ARMS IN CAMPTOSTROMA, AN ARCHAIC PENTARADIATE ECHINODERM

Abundant specimens of the pentaradiate echinoderm Camptostroma roddyi include a few that preserve details of the ambulacrum. The collections come from the basal Kinzers Fm., uppermost Cambrian stage 4 (latest Early Cambrian) of SE Pennsylvania. Most are natural molds of semi-articulated plate masses showing little more than an aboral disc and a pleated peripheral apron. Rarely, a specimen will be found overturned, with the oral surface oriented downward. On these, the ambulacra are sometimes well preserved.

Camptostroma roddyi was a large-bodied (up to 8cm diameter), free-living, biscuit-shaped organism with a skeleton composed of numerous small plates. The aboral surface was a heavily plated, relatively flat disc. The aboral plates were solid and imbricate. A prominent, deeply-pleated peripheral apron surrounded the aboral disc and extended onto the domed oral surface. Both the pleated apron and oral surfaces were lightly built. The oral surface was dominated by broad, pore-bearing interambs, separated by five narrow, curved ambulacra. In life, the weight of the aboral disc kept the animal oriented with the oral side up.

Five ambulacral food grooves, arranged in a 2-1-2 pattern extend to the edge of the oral surface. The oral frame is not exposed. All five ambulacra form high, curved ridges that continue beyond the oral surface onto short arm-like projections. The hydropore and periproct lay in the CD interamb. Coverplate sheets arch above each food groove. The platelets in each sheet are aligned in uniserial stacked sequence, with the lateral-most series much larger than the next medial elements. Floor plates are laterally exposed with internal ambulacral pores between sequential elements. Proximally, they are elongate, roughly alternating. More distally, they become short and wide. The sides of a few arms express epispires. Because these appendages are rarely preserved away from the theca, it is possible that only some individuals had free arms.

Camptostroma roddyi adds to the known range of early pentaradiate echinoderm morphology. The arms in C. roddyi are essentially distal ambulacral tips and nearby interamb regions that have grown beyond the theca and acquired edrioasterid-like floor plates. They are unlike blastozoan brachioles, but recall the coelom-bearing arms of the most archaic crinoids.