SIZE DISTRIBUTION AND TAPHONOMIC GAP OF THE BLASTOID PENTREMITES IN THE UPPER MISSISSIPPIAN GLEN DEAN FORMATION

The blastoid echinoderm Pentremites is an icon of North American Mississippian faunas, at times being the dominant macroscopic organism in a given locality. One such locality along the Pennyrile Parkway (Upper Mississippian (Chesterian) Glen Dean Formation near Hopkinsville, KY) contains copious numbers of Pentremites Blastoids. Four Pentremites species were found at this locality including Pentremites tulipiformis, and P. pyriformis, along with two new species. Pentremites tulipiformis and P. pyriformis comprise about 95 percent of specimens. Unbiased bulk samples were collected to examine blastoid size frequency distribution, after observing a distinct absence of blastoids less than 3 mm in thecal height. Bulk samples of shale were collected at 4m intervals along the locality in large one-gallon bags. Samples were weighed, rinsed with water, soaked in hydrogen peroxide, oven dried, and reweighed. All residues were retained above 105µm and sieved for ease of processing. All complete blastoid thecae were collected from residues greater than 2 mm. For sieve sizes of less than one mm, residues were divided multiple times using a sediment splitter and the total number of blastoids was determined by extrapolating the number of specimens found per gram. The largest size fraction (>2.0mm) contained between 2-12 specimens of Pentremites – approximately 1-6 blastoids per kg of bulk sediment. No complete thecae were recovered in size fractions between 2.0mm to 0.25mm, and rare blastoids in this size bin included only specimens in which the vault of the theca was fully disarticulated. However, in the size fraction between 0.25 mm and 0.105 mm, blastoids are abundant averaged between 13-15 thecae per gram of sediment residues – approximately 1300-1500 per kg of bulk sediment.

The results suggest that the blastoid Pentremites has extremely low survivorship, with nearly three orders of magnitude more microscopic specimen than larger specimen. Furthermore, the average thecal height of Pentremites at the locality is 0.3 mm. The data also suggest a taphonomic window in which thecae with a height between 0.25 mm and 2 mm are rarely preserved. This probably results from the poor suturing of the thecal vault plating before the ambulacra are well developed.