Saturday, April 3, 2021

[Paleontology • 2021] Ornatops incantatus • A New Brachylophosaurin (Dinosauria: Hadrosauridae) from the Upper Cretaceous Menefee Formation of New Mexico

Ornatops incantatus 
McDonald​, Wolfe, Freedman Fowler & Gates, 2021 
illustration: Brian Engh 

Brachylophosaurini is a clade of hadrosaurid dinosaurs from the Campanian of western North America. Although well-known from northern localities in Montana and Alberta, including abundant material of Brachylophosaurus canadensis and Maiasaura peeblesorum and the holotypes of Acristavus gagslarsoni and Probrachylophosaurus bergei, material from southern localities in Utah and Colorado is restricted to a partial skull referred to A. gagslarsoni and several indeterminate specimens. Here we describe Ornatops incantatus gen. et sp. nov., a new brachylophosaurin known from a partial skeleton from the Allison Member of the Menefee Formation in New Mexico. Ornatops is the first brachylophosaurin reported from New Mexico and the southernmost occurrence of the clade. Ornatops shares with Probrachylophosaurus and Brachylophosaurus a caudally expanded nasofrontal suture on the frontals, but also exhibits an autapomorphic nasofrontal suture morphology, with a horizontal rostral region and elevated caudal region with two prominent parasagittal bumps, which is different from other brachylophosaurin specimens, including juvenile and adult Brachylophosaurus. A phylogenetic analysis places Ornatops in a trichotomy with Probrachylophosaurus and Brachylophosaurus, with Maiasaura and Acristavus as successive outgroups.

Figure 7: Braincase of WSC 10058, holotype of Ornatops incantatus.
Braincase and digital 3D model in dorsal (A and B), ventral (C and D), and caudal (E and F) views.
Abbreviations: abs, alar process of the basisphenoid; bbpp, base of basipterygoid process; fm, foramen magnum; lcmp, partial caudomedial process of left squamosal; mls, midline suture between caudomedial processes of the squamosals; poc, paroccipital process. Scale bar equals 5 cm.

Figure 12: Reconstruction of the caudal region of the skull of WSC 10058, holotype of Ornatops incantatus.
 Orthographic views of a digital 3D model in (A) dorsal and (B) right lateral views. The right half of the braincase, right postorbital, right squamosal, and left quadrate have been mirrored to create this reconstruction. Scale bar equals 5 cm.

Systematic paleontology
Dinosauria Owen, 1842, sensu Baron, Norman & Barrett, 2017
Ornithischia Seeley, 1888, sensu Sereno, McAllister & Brusatte, 2005

Ornithopoda Marsh, 1881, sensu Butler, Upchurch & Norman, 2008
Iguanodontia Baur, 1891,  sensu Sereno, McAllister & Brusatte, 2005

Hadrosauridae Cope, 1869, sensu Prieto-Márquez, 2010a
Saurolophinae Brown, 1914, sensu Prieto-Márquez, 2010a
Brachylophosaurini Gates et al., 2011, sensu Freedman Fowler & Horner, 2015

Ornatops incantatus gen. et sp. nov.

Holotype: WSC 10058, associated skeleton including the partial right premaxilla, right postorbital, right squamosal, both quadrates, nearly complete skull roof and braincase, two partial dorsal vertebrae, a dorsal rib, ossified tendons, the right scapula, proximal end of the right humerus, right ulna lacking the proximal end, right radius lacking the proximal end, right metacarpals II and III, and incomplete pubis and ischium. While this article focuses on the cranial bones, the rest of the specimen will be completely described in a forthcoming publication (A.T. McDonald et al., 2021, in preparation).

Etymology: Ornatops is derived from the Latin word ornatus (ornate) and the Greek ops (face), in reference to the elaborate nasofrontal suture. The species name, incantatus, is a Latin word meaning “enchanted”, referring to the State of New Mexico, where the holotype was collected and which carries the motto “Land of Enchantment”. The binomen can be translated as “enchanted ornate face”.

Locality: WSC 10058 was collected in San Juan County, New Mexico, on land administered by the United States Bureau of Land Management (BLM). Precise locality data are on file at WSC and the BLM.

Horizon: WSC 10058 was collected at a single locality in the Juans Lake Beds (Miller, Carey & Thompson-Rizer, 1991), upper part of the Allison Member, Menefee Formation, approximately 120 meters below the overlying Cliff House Sandstone; middle Campanian, Upper Cretaceous, older than ~78.5 Ma based upon the occurrence of the ammonite index fossil Baculites perplexus in the overlying Cliff House Sandstone (Siemers & King, 1974; Molenaar et al., 2002; Lucas et al., 2005).

Specific diagnosis (as for genus by monotypy): brachylophosaurin distinguished by a single autapomorphy: nasofrontal suture on dorsal surface of frontals is horizontal rostrally and elevated caudally, ending in a pair of parasagittal bumps adjacent to the cranial midline. Furthermore, the nasofrontal suture extends caudally onto the dorsal surface of the frontals farther than in adult Probrachylophosaurus bergei, but not as far as in adult Brachylophosaurus canadensis.

Ornatops incantatus is a new genus and species of brachylophosaurin hadrosaurid known from the holotype partial skeleton from the Allison Member of the Menefee Formation in New Mexico. The morphology of the caudally expanded nasofrontal suture in O. incantatus indicates that it is closely related to and possibly intermediate between Probrachylophosaurus bergei and Brachylophosaurus canadensis, although additional material is necessary to fully explore the evolution of brachylophosaurins in southern Laramidia. The expanded nasofrontal suture of O. incantatus indicates the presence of a solid nasal crest, similar to P. bergei and B. canadensis, although the shape of the crest is unknown. O. incantatus is the first crested brachylophosaurin found in southern Laramidia, illustrating the potential of the Menefee Formation to inform future studies on dinosaur evolution in North America during the Campanian.

Andrew T. McDonald​, Douglas G. Wolfe, Elizabeth A. Freedman Fowler and Terry A. Gates. 2021. A new brachylophosaurin (Dinosauria: Hadrosauridae) from the Upper Cretaceous Menefee Formation of New Mexico. PeerJ. 9:e11084. DOI: 10.7717/peerj.11084