name but chicken-sized fellow theropod Anchiornis huxleyi, not so much. But though the bigger, beefier thunder lizards may dominate today’s pop culture, there’s growing evidence that smaller dinosaurs were far more prevalent and diverse than the fossil record indicates. Now, the discovery of a new small-bodied dinosaur strongly suggests that current estimates of the numbers and variety of these animals has been significantly underestimated. Acrotholus audeti, described today in Nature Communications, was a small-bodied pachycephalosaurid, one of the ornithischian dinosaurs that evolved thick, domed skulls. Based on fossils found in Alberta, Canada, and dated to the Santonian age (roughly 83-86 million years ago), A. audeti is the oldest pachycephalosaurid ever found in North America. Older specimens from sites outside the continent have been described based on teeth and are unsubstantiated, according to A. audeti’s research team, possibly making their find the oldest pachycephalosaurid in the world. The A. audeti specimen has a fully developed cranial dome and other derived pachycephalosaurid traits, including eye sockets which have been completely incorporated into the dome. This rewrites the evolutionary timeline of these thick-skulled dinosaurs: Previous theories suggested pachycephalosaurids evolved from post-Santonian flat-headed or partially domed species. Because the bones of larger, more robust animals are more likely to be preserved (smaller bones often are destroyed or displaced by weather or carnivores), determining the ratio of small- to large-sized dinosaurs in ecosystems of their time has been difficult. The limited number of fossils from smaller dinosaurs has also hampered efforts to understand how rapidly they evolved. However the fossil record of pachycephalosaurids---among the smaller ornithischians---is somewhat more complete due to the animals’ robust skulls, which are resistant to destruction. Most pachycephalosaurids, in fact, are known solely from cranial fossils. Paleontologists working on A. audeti believe the age and highly derived traits of the newly described species provide evidence that smaller dinosaurs were greater in number, and more diverse, than scant fossils suggest.