TAPHONOMIC BIAS IN TAXONOMIC ASSIGNMENT: A SEMI-LANDMARK APPROACH TO DISTINGUISHING SPECIES OF SILURIAN EURYPTERIDS

Eurypterids are generally rare in the fossil record owing to their lightly sclerotized exoskeleton. The multi-element eurypterid exoskeleton also makes them highly prone to pre-burial disarticulation. It has been suggested that certain elements, such as carapaces, can only be used to identify taxa to the genus level, in which case erection of species based on these elements could inflate eurypterid diversity through oversplitting. There is also a risk of elevating juvenile instars of known taxa to species status when relying on single elements. Within the upper Silurian genus Eurypterus are several taxa that have been established from limited material, including E. laculatus, known only from isolated carapaces. E. laculatus has been identified in several units including the Ellicott Creek Breccia Member (ECB) of the Fiddler’s Green Formation (Bertie Group) of New York/Ontario where it occurs with the more common E. remipes and E. dekayi. The ECB eurypterid assemblage is dominated by small, isolated carapaces with minimal taphonomic distortion, making it ideal for testing questions of taxonomy and ontogeny when faced with only partial remains. We employed a geometric morphometric approach, using a combination of fixed and semi-landmarks to characterize carapace shape. To test the ability to distinguish species using only isolated carapaces, we compared specimens from the ECB using a priori species designations. We also determined the influence of ontogeny on Eurypterus morphology when disassociated from size. To determine whether or not the small-sized ECB E. remipes population is morphologically unique, or simply part of an ontogenetic gradient, we compared it to a more normally distributed population from the contemporaneous Phelps Waterlime Member. Preliminary results indicate that ontogeny controls a significant amount of morphological variation in Eurypterus. The carapace shapes of adult E. remipes and E. dekayi appear to be morphologically distinct whereas E. laculatus’s status remains equivocal based on statistical analysis and PCA visualization. Notably, morphological overlap among juvenile-sized Eurypterus in this study precluded accurate taxonomic assignment at that growth stage. This suggests that identification of young conspecifics may not be possible when using only the carapace.