SEMI-LANDMARK ANALYSIS OF EURYPTERIDS: A NEW TOOL FOR ASSESSING ECOPHENOTYPY

Eurypterids occur in a number of depositional facies within the Appalachian basin of North America, including, frequently, those interpreted as hypersaline, peritidal environments. In the Upper Silurian Ellicott Creek Member of the Fiddler’s Green Formation of New York and Ontario, for example, hundreds of small eurypterids from a single genus (Eurypterus) occur on the same horizon as salt hoppers, in a chemically precipitated dolomite most likely deposited under evaporitic conditions. It seems unlikely, however, that eurypterids would have actively occupied these hostile environments based on comparisons with modern marine chelicerate analogs such as horseshoe crabs. Because most of these eurypterid fossils are considered to be molts rather than carcasses, it is more plausible that specimens were either transported into these environments after undergoing ecdysis elsewhere, or that areal salinity changed after ecdysis took place. Determination of ecophenotypic effects on development, such as stunting or dwarfism, which might result from living in a stressed, hypersaline environment, would aid in assessing whether eurypterid fossils found in these settings were derived from inhabitants, or represent transported remains. Ecophenotypy is typically difficult to prove in the fossil record of extinct groups, but morphometric analysis represents one method for quantifying environmental influences on morphology. Semi-landmark analysis, in particular, is well suited to the characterization of the eurypterid prosoma shape as this tagma lacks defined features necessary for traditional landmark analysis. A large population (N > 600) of well-preserved Eurypterus sp. collected from the Ellicott Creek Member makes an ideal eurypterid taxon on which to apply this technique for the first time and forms the basis of this study. Aside from providing a novel method for answering the question of whether eurypterids inhabited stressed environments, semi-landmark analysis also offers to provide new insights into eurypterid ontogeny and phylogeny, which have previously been quantified using standard linear metrics or traditional landmarks.