PHENOTYPIC PLASTICITY OR SPECIATION? PATTERNS OF MORPHOLOGIC CHANGE IN EOCENE SAND DOLLAR ECHINOIDS

The family Protoscutellidae consists of three nominal genera, Protoscutella, Periarchus, and Mortonella. The family was taxonomically delineated to distinguish this group of sand dollars prevalent in the Eocene of the eastern United States from all other scutelliform sand dollars. The presence of five gonopores affirms the monophyly of the family, distinguishing it from other contemporary or near-contemporary scutelliforms. This unusual trait suggests that the family originated from a small population that migrated into and subsequently became isolated in the region that is now the southeastern United States. Protoscutellid species are variously described on the basis of a series of morphological characters exhibited by adults that include size, shape, location and details of the apical system, location of the periproct, coronal plate patterning, and the development of food grooves. We explore the phylogenetic significance of these characters by focusing on their development during ontogeny and their variety of expression in local populations. The pattern that emerges regarding the evolution of the protoscutellids is that of a relatively large amount of phenotypic plasticity, with the appearance of disjunct polymorphism within and between contemporaneous local populations. The pattern may represent cryptic genetic variation that is repeatedly unbuffered in derived lineages, resulting in recurrence of morphological patterns through both time and space and a high degree of adaptability to the varied environments of the Gulf coast. We review a simplified taxonomy of the family based on this interpretation.