TAPHONOMY OF THE MOTHER'S DAY QUARRY, UPPER JURASSIC MORRISON FORMATION, SOUTH-CENTRAL MONTANA, USA

Abstract

The Mother's Day Quarry, in the lower part of the Upper Jurassic Morrison Formation of south-central Montana, contains a low-diversity assemblage dominated by immature diplodocoid sauropods. The sauropod bones lie within a ∼3-m-thick, massive, sandy siltstone containing scattered pebbles and carbonate nodules. Clay rip-up clasts indicate a high-energy depositional environment, and high-angle bone orientations are commensurate with a combination of the inferred high-energy burial event and postburial trampling of the assemblage. Since the Morrison Formation records deposition in an alluvial plain setting characterized by little topographic relief, heavy rainfall likely triggered the high-density flow event. Taphonomic analysis shows no indication that the Mother's Day assemblage was transported far by the entombing flow, though many elements may have been reworked. There is no statistically significant orientation of elongate elements and little apparent evidence of hydraulic sorting or transport damage. Remnants of soft tissue recovered from the quarry and the semiarticulated state of many of the bones demonstrate that the sauropod carcasses were buried a relatively short time after death. The cause of death of the sauropods is largely speculative; it is thought to have been related to drought based on the juvenile-dominated age profile of the assemblage and the inferred sequence of limb disarticulation. Comparison with other sauropod-dominated assemblages reveals that burial rates have little influence on the preservation of sauropod remains. Low-transport potential of sauropod bones implies that the sauropod component of dinosaurian assemblages should nearly always represent faunal elements derived from environments proximal to the site of deposition.