WERE EARLY JURASSIC DINOSAURS GREGARIOUS? NEW EVIDENCE FROM DINOSAUR FOOTPRINT RESERVATION IN HOLYOKE, MASSACHUSETTS

Dinosaur Footprint Reservation is located in the Lower Portland Formation (ca. 201 MA) of the Hartford Basin, and its constituent rocks were deposited in shallow water on a lake margin. The locality is well known for multiple, parallel trackways of large theropod dinosaurs. These trackways have been interpreted as evidence for gregarious behavior in these carnivores (Hitchcock 1848; Ostrom 1972), but previous studies were based only on a subset of the dinosaur tracks present on a single bed (the “Ostrom Bed”). Specifically, previous work neglected small- to medium- sized tracks, which are more difficult to identify on the heavily weathered surface. We restudied this classic locality and located nearly 1000 tracks, including those of small- and medium-sized theropods (Grallator isp. and Anchisauripus isp.), large theropods (Eubrontes isp.), ornithischians (Anomoepus isp.), and a single crocodilian (Batrachopus isp.). The position, length, and orientation of the tracks were recorded with a total station and/or Brunton Compass. Additionally, orientations of ripple marks were recorded as indicators of paleoshoreline orientation. Although we focused on the Ostrom Bed, dinosaur tracks were also sampled from four other beds located stratigraphically upsection. Preliminary analysis of data from the Ostrom Bed shows a bimodal distribution for the orientation of all dinosaur tracks. However, trackways of large theropods trend roughly parallel to ripple marks, whereas trackways of small theropods and ornithischians trend approximately normal to the ripples. Thus, large theropods were walking approximately parallel to the shoreline whereas smaller dinosaurs were walking toward or away from the lake. Although these initial results do not falsify the hypothesis that the animals were gregarious, they suggest that a geographical barrier (i.e., the paleoshoreline) probably controlled the animals’ directions of travel. Animal size was also a primary control on behavior, with large and small dinosaurs utilizing this habitat in different ways.