TAPHONOMY OF BROKEN HEART URCHINS IN THE CLAYTON FORMATION (PALEOCENE) OF ALABAMA

An exposure of the Pine Barren Member of the Clayton Formation (Paleocene:Danian) in southern Lowndes County, Alabama includes a large weathering surface covered with many thousands of disarticulated echinoid plates. This area is significant due to the rarity of fresh exposures in the formation. The source bed is medium gray sandy calcareous clay that also contains disarticulated plate fragments and rare whole specimens of the spatangoid echinoid Hemiaster stella. Collections of the weathered material were sieved and the echinoid fragments retained for analysis. Fragment size distributions were calculated by sieve analysis: the major axis ranging in size from less than 2mm (sand-sized) to 25mm, with an average of 8.128mm. The echinoid fragments comprise approximately 70% of the coarse material, with the remainder comprise primarily of molluscan fragments with minor amounts of bryozoan species. The echinoid fragments are predominantly monospecific, representing a species of Hemiaster (most likely H. stella) , but with minor fractions of other species.

The nature of the deposit represents two distinct fragmentation events: one roughly penecontemporaneous with deposition in the Paleocene, and another associated with the modern exhumation and weathering of the deposit. Microstructure of the plates, their separation and fracture surfaces, and, to some degree, their color serves to distinguish these distinct taphonomic episodes.