EXPLORING ANKYLOSAUR SOCIAL BEHAVIOR THROUGH GASTONIA ASSEMBLAGES IN UTAH'S CEDAR MOUNTAIN FORMATION

Herd behavior is a survival strategy observed in many modern animals, providing protection from predators, increased foraging efficiency, and social learning. Species like elephants, buffalo, and deer form mixed- or gender-segregated herds, with bachelor herds emerging when males separate from natal groups. These modern examples offer analogs for investigating the social behavior of extinct animals, such as ankylosaurs. Several dinosaur families, including sauropods, ceratopsians, and hadrosaurs show evidence of herd behavior through mass death assemblages, nesting sites, and trackways.

Mass death assemblages (MDAs) are groups of individuals of the same species found together due to catastrophic events resulting in the death of large numbers of individuals. Ankylosaurs are traditionally considered solitary because they are armored and typically occur as isolated specimens. However, MDAs are known for Pinacosaurus (Gobi Desert), Gastonia (Utah), and a mixed assemblage of Struthiosaurus and Hungarosaurus (Hungary), challenge this view. Pinacosaurus MDAs, composed exclusively of juveniles, may suggest bachelor herds as an anti-predator strategy, while the histology of Struthiosaurus and Hungarosaurus indicates subadults to adults in a mixed herd.

A newly discovered Gastonia MDA near Moab, Utah, in the Yellow Cat Member of the Cedar Mountain Formation (Barremian, Early Cretaceous), supports prior evidence of Gastonia MDAs from Dalton Well, Lorrie’s, and Yellow Cat quarries. This site contains a minimum of six Gastonia individuals alongside Moabosaurus (MNI = 1), a large allosauroid (MNI = 1), and Utahraptor teeth.

Histological analysis will be conducted to assess the ontogenetic stages of the Gastonia specimens, addressing limitations in age estimation based on size classes. Additionally, MDAs pose interpretive challenges, as they may result from environmental stress (e.g., drought) or post-mortem transport rather than social grouping in life. By integrating taphonomic, histological, and comparative analyses—including modern herd analogs—this research aims to refine our understanding of ankylosaur social behavior and contribute to broader discussions of dinosaur herd dynamics.