DESCRIPTION OF A LOWER MORRISON FORMATION DINOSAUR QUARRY FROM SOUTH-WESTERN MONTANA

While receiving less scientific attention than the more popularized formations within the state, the Morrison Formation (latest Oxfordian - early Tithonian) of Montana represents an overlooked Mesozoic ecosystem. These Montanan localities largely appear to be equivalent to the upper portion of the Salt Wash Member (Kimmerdgian), and the vertebrate fauna resemble typical Morrison assemblages - at least three sauropod species, one Stegosauria, and one allosauroid theropod. However one unusual feature in marked contrast to typical Morrison localities is the sheer abundance of immature sauropod material. One site from south central Montana represents a bone bed of over fifteen individual diplodocids (maximum femur length 120 cm). Another site, designated the O’Haire quarries, represents five immature sauropods belonging to the genera Diplodocus and Apatosaurus (the majority under two-thirds adult body length), and one individual of the theropod Allosaurus. The O’Haire locality exhibits a normal graded sequence containing alternating well cemented mudstone and fine grained, well cemented sandstone facies approximately 1-2 meters in thickness. The presence of mud rip-up clasts, cross-bedding, and other sedimentary structures within these facies are interpreted as representing a migrating point bar within a north flowing meandering fluvial system. Skeletal remains near the sediment interfaces exhibit abrasion and rounded edges (consistent with Fiorillo’s abrasion Stage 1), along with sub-horizontal orientation which are all taphonomically consistent with channel-lag deposits. This interpreted depositional history indicates that unlike more typical or famed Morrison localities, such as Dinosaur National Monument or Cleveland-Lloyd Dinosaur Quarry, the O’Haire quarries do not appear to represent mass catastrophic death assemblages; and are therefore representative of a more natural environmental depiction.