VARIATION IN EYE MORPHOLOGY OF THE TRILOBITE PACIPHOPS LOGANI BIRDSONGENSIS (DELO), DEVONIAN BIRDSONG SHALE MIDDLE TENNESSEE

The compound eye structure in Trilobites has been extensive studied. Variation in eye morphology has been used to suggest sexual dimorphism, polymorphism, growth patterns, and taxonomic status. In the case of Paciphops logani birdsongensis (Delo) eye morphology has been used to establish two morphological variants a large eyed form with an averge of 17 dorsal ventral files and an average of 73 lens and a small eyed form with 14-15 files with an average of 40 lens (Eldredge, 1973). With a nearly equal division between the morphologies. Previous studies have been largely based on museum collections form a variety of localities. This study re-examines variation in eye morphology from a large collection of P. logani birdsongensis from a single locality of the Birdsong Shale in Benton and Henry Counties , Tennessee. This collection consisting of more than 100 specimens allows for examination of eye morphology from a single paleoenvironmental setting. In addition collection of all identifiable specimens eliminates selectivity for the best preserved or morphological varied specimens. Preliminary analysis of eye morphology indicated the presence of only the large eyed morphology, average number of files 22 with average of 69 lens. The complete absence of the small eyed morphology is notable. Possible explanations for absence of the small eyed morphology include, a differing proportion of morphology in the population, environmental control on the distribution of the eye morphology, and skewed proportions in museum collections.