Systematics, functional morphology, and life modes of Late Ordovician edrioasteroids, Orchard Creek Shale, southern Illinois

Abstract

Five genera and species of edrioasteroids co-occur in the Orchard Creek Shale (Late Ordovician, Southern Illinois) including: Savagella illinoisensis (Miller and Gurley, 1895), Hystrichopsydrax sandersi n. gen. and sp., Euhydrodiskos diktyotos n. gen. and sp., ?Isorophus sp., and ?Pyrgocystis sp. The first three listed taxa are endemic to the Orchard Creek. Savagella and Hystrichopsydrax are unusual Agelacrinitidae Chapman, 1860; Euhydrodiskos is classified within the Lebetodiscidae Bell, 1976. Skeletal features of these three taxa imply unique functional and life mode adaptations in three areas: attachment, protection, and respiration. Functional and life mode inferences are supported by paleoecologic data. Savagella and Hystrichopsydrax are unique among post-Cambrian edrioasteroids in possessing a thickened proximal peripheral rim circlet having vertical mutual articulations resulting in a rigid peripheral thecal framework. This allowed exploitation of perishable, pliant, relatively flat surfaces but presumably inhibited attachment to rigid, changing, curved surfaces such as were utilized by most post-Cambrian edrioasteroids. Hystrichopsydrax was protected by thecal plates bearing large spinose processes. Savagella and Euhydrodiskos each had camouflage protection. Euhydrodiskos has an unusually large hydropore, perhaps implying enhanced respiratory capability compared with other lebetodiscids. Orchard Creek edrioasteroids occur in a thin silty shale to limestone transitional shallow marginal marine sequence and are accompanied by a sparse but diverse invertebrate macrofauna.