A new study has found that most large theropod dinosaur species rapidly acquired their massive stature immediately following the evolutionary development of elaborate cranial features like horns, crests and bumps.
Using computer modeling, researchers at North Carolina State University examined the fossils of 111 theropods exhibiting varying degrees of "bony cranial ornamentation" and compared how their overall body size changed over thousands of years of evolution.
What they found is that theropods weighing under 80 pounds did not sport any bony ornamental features, but the vast majority of theropods that weighed more than 80 pounds (20 out of 22 species) did exhibit protrusions on their heads. Any large theropods that did not develop these pronounced features, like the Acrocanthosaurus, evolved into their gigantic mass at a much slower rate than more ornamental theropods.
"Body size evolved directionally toward phyletic giantism an order of magnitude faster in theropod species possessing ornaments compared with unadorned lineages," the researchers write in the journal Nature Communications. "In addition, we [found] a body mass threshold below which bony cranial ornaments do not originate."
These horns, crests and bumps likely served as "socio-sexual display
mechanisms," which allowed the dinosaurs to communicate signals for initiating mating, guarding territory and defending themselves.
Terry Gates, a researcher at North Carolina State University and one of the study's authors, says he and his team were "surprised to find such a strong relationship between ornaments and huge body size in theropods. Something about their world clearly favored bling and big bods."
The study also examined velociraptors and other maniraptoriforms,
which is the lineage of dinosaurs that led to modern birds. Though many of these creatures weighed more than 80 pounds, they
never evolved to form extravagant cranial features like the large
theropods. What prompted these maniraptoriforms to buck such a
well-established evolutionary trend among large dinosaurs?
According to Gates, "the best explanation is that the long stiff feathers, which originated in this group of dinosaurs and were similar to modern bird feathers, could perform equally well as social signals when compared to the bony displays in T. rex or Dilophosaurus."