SUMMIT STRUCTURES AND ABNORMALITIES IN THE BLASTOID PENTREMITES FROM THE UPPER MISSISSIPPIAN GLEN DEAN FORMATION

Summit structures - cover plates over the mouth, ambulacra and spiracles if present – are features common to all blastoids. Because these structures are composed of small and fragile platelets, summit structures and cover plates are rarely preserved even under very favorable environmental conditions. Because these structures cover body openings including the mouth, anus, gonopore and respiratory structures, they have the ability to open and close.

The Pennyrile Parkway locality of the Upper Chesterian (Mississippian) Glen Dean Formation is known for its high abundance and striking preservation of blastoids. To date, we have identified two genera and five species of blastoids in collections numbering approximately 1,500 specimens. Of this population, approximately 63% are Pentremites tulipiformis, 33% are P. pyriformis, less than 4% are two new Pentremites species, and less than 0.1% are an undescribed Diploblastus species. Summit structures have been found on two species, P. tulipiformis and P. pyriformis. Here, we reevaluate and clarify the classification of pentremitid summit structures presented by Beaver (2000) based on new material. Summit structures of both species are rather similar, including oral cover plates, spiracle cover plates, and a very pronounced anal pyramid. Cover plates over the primary food groove are biserial, fit tightly between the biserial spiracle cover plates, and traverse the entire length of the ambulacra. However, the spiracle cover plates differ between the two species, being flat and bladed in P. tulipiformis and long and toothlike in P. pyriformis.

We have also identified multiple blastoids (<0.2%) that exhibit pathologies and other anomalies. Several specimens have one ambulacrum that failed to develop; lacking a lancet and side plates, and the radial plate lacks a sinus. These specimens look four sided, however all of the thecal plates are present, though oddly malformed. In other specimens pathologies are evident in the exposure of the hydrospires at the ambulacral tips. Oddly rugose distal ambulacra with a pillar like projection previously interpreted as pathologic, is normal for P. tulipiformis and occurs in all ambulacra of all specimens.