Exploring possible ontogenetic trajectories in tyrannosaurids using tracks from the Wapiti Formation (upper Campanian) of Alberta, Canada

ABSTRACT

Fossil tracks should theoretically capture differences in pedal anatomy between growth stages of the same taxon, particularly those related to the soft tissue of the foot, providing a more realistic view of pedal ontogeny than skeletal material alone. However, recognizing these ontogenetic trajectories is complicated by the influence of preservation and kinematics on track morphology, as well as the inherent difficulty of referring different tracks to a single taxon. Here, we explore differences in track morphology from a collection of tracks attributed to tyrannosaurids from Unit 4 of the Wapiti Formation (upper Campanian) in western Canada. Along with morphology, close geographic and stratigraphic associations suggest that the tracks pertain to similar tyrannosaurid trackmakers. A geometric morphometric analysis of the track outlines reveals size-dependent increase in relative track robusticity, driven primarily by an increase in ‘heel’ breadth and surface area. This relationship is lost when the dataset is expanded to include tyrannosaurid tracks globally, which we attribute to increased stratigraphic and taxonomic ‘noise’ within the global dataset that masks the tightly constrained patterns obtained from the Wapiti Formation tracks. Although there is some substrate and kinematic influence on certain aspects of track morphology, we hypothesize that the observed size-dependent relationship reflects genuine expansion in the breadth of the heel soft tissues and probably their overall surface area associated with growth. Increased pedal robusticity likely assisted with weight bearing and locomotor stability as body mass increased over ontogeny, supporting previous hypotheses that some tyrannosaurids underwent a growth-related reduction in relative agility and/or cursorial performance.

ACKNOWLEDGMENTS

We thank R. T. McCrea and L. G. Buckley for their on-site assistance with track identifications; D. Larson for providing photographs of UALVP 53475 and access to field equipment belonging to the Philip J. Currie Dinosaur Museum; G. Behuniak, C. Coy, S. Kagan, and W. Yan-yin for their help with on-site track documentation during the BADP2018 field season; B. Longley-Holland and D. Evans for each discovering large theropod tracks near Red Willow Falls (the BADP2019 natural cast and UALVP 53475, respectively); D. Henderson for providing a length measurement of TMP 1967.019.0145; members of the University of New England Paleoscience Research Centre for their helpful discussions and support while conducting this work; T. Cullen for clarifying aspects of his study (Cullen et al., 2013:fig. 2C); and the Grande Prairie Regional College (GPRC) for providing logistical support during BADP fieldwork. Finally, we thank M. Belvedere, J. Lallensack, and S. W. Salisbury (Handling editor) for their careful reviews that significantly improved the final version of this paper. Funding for this research was provided by: the University of New England, Australia; a Research Training Program scholarship from the Australian Government to N. J. Enriquez; the River of Death and Discovery Dinosaur Museum Society in support of the Philip J. Currie Professorship in Vertebrate Palaeontology at the University of Alberta, Canada, and a Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2017-06246) and start-up funding to C. Sullivan; and Dinosaur Research Institute Fieldwork in Alberta Grants to N. E. Campione and M. J. Vavrek.