A New Crested Brachylophosaurin (Dinosauria: Hadrosauridae) from the Upper Cretaceous Menefee Formation of New Mexico and Judith River Formation of Montana
MCDONALD, Andrew T; Western Science Center, Hemet, CA, USA. WOLFE, Douglas G; Zuni Dinosaur Institute for Geosciences, Springerville, AZ, USA. FREEDMAN FOWLER, Elizabeth A; Dickinson State University, Dickinson, ND, USA.
Abstract
The lower – middle Campanian Menefee Formation forms extensive badlands in the San Juan Basin of New Mexico, but apart from the partial skeleton of a centrosaurine reported in 1997, the unit’s dinosaurs are fragmentary. New discoveries by an ongoing project conducted by Western Science Center (WSC) and Zuni Dinosaur Institute for Geosciences, with volunteers from Southwest Paleontological Society, are fleshing out the dinosaur assemblage, including the new nodosaurid Invictarx and tyrannosaurid Dynamoterror described in 2018.
WSC 10058, the partial skull and right forelimb of a large hadrosaurid, was collected in the upper Allison Member on BLM land in 2018. Based upon quadrate height, the skull is 98% the size of Museum of the Rockies (MOR) 794, an adult-sized Brachylophosaurus skull. Phylogenetic analysis places WSC 10058 in Brachylophosaurini, forming a trichotomy with Probrachylophosaurus and Brachylophosaurus, with Maiasaura and Acristavus as successive outgroups.
WSC 10058 shares with P. bergei and B. canadensis a corrugated nasofrontal suture that extends caudally onto the dorsal surface of the frontals. The nasofrontal suture of WSC 10058 extends farther caudally than in the ontogenetically mature holotype of P. bergei, but not as far as in adult-sized B. canadensis, in which the suture covers the entire dorsal surface of the frontals. The suture of WSC 10058 is also distinctive in forming an elevated concave ramp along the midline and ending caudally in two parasagittal bumps.
MOR 6636, an isolated brachylophosaurin right nasal from the lower Judith River Formation (= lower Oldman Formation, 24 m above the Marker A coal of the Taber Coal Zone) in Kennedy Coulee, Montana, mirrors the nasofrontal suture of WSC 10058, with a convex ventral surface ending in a deep parasagittal pit. MOR 6636 was 3D-printed at WSC and fits closely onto the frontals of WSC 10058, suggesting the two specimens might pertain to the same taxon. MOR 6636 was collected at least 6 m higher than the holotype of P. bergei and lower than specimens of B. canadensis. Its nasal crest is longer than the short wedge-shaped crest of P. bergei, but shorter than the elongated paddle-shaped crest of B. canadensis.
If WSC 10058 and MOR 6636 represent the same new genus and species of brachylophosaurin, morphologically and stratigraphically intermediate between P. bergei and B. canadensis, then it might help constrain the age of the upper Allison Member to between 79.2 Ma (based on P. bergei and MOR 6636) and 78.5 Ma (based on Baculites perplexus in the Cliff House Sandstone, overlying the Menefee).