Intraspecific facial bite marks in tyrannosaurids provide insight into sexual maturity and evolution of bird-like intersexual display

Abstract

Intraspecific aggression, or agonism, is a widespread intrasexual selective behavior important to understanding animal behavioral ecology and reproductive systems. Such behavior can be studied either by direct observation or inferred from wound/scar frequency in extant species but is difficult to document in extinct taxa, limiting understanding of its evolution. Among extant archosaurs, crocodylians display extensive intrasexual aggression, whereas birds show extreme visual/vocal intersexual display. The evolutionary origin of this behavioral divergence, and pattern in non-avian dinosaurs, is unknown. Here we document the morphology, frequency, and ontogeny of intraspecific facial bite lesions (324 lesions) in a large sample of tyrannosaurids (202 specimens, 528 elements) to infer patterns of intraspecific aggression in non-avian theropods. Facial scars are consistent in position and orientation across tyrannosaurid species, suggesting bites were inflicted due to repeated/postured behavior. Facial scars are absent in young tyrannosaurids, first appear in immature animals (~50% adult skull length), are present in ~60% of the adult-sized specimens, and show aggressor:victim size isometry. The ontogenetic distribution of bite scars suggests agonistic behavior is associated with the onset of sexual maturity, and scar presence in approximately half the specimens may relate to a sexual pattern. Considered in a phylogenetic context, intraspecific bite marks are consistent and widely distributed in fossil and extant crocodyliforms and non-maniraptoriform theropods, suggesting a potential plesiomorphic behavior in archosaurs. Their absence in maniraptoriform theropods, including birds, may reflect a transition from boney cranial ornamentation and crocodylian-like intrasexual aggression to avian-like intersexual display with the evolution of pennaceous feathers.

Observations on Paleospecies Determination, With Additional Data on Tyrannosaurus Including Its Highly Divergent Species Specific Supraorbital Display Ornaments That Give T. rex a New and Unique Life Appearance

“…And suggests that the species were probably internally dimorphic. The possession of such display organs is in line with the evidence for intraspecific combat recorded on the skulls of the genus (Brown et al 2022).…”

A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana

“…10 ); the edges of these grooves are beveled and are likely pathological. They may represent bite marks, as have been described on the craniofacial bones of other tyrannosaurids ( Voris et al, 2020 ; Brown, Currie & Therrien, 2022 ).…”

Celebrating dinosaurs: their behaviour, evolution, growth, and physiology

“…Global Positioning Systems (GPSs) have certainly improved the ability to pinpoint sites (MacDonald et al 2005) with greater efficiency and accuracy, and drones are making a huge difference in mapping the distribution of skeletons (Fanti et al 2018a). Quarry mapping has improved dramatically using high-resolution GPS (Brown et al 2022), lidar, and photogrammetry (Matthews et al 2016). Geochemical fingerprinting of specimens and sediments (Fanti et al 2018b) is now possible because of portable X-ray fluorescence (XRF) analysers (Fig.…”

“…Behaviour is, of course, a very broad topic that takes in an almost infinite array of potential topics. It covers everything from mating (Tanke and Currie 2000;Brown et al 2022) and nesting behaviours (Dong and Currie 1996), to fighting and foraging/hunting behaviours (Farlow 1976b;Currie 2000;Currie and Eberth 2010), to migration (Currie 1992), to social behaviour and so much more.…”