Academic literature on the topic 'Parasaurolophus'

The lambeosaurine hadrosaurid Parasaurolophus is known from rare occurrences in Campanian deposits of western North America. A previously undescribed large hadrosaurid braincase from the Dinosaur Park Formation (Alberta, Canada) is assigned to the genus Parasaurolophus on the basis of several derived characters associated with the frontal–nasal articulation at the base of the crest. This identification is supported by two separate phylogenetic analyses, in which the specimen clusters with other more completely known Parasaurolophus exemplars. If correctly identified, the specimen represents the third and largest cranial specimen of the genus from the Late Cretaceous of Alberta. The specimen occurs in the same deposits as the holotype specimen of Parasaurolophus walkeri and may represent a late ontogenetic stage of this taxon. As opposed to a small frontal dome in the holotype of P. walkeri , the external contribution of the frontal to the skull roof is obliterated in the new specimen. If these hypothesized ontogenetic changes in the skull roof correlate with the size and posterodorsal development of the crest, as in other lambeosaurines, it suggests that the crest had not reached its full expression in the holotype. When placed into a detailed biostratigraphic context for the first time, the limited Parasaurolophus material from the Belly River Group is distributed in the lower half of the Dinosaur Park Formation at Dinosaur Provincial Park. This suggests that Parasaurolophus may be associated with the lower Centrosaurus – Corythosaurus assemblage zone and may have preferred more inland environments than other hadrosaurids, such as Lambeosaurus and Prosaurolophus .

For nearly 60 years, skulls of Parasaurolophus species have been differentiated primarily on the basis of crest shape rather than on unique morphologic characters of other cranial elements. Complicating matters is the fact that crests dramatically change shape throughout ontogeny. Without a complete growth series, it has become difficult to assess the taxonomic distinctness of each species through the lens of allometric growth. Parasaurolophus cyrtocristatus has proven to be especially troublesome to assess because of the poorly preserved nature of the type and only skull. A new, partial skull from the Fossil Forest Member of the Fruitland Formation—the same geologic unit as the type specimen—is the first opportunity to re-diagnose this species as well as redefine the genus with many new traits. An undescribed, short-crested subadult skull from the Kaiparowits Formation of Utah previously assigned to cf. P. cyrtocristatus allows detailed comparisons to be made between the unnamed Utah taxon and the material of this species from the type locality. We find that several characteristics of the squamosal, supraoccipital, and premaxilla shared between the referred skull and the type skull are unique to P. cyrtocristatus (senso stricto) within the genus, irrespective of the overall crest shape. A phylogenetic analysis that includes six new characters posits that P. cyrtocristatus and P. tubicen are sister taxa, and that the latter does not share a closest common ancestor with the long-crested P. walkeri as previously hypothesized. This result helps to explain why both taxa are found in northeastern New Mexico, USA and in sequential geologic units (Fruitland Formation and Kirtland Formation, respectively). Additionally, the exquisitely preserved new skull provides the first opportunity to unequivocally identify the osteological make-up of the Parasaurolophus cranial crest. Unlike in previous reconstructions, the crest composition in Parasaurolophus follows what is seen in other lambeosaurines such as Corythosaurus, where the dorsal process of the premaxilla dominates the crest, with the nasal forming 80% of the ventral paired tubes, and the lateral premaxillary process acting a lateral cover between the dorsal and ventral tubes. The skull of P. cyrtocristatus is still incompletely known, so more complete material will likely reveal new features that further differentiate this species and aid in determining the pace of ornamental crest evolution.