MORPHOLOGIC CHARACTERIZATION OF THE “PRIMITIVE” ECHINOID FIBULARIA TEXANA (CLYPEASTEROIDA:SCUTELLINA)

The clypeasteroid echinoid genus Fibularia has been characterized as an extremely miniaturized “micro-echinoid”, but rather than “primitive” in a phylogenetic sense the genus is considered to comprise highly reduced, paedomorphic forms. The paedomorphic hypothesis implies that rate of evolutionary change in skeletogenesis has reached a stage of stasis (lacking any terminal addition of characters) and may shed light on general principles of early ontogenetic development in clypeasteroids. The spherical shape of the corona, the lack of internal skeletal supports, and simple jaw apparatus in adults of this genus approach the general characteristics of the imago or very early post-metamorphic stage of development seen in the development of the flattened “sand dollar” clypeasteroid bauplan. Fibularia texana (Twitchell), recognizing the synonymy of F. alabamensis Cooke and Echinocyamus? meyeri Aldrich, ranged from the Early through Late Eocene and was endemic to what is today the Gulf of Mexico coastal plain. The occurrence in Lower Eocene rocks (Bashi Marl) of Alabama makes it the oldest definitively known clypeasteroid echinoid from the Western Hemisphere. The paleogeographic endemism and wide temporal range (~18 MY) combine to make this species a model for exploring the degree of stasis of the morphological characters.

Abundant fossil material and excellent preservation of some examples of F. texana permit detailed description of: the heterognathus Aristotle’s lantern; lantern supports comprised of asymmetrically arranged interambulacral apophyses (an interpretation contrasting with the generally accepted hypothesis of inter-radially positioned fused auricles); stereom microstructure; exposed (naked) sphaeridial pits; double buccal pores; non-respiratory ambulacral and accessory pore development; structure of the monobasal apical system; and differentiation of plate patterning inside the petalodium. Details of these characters serve to distinguish F. texana from modern species (including the type species) of Fibularia and it may represent an as yet unnamed genus. The chronostratigraphic age of this species suggests that it is very near the basal neotenic form, and can be interpreted as retaining the characters of a juvenile cassiduloid echinoid rather than that of a derived clypeasteroid.