19-6 Early evidence of Brachylophosaurini (Hadrosauridae: Saurolophinae) from the lower Wahweap Formation of Grand Staircase-Escalante National Monument, southern Utah, USA

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia

Presenting Author:

Bryce Cassiano

Authors:

Cassiano, Bryce¹, Sertich, Joseph², Watkins, Madison³, Toth, Natalie⁴, Bastien, Salvador⁵, Titus, Alan⁶

(1) Department of Earth Sciences, Denver Museum of Nature & Science, Denver, CO, USA; The Montana Dinosaur Center, Bynum, MT, USA, (2) Department of Geosciences, Colorado State University, Fort Collins, CO, USA; Bureau of Land Management, Kanab, UT, USA, (3) College of Geosciences, Southern Utah University, Cedar City, UT, USA, (4) Department of Earth Sciences, Denver Museum of Nature & Science, Denver, CO, USA, (5) Department of Earth Sciences, Denver Museum of Nature & Science, Denver, CO, USA, (6) Bureau of Land Management, Kanab, UT, USA,

Abstract:

Despite the high levels of diversity and abundance of fossilized material found of hadrosaurids from the middle Campanian to the end of the Maastrichtian, little is known about the earliest hadrosaurids (Late Santonian-Early Campanian). Laramidian Formations that have produced hadrosaurid fossils from that time are few in number, with notable examples including the Aguja Formation, Mancos Shale, Menefee Formation, and Wahweap Formation. Recently, new models were used to redate the Wahweap Formation, resulting in its most basal section, the Last Chance Creek Member, being shown to be near the Santonian-Campanian boundary (between 83.64 and 81.54 Ma). This suggests that it is contemporaneous with the lower Aguja Formation and indicates that any hadrosaurs found here will be amongst the oldest in the region, giving greater insight into the early evolution and distribution of hadrosaurs.

Within the last six years four partial hadrosaur skeletons have been collected from this member. Three of these preserve a mixture of appendicular and vertebral elements, while one preserves cranial and vertebral elements. These possess characteristics that have affinity with Brachylophosaurini. The most notable postcranial element in this regard would be the pubes, which are preserved in two of the specimens. Their prepubic blades possess the characteristic shape found in Brachylophosaurini: ventrally deflected, wider than tall, rounded margins, and approximately the same length as the proximal constriction. The cranial specimen is of particular interest, as its maxilla shares characteristics with both Probrachylophosaurus and basal hadrosaurs such as Aquilarhinus. Notably, the state exhibited by its jugal process very closely resembles the latter, while many other characteristics including the slope of the ectopterygoid shelf, are more reminiscent of brachylophosaurins. Additionally, unlike brachylophosaurins, kritosaurins, and most basal hadrosaurids, this specimen only has one functional tooth per alveolus and a discontinuous ectopterygoid ridge with a shallow canal. Preliminary phylogenetic analyses recover the cranial remains as either being close to Probrachylophosaurus or Aquilarhinus. Postcranial specimens are unable to be distinguished from more basal forms unless they are combined into an OTU, in which case it is recovered within Brachylophosaurini. Additionally, if all four specimens are combined, the resulting taxon is recovered as a basal member of Brachylophosaurini. This placement within the tribe is significant, as it would extend their temporal range by several million years.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-14696

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