Spiraclavus nacoensis, a new species of clavate agelacrinitid edrioasteroid from central Arizona

Twenty-four fragmentary specimens and numerous disarticulated plates of a new clavate agelacrinitid edrioasteroid were collected from the Middle Pennsylvanian Naco Formation in central Arizona. Spiraclavus nacoensis n. gen. and sp. is similar to Lepidodiscus laudoni (Bassler, 1936) and Discocystis kaskaskiensis (Hall, 1858), but differs in the direction of ambulacral curvature and in the number and arrangement of ambulacral cover plates. Numerous well-preserved, disarticulated plates allow a full description of plate morphologies from various parts of the theca. The entire oral surface is rigidly constructed of tessellate plates. Interlocking serrate sutures between successive floor plates add strength. Plate sutures in the flexible and telescoping portions of the theca (recumbent zone and pedunculate zone, respectively) are beveled, allowing considerable imbrication and potential for thecal extension. The associated fauna indicates that the unit containing the edrioasteroids was deposited on an offshore, subtidal, marine carbonate shelf with occasional siliciclastic influxes. The specimens of Spiraclavus nacoensis were buried by siliciclastics, probably during storm events.