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| Parasaurolophus cyrtocristatus (Ostrom, 1961) in Gates, Evans & Sertich, 2021. DOI: 10.7717/peerj.10669 Illustration: Andrey Atuchin, Denver Museum of Nature & Science. |
Abstract
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.
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| Life reconstruction of the head of Parasaurolophus cyrtocristatus based on newly discovered remains. Illustration: Andrey Atuchin |
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| Figure 4: Skull of Parasaurolophus cyrtocristatus (DMNH EPV.132300). (A) Photograph of right lateral side; (B) illustration of right lateral side; (C) photograph of left lateral side; and (D) illustration of left lateral side. Abbreviations: Bso, Basioccipital; Bsp, Basisphenoid; Exo, Exoccipital; F, Frontal; La, Lacrimal; Lsp, Laterosphenoid; Na, Nasal; Osp, Orbitosphenoid; Pa, Parietal; Pmd, premaxilla dorsal process; Pml, premaxilla lateral process; Po, Postorbital; Pr, Prootic; Prf, Prefrontal; Ps, Presphenoid; Sq, Squamosal. |
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| Life reconstruction of Parasaurolophus group being confronted by a tyrannosaurid in the subtropical forests of New Mexico 75 million years ago. Illustration: Andrey Atuchin, Denver Museum of Nature & Science. |
Systematic paleontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1888
Ornithopoda Marsh, 1881
Iguanodontia Baur, 1891 sensu Sereno, 1986
Hadrosauridae Cope, 1870 sensu Prieto-Márquez, 2010
Lambeosaurinae Parks, 1923
Parasaurolophini Brett-Surman, 1989
Parasaurolophus Parks, 1922
Parasaurolophus cyrtocristatus (Ostrom, 1961)
Holotype: FMNH P-27393: Partial skull composed of nearly complete right premaxilla, posterior portion of left premaxilla, mostly complete right nasal, skull roof, partial braincase, and mostly complete postcranial skeleton.
Terry A. Gates, David C. Evans and Joseph J.W. Sertich. 2021. Description and Rediagnosis of the Crested Hadrosaurid (Ornithopoda) Dinosaur Parasaurolophus cyrtocristatus on the Basis of New Cranial Remains. PeerJ. 9:e10669. DOI: 10.7717/peerj.10669
