Unionids are a group of freshwater mussels whose fossils are the most common animal represented at the Carnegie Quarry. Although the bones of dinosaurs in the Carnegie Quarry are found in the bottom of an ancient river channel, they do not tell scientists much about the ecology of this Late Jurassic river.
Modern unionids inhabit river ecosystems. They have a unique method of dispersal, which relies on their larvae being parasitic to fish. The larvae are ingested or attach onto the gills of fish, which travel upstream, and detach or are excreted, and then bury into the substrate.
Unionids have a musculature that connects the two shells which contracts to close the shell, and after death detracts leaving the shell open. Thus, in the fossil record, Unionid shells are rarely found attached together.
In modern ecosystems, unionid death corresponds to several environmental conditions. The mass mortality of these mussels can be caused by low water flow, higher water temperature, low dissolved oxygen, higher concentrations of pollutants in the water, and numerous other factors any of which may have led to Unionids in the fossilized Morrison ecosystem (Carpenter, 2013).
Their intermittent dispersal through the sedimentary layers of the quarry supports the idea that the Morrison environment experienced repeated climatic shifts, such as a repeating switch from wet to arid conditions. The unionids record support the hypothesis that the Morrison fossil assemblage observed at the Carnegie Quarry was caused by drought. The animals would desiccate in the arid environment before their remains would be transported, scattered, and buried on the bottom of the river.
Specimens of Unionids from the Carnegie Quarry:
Carpenter, K. 2013. History, sedimentology, and taphonomy of the Carnegie Quarry,Dinosaur National Monument, Utah. Annals of Carnegie Museum, 81(3): pp. 153-232. 2013.
Information to be added as it becomes available