CATASTROPHIC MASS MORTALITY OF A HERD OF YOUNG DIPLODOCID SAUROPODS FROM THE MORRISON FORMATION OF MONTANA

The Mother’s Day Site, located in the Salt Wash member of the Upper Jurassic Morrison Formation in south-central Montana, preserves the remains of a herd of young diplodocid sauropods. The unit containing the sauropod assemblage is a muddy fine-grained sandstone, representing a levee or overbank deposit. All of the specimens recovered from the site belong to either juvenile or subadult individuals, as indicated by the small size of the elements and the unfused neurocentral sutures of the dorsal vertebrae. The only non-sauropod material recovered from the site consists of three isolated theropod teeth that are likely reworked and not directly associated with the sauropod elements. Bone modification features that would indicate some interval of exposure prior to burial (e.g. weathering cracks and trample marks) are not in evidence in the small sample of bones prepared thus far. This lack of bone modification, in conjunction with the monospecific nature of the site, suggests that the assemblage represents a single, catastrophic mass mortality.

Although miring has been previously proposed as the agent of mortality for the Mother’s Day assemblage, new evidence suggests that such scenario is unlikely. Long elements in the quarry are strongly oriented in an E/W direction, with a shallow dip to the west; this orientation and imbrication is likely the result of a current flowing to the east. In spite of the strong current orientation signal, elements commonly found articulated within the quarry, such as caudal vertebrae and pes units, show that decomposition of the carcasses was incomplete at the time of transport and burial. Additionally, the presence of preserved soft tissue in the form of skin impressions indicates relatively fast burial and shows that the carcasses were not remobilized after they were interred. The remnants of skin and other soft tissue would have increased the buoyancy of the bone to which they were attached, explaining the hydraulic inequivalence of the larger elements with the relatively fine-grained sediment of the bonebed unit.