Description of juvenile specimens of Prosaurolophus maximus (Hadrosauridae: Saurolophinae) from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada, reveals ontogenetic changes in crest morphology

ABSTRACT

Three juvenile specimens of Prosaurolophus maximus, represented by articulated to disarticulated skeletons, are the smallest known individuals for the taxon. Cranial anatomy of the juvenile specimens indicates that diagnostic characters of P. maximus are ontogenetically variable. In the smallest individual, the crest and deeply excavated fossa at the caudal margin of the circumnarial depression are poorly developed or absent. The crest approaches adult-like morphology in large juveniles, whereas crest robusticity and the deep excavation of the circumnarial depression occur only in subadult and adult individuals. The shape of the caudal margin of the circumnarial depression is consistent between juvenile and adult individuals, potentially making this feature a reliable character for taxonomic identification at younger ontogenetic stages. The crest of P. maximus grows isometrically during ontogeny, unlike the positive allometric growth of lambeosaurine hadrosaur crests, suggesting that this taxon may have had soft tissue structures associated with the narial-crest region, rather than the bony crest itself, selected for sexual display. Recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n, the juvenile specimens are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation (Alberta) and contemporaneous with most specimens from the Two Medicine Formation (Montana), extending the temporal range of the taxon to 75.7–74 Ma. The occurrence of P. maximus in well-drained terrestrial deposits of the Dinosaur Park and Two Medicine formations and marine sediments of the Bearpaw Formation indicates that this taxon inhabited various paleoenvironments in western North America.

ACKNOWLEDGMENTS

For access to collections and specimen information, we thank B. Strilisky, T. Courtenay, B. Sanchez, R. Russell, and H. Feeney of the Royal Tyrrell Museum of Palaeontology, K. Seymour of the Royal Ontario Museum, and M. Currie of the Canadian Museum of Nature. We would also like to thank R. Trudel (Korite International), D. Braman (RTMP), and D. Brinkman (RTMP) for the information they provided on the stratigraphic position of the juvenile specimens. We would also like to acknowledge A. Kilmury, C. Carbone, and D. Macleod for the preparation of TMP 2016.037.0001. We also wish to thank Korite International staff for their diligence in recognizing and reporting vertebrate fossils discovered during their mining activities and for their assistance with the excavation and recovery of fossils. Finally, we thank Andrew Farke, Elizabeth Freedman Fowler, and Michael D’Emic for comments that improved the manuscript. This research was supported by grants from the Royal Tyrrell Museum Cooperating Society, Alberta Culture and Tourism, the Alberta Lottery Fund, and an Natural Sciences and Engineering Research Council of Canada Discovery Grant (to D.K.Z.). Part of this research was conducted in the context of E. Drysdale’s master’s thesis at the University of Calgary Department of Geoscience.