General discocystinid morphology

Discocystinid are a group of highly derived edrioasteroids known primarily from the Mississippian and Pennsylvanian of North America. Their anatomy is fairly well understood and their taxonomy has recently been thoroughly revised (Sumrall, 1992, 1996; Sumrall and Bowsher, 1996). The basic discocystinid theca (or body wall) is between 20 and 90 mm in diameter and is clavate (mushroom-shaped). The oral surface is circular in outline with a mosaic of adjacent, polygonal plates in the interambulacral areas that are divided by five narrow ambulacra radiating from the peristome (or mouth). The ambulacra typically extend to the edge of the oral surface and follow it around to a blunt termination. Near the peristome on the anal side of the oral surface is the hydro-gonopore structure formed from a series of anterior, posterior and left plates (Sumrall, 1996). In the center of the interambulacrum on the anal side is an advanced valvular anal structure, or anal pyramid, formed from an inner and outer set of lath-shaped plates.

The aboral surface of the theca is divided into three zones. The zone nearest the edge, the recumbent zone, is formed from a series of somewhat irregularly arranged plates. These plates generally decrease in size proximally and are generally slightly imbricate. In Giganticlavus, however, the recumbent zone is much wider than that of other genera and is composed of thicker adjacent plates. The next zone, the pedunculate zone, is made up of 28-32 columns of highly imbricate plates. These columns have plates arranged into between 28 and 75 regular plate circlets. Although thought unlikely by Sumrall (1996) there does appear to be some evidence for a reduction in the number of columns in the most distal portions of the pedunculate zone. Distal to the recumbent zone is the peripheral rim that is typically less than 12 mm in diameter and acting only as a small holdfast. It is composed of a series of irregular circlets of highly imbricate plates that decrease in size distally. Plates in the most proximal circlet are generally transversely elongate, equidimentional in the middle circlets, and transversely elongate in the most distal circlets.

The discocystinid Hypsiclavus priesti in the retracted posture.

The discocystinid Hypsiclavus huntsvillensis in the extended posture.

Discocystinid clavate thecae are highly extendible and retractable (Bell, 1976, 1977; Sumrall, 1994) and are generally found preserved in one of two postures. In the retracted posture the pedunculate zone is retracted and the oral surface is pressed against the aboral surface forming a disk. In the extended posture the theca is generally preserved in with the oral surface folded over and the pedunculate zone preserved as a stalk giving the animal a mushroom shape.

REFERENCES

BELL, B. M. 1976a. A study of North American Edrioasteroidea. New York State Museum Memoir 21, 446p.

_____. 1977. Respiratory schemes in the class Edrioasteroidea. Journal of Paleontology, 51:619-632.

SUMRALL, C. D. 1992. Spiraclavus nacoensis, a new clavate agelacrinitid edrioasteroid from central Arizona. Journal of Paleontology, 66(1):90-98.

_____ .1994 (1993). Basic designs in Isorophinid edrioasteroids. Lethaia, 26:289-302.

_____. 1996. Late Paleozoic edrioasteroids (Echinodermata) from the North American midcontinent. Journal of Paleontology, 70(6):969-985.

_____. and A. L. Bowsher. 1996. Giganticlavus, a new genus of Pennsylvanian edrioasteroid from North America. Journal of Paleontology, 70(6):986-993.