|ANALYSIS OF THE COMPLETENESS OF EURYPTERID REMAINS
O. Erik Tetlie, Samuel J. Ciurca, Jr.
Taphonomy is the study of conditions and processes that happen to an organism during fossilization. One taphonomic process, with a potential of influencing the completeness of fossils of the extinct eurypterids, is transportation. Most eurypterid remains at most localities are found disarticulated. More rarely, 'complete' individuals are found and these are often the specimens (selectively) preserved in museum collections. As arthropods, eurypterids molted their exoskeleton during growth and provided several exoskeletons that had the possibility of preservation as a fossil eurypterid, perhaps more than 10 molts in a eurypterid's lifespan. For example, there are about 200 parts that could be preserved from just one molt of the most common eurypterid in New York, Eurypterus, giving at least 2000 potential fossils from one individual. Since most of these are tiny spines on the legs, they are rarely seen, and even a 'complete' individual would rarely exhibit more than a tiny fraction of all possible parts. The number of leg spines also varies from taxon to taxon, and to be able to empirically compare the completeness of eurypterids from different localities, horizons and taxa, we propose to calculate a 'eurypterid completeness index' (ECI). The ECI provides a weight of 1 to each of the following elements when they are articulated: carapace, 12 tergites (dorsally), 6 tergites, 4 pairs of sternites, metastoma and genital operculum (ventrally), 10 appendages (II-VI) and telson to circumvent preservational and morphological differences. Incomplete elements that can be identified are given the weight of 0.5 and those that are not identifiable 0.1. This gives a eurypterid fossil a potential maximum score of 24 and a minimum score of 0.1. The ECI is calculated by summarizing the scores of all individual fossils and dividing on the number of specimens, thus giving an average value for the specimen completeness.
We are currently retrieving data from tens of thousands of partial and nearly entire individuals, from more than a hundred localities, now in the collections of the Yale Peabody Museum. Are all eurypterid remains the result of molting followed by transportation and disarticulation of molted parts? Are the 'complete' specimens nicely preserved molts or carcasses? Is preservation due to exceptional conditions, e.g. hyupersalinity or storms? Where did eurypterids actually live? Can we understand biological and sedimentary facies changes from the stratigraphic record preserved?
Preliminary ECI values have so far been computed for three localities with number of specimens indicated: Kokomo Limestone, Wabash Formation, Kokomo, Indiana ECI 6.69 (27.8% completeness, N=13), Split Rock Quarry, near Syracuse, New York (Olney Limestone) ECI 1.69 (7.0%, N=187), and Phelps Member, New York State Thruway, Phelps, New York ECI 1.52 (6.3%, N=96).
|ABSTRACT (below) reprinted from the Rochester Academy of Science Fall 2005 Scientific Papers Day hosted by Finger Lakes Community College and the Department of Environmental Conservation and Horticulture, Canandaigua, New York|
|ABOVE: Not an atypical find. The Bertie Group bears abundant examples of windrows and concentrations of fossil remains of eurypterids and many other organisms. This photo exhibits molted and disarticulated remains of many eurypterid parts - note particularly the two carapaces in the center field. Recognizable parts (coxae, etc.) are present as well as unrecognizable arthropod integument. This is most probably a tempestite, a storm deposit preserved within the Williamsville Waterlime. The storms appear to have sent easily floated (buoyant) material past the tracts of stromatoporoids (Akron-Cobleskill) and stromatolites (where present) into shallow lagoons and near-shore regions.
Strata consisting of waterlime appear to have formed in shallow waters where extensive mudflats could develop at one extreme, and in somewhat deeper water farther from shore where microbialite and stromatoporoid biostromes and bioherms developed.
|LEFT: Eurypterus remipes. Unusual preservation that presents a sideview of this common eurypterid of the Fiddlers Green Formation. Note anterior appendages and dislocation of tergites in front half of the specimen.
Phelps Member at Passage Gulf, New York.