Juvenile specimens of Prosaurolophus maximus (Hadrosauridae, Saurolophinae) from the marine Bearpaw Formation (upper Campanian, Upper Cretaceous) of southern Alberta, Canada reveal ontogenetic change in crest morphology
Date
2019-01-25
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Abstract
Three juvenile specimens of Prosaurolophus maximus, represented by both articulated and disarticulated skeletal material, are the smallest known individuals for the taxon. Their cranial anatomy indicates that the diagnostic characters of P. maximus are ontogenetically variable. In the smallest specimen, the crest and deeply-excavated fossa at the caudal margin of the circumnarial depression are poorly-developed or absent. In the larger specimens, the crest approaches the morphology observed in subadult and adult specimens but lacks the robusticity and deep excavation of the circumnarial depression typical of P. maximus. In contrast, the shape of the caudal end of the circumnarial depression is ontogenetically consistent, potentially making it a reliable character for taxonomic identification. Histological sections from the three juvenile specimens and one large individual indicate that the minimum biological age of the known specimens of P. maximus ranges from two to five years. Given that the large P. maximus individual has yet to reach skeletal maturity, the species may reach a maximum body size larger than represented by currently known specimens, perhaps with a skull length approaching the size of Saurolophus angustirostris. The lack of positive allometric growth associated with the P. maximus crest, which undergoes isometric growth from age three onward, differs from that observed in lambeosaurine hadrosaurs suggesting that the crest may not have been the result of sexual selection. However, positive allometric growth of the circumnarial region and the rugosity of the bony crest suggests that soft tissue structures associated with the narial-crest region, rather than the bony crest itself, may have been selected for sexual display in P. maximus. The juvenile specimens were recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n. They are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation, and contemporaneous to 100,000 years younger than specimens from the Two Medicine Formation, extending the taxon’s temporal range. The occurrence of P. maximus in the marine deposits of the Bearpaw Formation and well-drained upland terrestrial deposits of both the Dinosaur Park and Two Medicine formations suggests that this taxon inhabited various palaeoenvironments.
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Keywords
Dinosaur, Palaeontology, Ontogeny, Hadrosaur, Prosaurolophus, Juvenile, Bearpaw, Histology, Morphometrics
Citation
Drysdale, E. T. (2019). Juvenile specimens of Prosaurolophus maximus (Hadrosauridae, Saurolophinae) from the marine Bearpaw Formation (upper Campanian, Upper Cretaceous) of southern Alberta, Canada reveal ontogenetic change in crest morphology (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.