Bony cranial ornamentation linked to rapid evolution of gigantic theropod dinosaurs

Abstract

Exaggerated cranial structures such as crests and horns, hereafter referred to collectively as ornaments, are pervasive across animal species. These structures perform vital roles in visual communication and physical interactions within and between species. Yet the origin and influence of ornamentation on speciation and ecology across macroevolutionary time scales remains poorly understood for virtually all animals. Here, we explore correlative evolution of osseous cranial ornaments with large body size in theropod dinosaurs using a phylogenetic comparative framework. We find that body size evolved directionally toward phyletic giantism an order of magnitude faster in theropod species possessing ornaments compared with unadorned lineages. In addition, we find a body mass threshold below which bony cranial ornaments do not originate. Maniraptoriform dinosaurs generally lack osseous cranial ornaments despite repeatedly crossing this body size threshold. Our study provides novel, quantitative support for a shift in selective pressures on socio-sexual display mechanisms in theropods coincident with the evolution of pennaceous feathers.

Figure 1. Phylogenetic t-test data distribution performed in BayesTraits.

(a) Blue data points represent unornamented theropods, red represents ornamented species, open circles are non-maniraptoriform theropods, whereas closed circles are species found within Maniraptoriformes, total sample size in phylogenetic t-test is 111 theropod species. Horizontal black line shows the mean log_e body mass (4.207) among all theropods in the sample. Boxes within the box plot shows the first and third quartiles of the data; whiskers expand through the 95th quartile. The estimated posterior distribution for (b) slope, (c) punctuation and (d) phylogenetic signal shown on right.

Figure 2. Phylogeny of non-avian theropods used in this study.

(a) Density map of Bayesian stochastic character probabilities of unornamented (blue) and ornamented (red) character states. (b) Body mass estimates for non-avian theropods used in this study. Green shading shows the distribution of pennaceous feathers among theropods.