DID PROTOSCUTELLID SAND DOLLAR ECHINOIDS POSSESS GREGORY’S DIVERTICULUM?

The protoscutellid sand dollar echinoids (Echinoidea: Scutelliformes: Protoscutellidae) of the southeastern United States are a problematic group. The family comprises 3 nominal genera (Protoscutella, Periarchus, and Mortonella), not always readily distinguished from one another, with about 15 nominal species and subspecies which range from middle to late Eocene in age. They were a common and often abundant faunal element in shallow littoral environments across the region until their disappearance at the end of the Eocene and seem to have been replaced in these habitats by more morphologically robust species of the clypeasterine echinoid Clypeaster. All modern Scutelliformes, with the exception of the highly derived Astriclypeidae and the enigmatic Rotulidae, possess Gregory’s diverticulum, an accessory organ of the digestive tract. Recent studies have shown that juvenile Scutelliformes selectively fill Gregory’s diverticulum with heavy minerals (primarily magnetite, ilmenite, rutile, and zircon). Although soft-part morphology is rarely preserved in fossils, selective concentration of heavy minerals within the test has been shown to be retained post-mortem under rapid burial conditions. The aim of the present study is to improve understanding of the evolution of such a complex behavior. Optical, X-ray imagery, computed tomography, and SEM/EDS methods were used to examine sediment contained within juvenile (1-10mm test length) Protoscutella and Periarchus fossils collected from Texas, Mississippi, Alabama, and North Carolina. Specimens were found in sediments including argillaceous quartz-glauconite sand, argillaceous quartz sand, and quartz sandy carbonate, all with minor fractions of heavy minerals. Although most specimens analyzed retained the Aristotle’s lantern and were usually filled with sand-sized mineral grains, no heavy mineral concentration was detected. These data suggest that protoscutellid sand dollars, even though representing stem Scutelliformes, did not sequester heavy minerals and by inference did not possess Gregory’s diverticulum.