Decapoda (Crustacea) of the Coon Creek Formation (Maastrichtian) of Mississippi and Tennessee

Abstract

The Upper Cretaceous (Maastrichtian) Coon Creek Formation of Mississippi and Tennessee possesses a diverse and abundant assemblage of decapods including lobsters, ghost shrimp, and crabs. The formation lies in a temporally and paleogeographically significant location, situated between the Atlantic Coastal Plain and the Western Interior Seaway, shortly before the closing of the seaway and the K-Pg mass extinction. Coon Creek decapods have been little studied since the fauna was first described in the 1920s. A large collection of specimens, ranging in preservation from poor to excellent, has recently become available for study. Analysis of the elemental composition of the sediment and cuticle of six species of decapods from six families (Palinuridae, Nephropidae, Callianassidae, Dakoticancridae, Raninidae, and Retroplumidae) collected at the Blue Springs locality in Mississippi reveals phosphatic replacement of cuticle and trace amounts of iron and sulfur in the surrounding sediment. Concretions bearing decapods and a decapod burrow were observed in thin section and were mapped for elemental distribution using Energy-dispersive X-Ray spectroscopy and dot mapping indicating consistent elemental results with the surrounding sediment. The six species of decapods were analyzed using the same techniques and suggest diagentic affects in cuticlar structure and the presence of silica forming on the cuticle of a single specimen. Taphonomic data supports preservation ranging from well preserved phosphatization to secondary alteration to silica-rich exterior and weathering clay minerals. Because the silica is not replacing the phosphatized exocuticle and microscopic structure of cuticle in cross section is not preserved in the silica layer, the cause of this alteration is uncertain. The systematics and taxonomy of the decapod species from the Coon Creek Formation were reassessed. Sixteen species are identified, including two new species: Hoploparia tennesseensis, Hoploparia mcnairyensis, Linuparus keyesi sp. nov., Linuparus sp., Palaeopetrochirus enigmus, Seorsus wadei, Bournelyreidus ericksoni sp. nov., Cristipluma mississippiensis, Lithophylax flectus new combination, Hoploparia georgeana, Mesostylus mortoni, Tetracarcinus subquadratus, Avitelmessus grapsoideus, Cretacoranina testacea, Dakoticancer australis, Prehepatus harrisi, and ?Latheticocarcinus atlanticus, as well as two fragments belonging to the Majidae. This decapod fauna has species in common with correlative units of the Western Interior Seaway, the Gulf Coastal Plain, and the Atlantic Coastal Plain, supporting the hypothesis that the Mississippi Embayment is an ecotone for North American decapods.