Ampheristocystis , a Silurian (Lower Wenlock) eocrinoid (Echinodermata, Blastozoa) from Indiana

The enigmatic blastozoan echinoderm Ampheristocystis concentricus , based on four specimens from the upper part of the Osgood Member, Salamonie Dolomite (Upper? Llandovery–Lower Wenlock), southeastern Indiana, is most likely an eocrinoid. Supporting this assignment are the five basals(?), five symmetrical appendage facets (possibly bearing biserial brachiolar trunks) and details of the thecal plating (moderate number of semiregularly arranged thecal plates). Some features (poorly known subvective appendages and poreless thecal plates) could indicate paracrinoid affinities, and suggest that the classes Eocrinoidea and Paracrinoidea may be more closely related than indicated by current taxonomy. Whatever the class disposition, the genus is the youngest representative yet known. Placement in the Blastozoa, based on holoperipheral plate growth, morphology of the brachiole‐like appendages and peristome, among other features, is unequivocal. The sharply discriminated oral and thecal plating, and protuberant anal opening, are unusual features for either paracrinoids or eocrinoids. Separate familial status is advocated for Ampheristocystis , but the family is not assigned to an order. The oral surface of the genus, consisting of regular orals and suborals, and fixed ambulacrals, presents an arrangement unique among eocrinoids, but analogous to the tegmen of camerate crinoids and of the rhombiferan family Caryocrinitidae. Development of a regular tegmen, fairly regular thecal plating and elongate fusiform shape indicate Ampheristocystis is an evolutionarily convergent, aberrant blastozoan, perhaps competitive with the camerates that generally dominated Silurian echinoderm faunas, and with the contemporary, equally successful blastozoan Caryocrinites . Ampheristocystis is not closely associated with hard surfaces or debris piles, as are many Osgood echinoderms, but preferred soft silt or mud bottoms. Associated echinoderms include several diploporites, coronoids, a few large camerates (e.g. Eucalyptocrinites ) and Caryocrinites . Two specimens have evidently decollated the stem; one retains a sizeable, crinoid‐like stem. Hence, it is presently unclear whether the life habits of this genus were more like that of the common Osgood diploporites or stemmed contemporaries such as Caryocrinites and Eucalyptocrinites . Alternatively, it could perhaps exploit both habits equally well. Copyright © 2005 John Wiley & Sons, Ltd.