Cretaceous Decapod Lagerstatten of the Western Interior Seaway

Abstract

Decapod lagerstätten of the Western Interior Seaway preserve ~10 repeated assemblages of decapods superimposed on molluscan assemblages. Decapods are preserved as hundreds or thousands of threedimensional, phosphatic concretions that are scattered through several meters of shale over areas up to 1500 km2. Decapod assemblages are interpreted as preserved community fractions, providing “faunal snapshots” during the Cretaceous. These “snapshots” can be dated by biostratigraphic zonation or by absolute dating of the bentonite beds; then assembled in sequence to provide powerful evidence of significant changes in the composition of the decapod faunas, evolution of individual taxa, and changing food webs. Thirty-eight years of collecting produced 77 collections of decapod Rocky Mountain Section – AAPG: 2019 Annual Meeting 46 crustaceans of ~22,532 specimens; forming “The Bishop Decapod Collection.” These collections are intimately tied into the biostratigraphic ammonoid zonation of W.A. Cobban and his colleagues at the U.S. Geological Survey and Museums around the World, giving them a utility unknown before in decapod evolution. One fauna,“The Baresch Collection,” a small collection of ~ 23 concretions from the Exiteloceras jenneyi Zone, contains the earliest known Dakoticancer overanus in the WIS; representing an evolutionary event, the invasion of the northern Western Interior Seaway, by Dakoticancer, which subsequently became the dominant crustacean in seven subsequent faunas. This relationship, was predicted in an earlier publication; “It is suggested that sulphur and phosphate mediating bacteria similar to Thiomargarita, if not ancestral to it, were intimately involved in generating the phosphate necessary to form the concretions preserving the phosphatic Dakoticancer lagerstätten of South Dakota.” (Bishop, 2007:16). W. A. Cobban assisted in helping locate The Baresch Locality on a sketch map showing the outcrop of the E. jenneyi zone where it crossed Old Highway 85. Delores Baresch subsequently accompanied GAB to the locality, validating its location and documenting it as the source of her collection; placed in the SDSMT Museum of Geology. Several additional years of collecting were added to her small collection and the combined collection documents the invasion of the Western Interior Basin by Dakoticancer overanus Rathbun, 1917 at 74 mybp.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019