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.

See Materials and Methods sections of the paper for methods details. Supplemental data consists of: supplementary text document, including 11 supplementary figures (Figures S1–S11), 11 supplementary tables (Tables S1–11), legends for the eight supplementary datasets (Datasets S1 to S8) (as external files: .xls, .pdf), and supplementary references. 

Supplementary Datasets:

Dataset S1 (separate .pdf file): Line drawings of tyrannosaur cranial elements (1-79) showing position of healed/healing intraspecific bite mark lesions.  Red indicates marks that are incised and porous while orange indicated marks that are smooth or raised.  Multiple marks on an element are numbered using roman numerals from rostral to caudal.  Grey areas indicate reconstruction or broken surfaces. See Supplemental Dataset S2 for quantitative data for each specimen and mark.  Only specimens with lesions are illustrated. Number corresponds to column I in Dataset S2. Data used to generate Figures 4, S4.  All scale bars equal 10 cm.

Dataset S2 (separate .xls file): Quantitative data on tyrannosaur cranial elements with and without intraspecific bite mark lesions. Sheet 1 sorted by specimen. Sheet 2 is sorted by element. Sheet 3 is sorted by taxon (maxilla and dentary only). Data used to generate Tables 1, S4-S6 and Figures 3, 6-9, S4, S5. Column I indicates number in Dataset S1

Dataset S3 (separate .pdf file): Line drawings of Alligator cranial elements showing position of healed/healing intraspecific bite mark lesions.  Red indicates marks that are incised and porous while orange indicated marks that are smooth or raised. Grey areas indicate reconstruction or broken surfaces. See Dataset S4 for quantitative data for each specimen and mark.  Only specimens with lesions are illustrated. Data used to generate Figures 10, S6, S7. All scale bars equal 10 cm.

Dataset S4 (separate .xls file): Quantitative data on Alligator cranial elements with and without intraspecific bite mark lesions. Data used to generate Figure 10, Table S11.

Dataset S5 (separate .xls file): Literature-derived data on intraspecific bite marks/tooth mark frequency in extant animals. Data concentrate on reported results for males and females individually.  Some data (highlighted) are excluded due to incompleteness or marks made by non-homologous structures. Data are used to generate Figure 13.

Dataset S6 (separate .xls file): Data for regression of tooth for length and mean alveolar size. Dataset of tyrannosaur dentary, maxilla, and premaxilla elements preserving a complete tooth row, and the mean alveolar length. Data are used to generate Figure S1, Table S1.

Dataset S7 (separate .xls file): Data set of adult sex ratio in extant bird and crocodylian species/populations. Some crocodylian datasets do include subadult individuals. Data are used to generate Figure S10.

Dataset S9 (separate .xls file): Data of intraspecific bite mark lesion occurrences in Theropoda. Data used to generate Figure 14.