Thursday, September 9, 2021

[Paleontology • 2021] Intraspecific Facial Bite Marks in Tyrannosaurids provide insight into Sexual Maturity and Evolution of Bird-like Intersexual Display

Artistic reconstructions of hypothesized face-biting behaviour in Tyrannosauridae.
Left, two Gorgosaurus individuals aggressively square off.
Right, the aftermath of face-biting behaviour, 
showing recent wounds and older scars. 

in Brown, Currie & Therrien, 2021.

Image © Royal Tyrrell Museum, illustration by Julius Csotonyi.

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.

Skull bones with tooth-strike scars across tyrannosaur specimens of increasing sizes.


Presence of facial tooth marks, ornamentation of the skull, and presence of feathered forelimbs, across various members of Theropoda.

Caleb M. Brown, Philip J. Currie and François Therrien. 2021. Intraspecific Facial Bite Marks in Tyrannosaurids provide insight into Sexual Maturity and Evolution of Bird-like Intersexual Display. Paleobiology. FirstView. pp. 1 - 32. DOI: 10.1017/pab.2021.29

Clash of the Titans: Facial Scars as Indicators of Aggressive Behaviour and Sexual Maturity in Tyrannosaurs