Echinoid concentration beds: two examples from the stratigraphic spectrum

Echinoid concentrations provide insight into the role of shell composition and taphonomic resilience in the formation of shell bed types. The two groups of echinoids, regular and irregular, have very different responses to taphonomic processes and result in different types of fossil accumulations, on which relatively little previous research has been conducted. Such concentrations provide important information about the preservation of taphonomically fragile echinoid skeletons. To better understand the unique characteristics and depositional histories of echinoid beds, the stratigraphy, sedimentology, petrology and paleontology of two echinoid beds, a sand dollar bed from the Miocene Buttonbed Sandstone of California and a sea urchin spine bed from the Lower Triassic Virgin Limestone of Nevada, were studied. Both the Virgin Limestone echinoid bed and the Buttonbed coquina are nearly monospecific echinoid accumulations and therefore represent extreme examples of echinoid-rich deposits. Similarly, these two beds provide distinctly different examples of fossil echinoid beds because: (1) they are composed of different types of echinoids; (2) they were deposited in different marine environments; and (3) they represent very different stages in the evolutionary history of echinoids. The Buttonbed sand dollar bed, deposited in a shallow marine siliciclastic environment, was formed by both biogenic and sedimentologic processes. In contrast, the Virgin Limestone spine bed, deposited in a distal carbonate shelf setting, was subject to diagenetic as well as biogenic and sedimentologic processes during formation.