Preliminary investigation of Allosauroidea facial integument and the evolution of theropod facial armor

Abstract

In amniotes, the growth of specialized facial integument, or external skin tissues, has a close relationship with the texture and morphology of underlying cranial bones. Osteological correlates of facial skin structures have been used before to reconstruct the integument of several extinct dinosaurian lineages but not for theropods from the clade Allosauroidea, whose members exhibit exceptionally rugose and heavily ornamented skulls. This study aims to investigate, in a preliminary sense, the facial integument of Allosauroidea by examining osteological correlates from high-resolution photographs of fossil material. We compared the inferred integument of allosauroids to analogous structures in modern animals in order to discuss potential biological and behavioral implications, with soft-tissue adaptations suggesting headbutting or sparring in some taxa. Allosauroids display an evolutionary trend for increasing cranial cornification throughout their history, and their most derived members exhibit greater development of facial armor than other tetanurans. We developed three hypotheses to explain the selective pressures that may have driven the independent evolution of this dermal armor in several unrelated theropod clades, namely abelisaurids, carcharodontosaurids, and tyrannosaurids. Empirical tests of paleoecological data did not support any of the three hypotheses of intraspecific conflict, competition from other carnivores, or dangerous prey items as the main pressure that drove the evolution of theropod facial armor. We suggest that more sampling of armor-faced theropods and their associated ecosystems, especially those from the Southern Hemisphere, is needed to reveal the reasons behind the convergent evolution of this integument.