Procambarus primaevus Green River Fossil Crayfish

Name: Order: Decapoda; Family: Cambaridae; Procambarus primaevus

Age: Eocene

Size: mm (25.4mm=1 inch): 60 mm in length (tip of claws to tip of telson); on a 75 mm by 100 mm matrix

Location: Split Fish Layer Green River Formation, Fossil Lake, Kemmerer, Wyoming

Procambarus primaevus Green River Fossil CrayfishDescription: This 50 million year old, Eocene-Era fossil crayfish comes from one of the world's famous Laggerstatten, the Green River Formation in Wyoming. A small portion of the fossils from Green River exhibits such fine preservation. Most fossils offered are various fish, but other fossils are also found.

This crayfish has wonderful details present, and represents the finest degree of preservation. Notice that ebven the annulation of the antennae is evident. Crayfish have a spotty fossil record beyond 30 million years, but Procambarus from the Green River Formation and Palaeocambrus from the Yixian Formation of China are exceptions.

About the Green River Formation: Class Actinopterygii, the ray-finned bony fishes, comprise almost half of all known species of vertebrates, some 20,000 extant species. There are numerous locations worldwide that are noted for wondrous preservation of bony fishes, and the Green River formation that covers some 25,000 square miles of SW Wyoming, west Colorado and east Utah is one of the premier examples. The formation is one of the largest lacustrine sedimentary accumulations in the world, and spans the period from 40 to 50 million years ago during the Eocene Epoch.

During the Eocene, based on the fossil record, the region was sub-tropical to temperate. Some 60 vertebrate taxa have been described from the formation, including crocodiles, boa constrictors, and birds, as well as abundant invertebrates and plants. The unusually excellent preservation of the Green River fossils is usually attributed to a combination of two factors: 1) a cold period during the Eocene that would have caused dead fish to sink faster due to a less inflated swim bladder; and 2) the great depth of the lakes and the consequent anoxic conditions that would have often prevented scavengers from disturbing the carcasses.

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