NEW PARACRINOID (BLASTOZOAN ECHINODERM) FROM THE MCLISH FORMATION (DARRIWILIAN; LATE MIDDLE ORDOVICIAN), IN SOUTHERN OKLAHOMA

A new paracrinoid, a stemmed blastozoan echinoderm, was found by the junior author in the McLish Formation (late Middle Ordovician) on a ranch in the Arbuckle Mountains of southern Oklahoma. The single specimen is a broken half-theca with 31 plates preserving the oral surface, most of one side, and part of the lower theca. The theca is large, ovoid, and has large thin plates with slightly raised centers but no pore- or fold-like respiratory structures. The exposed uppermost side of the theca was extensively weathered, but because the thecal interior had been filled with calcite crystals during diagenesis, the plate sutures were still preserved on this eroded surface. The broken lowermost surface developed a thick layer of caliche during weathering, but this helped to hold most of the preserved theca together.

The oral surface has a central elliptical mouth with two strongly curved ambulacra extending in opposite directions from it (probably representing B-C and D-E), producing a compact S-shaped ambulacral system. The short clockwise ambulacra have 10-11 uniserial floor plates on the outside edge bearing facets for small erect appendages, probably uniserial brachioles, and a food groove extending from the facets to the mouth along the inner edge. A small hydropore appears to be right below the mouth in the CD interray, and a partial periproct opening is in the BC interray adjacent to the right ambulacrum. These summit features are similar to several Late Ordovician paracrinoid genera from the eastern USA and southern Canada, especially Canadocystis, but this older McLish specimen has a much larger, more elongate theca, larger but thinner plates, and the mouth is not offset to one side with the periproct opposite the stem facet, a feature present in Canadocystis and many other paracrinoids. Thus, this McLish specimen probably represents a new paracrinoid genus.