TRIARTHRUS FROM THE UPPER ORDOVICIAN OF NEW YORK REVISITED: TAPHONOMIC, TAXONOMIC AND MORPHOMETRIC ANALYSIS OF TRIARTHRUS BECKII GREEN AND TRIARTHRUS EATONI (HALL)

The taxonomic status of species of the olenid trilobite Triarthrus from the Upper Ordovician of New York has long been debated. Triarthrus beckii and T. eatoni have been interpreted as ecophenotypic morphotypes of a single species and have also been recognized as two distinct species with T. beckii possessing a shorter palpebral lobe and less developed thoracic spines than T. eatoni. Morphometric analyses were performed on 144 Triarthrus specimens from seven localities in the Mohawk and Black River Valleys of New York from the Flat Creek, Dolgeville, Indian Castle, Frankfort, and Deer River stratigraphic units spanning four graptolite zones (O. americanus, O. ruedemanni, C. spiniferus, and G. pygmaeus). The collection, dominated by disarticulated exuviae with cranidia and thoracic segments, are from similar taphofacies and were likely transported and deposited in thin-bedded distal turbidities in a dysoxic and anoxic basinal setting. Thirteen landmarks were digitized from holaspid cranidia and subjected to Procrustes superimposition prior to PCA and thin-plate spine analyses. Graphical comparison of PC scores showed distinct clusters of specimens separated by stratigraphic level. Pairwise comparison of PC scores between all four stratigraphic levels show significant differences (p < 0.02) suggesting directional morphologic change of an evolving lineage through the entire series. However, the comparison of PC1 scores between Triarthrus from the C. spiniferus and G. pygmaeus zones yielded the most significant difference (p < 0.001). Principle component loadings and thin-plate spine visualizations showed major variation of eye position and shape of palpebral lobe between these stratigraphic levels suggesting separation between T. beckii from T. eatoni between the C. spiniferus and G. pygmaeus zones. Following some previous authorities, we conclude that T. beckii is an older species of Triarthrus and morphologically distinct from T. eatoni. The position of the eye in T. beckii moves posteriorly and the palpebral lobe becomes much narrower through younger stratigraphic levels as it evolves into T. eatoni. This transition most abruptly occurs amid the C. spiniferus and G. pygmaeus zones at a stratigraphic level higher than previously recognized.