PROTOSCUTELLID SAND DOLLAR ECHINOID SUCCESSION, MIDDLE TO LATE EOCENE, TEXAS TO NORTH CAROLINA

Although located at the two geographic extremes of the coastal southeastern United States (in the wider sense) and represented in one case by clastic deposits and the other by carbonates, the succession of sand dollar echinoid species in Texas and North Carolina are surprisingly similar.  In the Weches, Cook Mountain, and Caddell Formations of east Texas the succession is from Protoscutella mississippiensis mississippiensis to P. tuomeyi and finally Periarchus lyelli.  In the Castle Hayne Formation of North Carolina the succession is Protoscutella mississippiensis rosehillensis, P. conradi, P. plana, and finally Periarchus lyelli.  The oldest sand dollars known from the southeast region are found in the Tallahatta Formation in southwestern Alabama, and may represent a separate subspecies of Protoscutella mississipiensis.

While some vicariance may be represented by the older Protoscutella mississippiensis s.l., the widespread nature of the other species indicate that there was free exchange of larvae throughout the latter part of the Middle Eocene and early part of the Late Eocene, and furthermore that larval recruitment was relatively insensitive to substrate.

Relative dating of the various deposits is difficult, but the geographic succession appears to be Alabama-North Carolina-Texas (and Mississippi) for Protoscutella mississipiensis s.l., and Texas-South Carolina-North Carolina for the closely related (and possibly synonymous) species P. tuomeyi, P. conradi, and P. plana.  The oldest specimens of Periarchus lyelli are found in the Middle Eocene Cook Mountain Formation in east Texas, probably followed by Mississippi and Alabama, then the Carolinas and Georgia, with a separate subspecies floridanus found in the lower part of the Ocala Limestone in Florida.  Erosion on the north shore of Lake Sam Rayburn in Texas caused by Hurricane Rita in September, 2005 created fresh exposure of Caddell Formation (Upper Eocene) and the opportunity to collect relatively well-preserved P. lyelli from this section (an example of an ill wind that blew some good). These are the “Chinese-hat” variant P. lyelli pileussinensis.  The Caddell can be traced into the Moodys Branch Formation in Louisiana, which is the zone most commonly associated with P. lyelli farther east.