Author(s): Madison Watkins
Mentor(s): Alan Titus, Grant Shimer, Arianna Harrington
Institution SUU
The early evolution of Hadrosauridae remains obscure, with identifiable materials currently restricted to the Milk River (Alberta), Menefee (New Mexico), Mooreville Chalk (Alabama), Aguja (Texas), and Wahweap (Utah) formations. Overall, the Wahweap Formation (Fm) has arguably yielded the largest volume of material, but diagnostic cranial material is rare. Redating of sediments were previously used to create a Bayesian model that projects the base of the Wahweap Fm at 82.17 Ma (uncertainty of +1.47/-0.63 Ma), placing these among the oldest North American hadrosaurids. To seek insight into hadrosaurid origins, we analyzed the stratigraphy, facies, taphonomy, and morphological diversity of eight hadrosauroid localities from the Last Chance Creek Member and two localities from the Reynolds Point Member, with all localities restricted to the lower 50 meters of the formation. Most specimens share morphologic features with Brachylophosaurini, though also show characteristics of other early hadrosauroids. Six localities were mudstone-hosted, two were in a sandy mudstone, and two were in a channel sandstone, likely reworked from floodplain deposits, possibly indicating that lower Wahweap hadrosauroids preferred quiet water settings. There is scant evidence of pedogenic overprint at the localities, an atypical condition for most Wahweap Formation mudstones. Individual bones show very little wear or evidence of scavenging, but the degree of disarticulation, sorting, and current orientation at six localities indicates burial was not immediate postmortem. The vast majority of bones are WS 1 and WS 2 on the Behrensmeyer Scale. Seven localities contain elements from Voorhies groups I and II. Three localities contain material from all three Voorhies groups, although one locality only has minimal group II and III elements, suggesting most localities have been influenced by low to medium velocity currents. Bone orientations confirm current influence, with four localities showing moderate current alignment, two localities showing a strong current alignment, and three localities that were completely random with no current signature. The remaining locality was an articulated partial skeleton from which we could derive no current data. Our taxonomic identifications and sedimentary interpretations suggest that the early Campanian radiation of hadrosaurids in southern Laramidia consisted of members of both Brachylophosaurini and Kritosaurini that lived in or around persistent bodies of water large enough for moderate velocity currents.