SUBSTRATE-CONTROLLED VARIABILITY WITHIN ATTACHMENT STRUCTURES OF CARYOCRINITES (ECHINODERMATA: RHOMBIFERA) FROM THE MIDDLE SILURIAN OF SOUTHEASTERN INDIANA

The hemicosmitid rhombiferan Caryocrinites is a common and conspicuous faunal element in a variety of Silurian marine environments. Where preserved, the dendritic radicular attachment structures of Caryocrinites can reliably be identified by the presence of a distinctive trilobate lumen and solid pseudocirrate radicles. A hardground surface within the Wenlock-age (Sheinwoodian) Massie Formation, exposed at the New Point Stone quarry near Napoleon, southeastern Indiana, is encrusted by a diverse assemblage of crinoid and blastozoan attachment structures, including structures that can be confidently attributed to Caryocrinites. This hardground is irregular, with slightly elevated, well-sorted, well-winnowed crests and more poorly sorted, coarser troughs. The surface is also host to fistuliporoid bryozoan-dominated microbioherms. The morphology of Caryocrinites attachments structures reflects local substrate conditions on this microbiohermal hardground: holdfasts on hardground crests are simple, approaching conical, and often cemented to other macrofossils, commonly diploporite thecal attachments; holdfasts on hardground troughs are more “typical,” comprising laterally branching, but thin, dendritic radix structures; holdfasts on microbioherms are extremely thickened by secondary stereom secretion—this undifferentiated stereom envelops the initial attachment site and proximal radicles. This segregation of attachment structure morphology is related to substrate properties. The stable and winnowed hardground crests require few radicles for stabilization, in contrast to the shifting, unstable bioclastic rubble of hardground troughs, which require greater surface area (i.e., lateral branching). The extreme thickening of structures on microbioherms is more enigmatic. Secretion of secondary stereom to prevent interaction with anoxic mud is unlikely given the diverse benthic fauna, but the swelling may be a response to interactions with microbes or bryozoans on the microbioherms. An interesting alternative involves purposeful growth of secondary stereom in order to prevent dislodgement from advantageous positions atop elevated microbioherms.