Taphonomy of the Sun River Bonebed, Late Cretaceous (Campanian) Two Medicine Formation of Montana
Scherzer, Benjamin Andrew.
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In the summer of 1998, a bonebed of juvenile dinosaur material was found in beds of the Two Medicine Formation along the Sun River in Teton County, Montana. Initial inspection of the "Sun River Bonebed" indicated unique dominance by juvenile remains and unusually high concentration of fossil material. Modern exposure of both the bonebed and surrounding strata allowed for a detailed taphonomic study of the assemblage. In the summer of 2004, the bonebed was excavated in a fashion that recorded depthwise taphonomic data of fossil material and surrounding sediment, and allowed for three-dimensional reconstruction of the bonebed. The lithology of the surrounding beds was documented for 120 m immediately below and 40 m immediately above the bonebed to interpret the overall depositional environment of the area. Lastly, the paleobiologic and sedimentologic interpretations made for the bonebed were compared to bonebeds of similar fauna in the Two Medicine Formation. Taphonomic attributes of the fossil material indicate a mass death assemblage of late juvenile lambeosaurines subject to post-mortem bioturbation and possible fluvial transport. A number of elements from the assemblage exhibit a rare form of taphonomic modification known as "wet rot," currently documented in only one other dinosaur bonebed. Sedimentologic and additional taphonomic data in the assemblage indicate entrainment of the vertebrate material in a cohesive debris flow and ultimate deposition in the respective flow deposit. The sedimentology of the surrounding beds indicates ultimate deposition in an ephemeral fluvial environment. The restricted age class representation of the assemblage lends credence to existing paleobiologic interpretations of hadrosaurids in the Late Cretaceous of Montana, and the Sun River Bonebed is significant in its exhibition of "wet rot" modification and in being one of a restricted number of documented debris-flow hosted vertebrate bonebeds.