MORPHOMETRIC ANALYSIS ON THE EVOLUTION AND HETEROCHRONY IN THE MIDDLE AND LATE ORDOVICIAN OLENID TRILOBITES TRIARTHRUS BECKII AND TRIARTHRUS EATONI FROM NEW YORK

Morphometric analyses were performed on 223 Ordovician Triarthrus specimens from the Mohawk and Black River Valleys of New York, spanning five graptolite zones (O. americanus, O. ruedemanni, C. spiniferus, G. pygmaeus, and C. manitoutinensis). The collections 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 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 five levels show significant differences (p < 0.02) suggesting directional morphologic change of an evolving lineage through the entire series. Principle component loadings and thin-plate spine visualizations showed greatest shift of eye position, shape of the palpebral lobe between the C. spiniferus and G. pygmaeus zones suggesting separation between T. beckii from T. eatoni. Further analyses on the ontogeny of T. becki and T. eatoni suggests heterochrony through peramorphosis was involved in this evolutionary event. Juveniles of T. eatoni exhibit similar morphology to adult T. beckii whereas holaspids of T. eatoni and T. beckii are morphometrically distinct. Triarthrus eatoni and T. beckii both exhibit allometric growth through ontogeny, in which the position of the eye moves posteriorly in both. In T. eatoni, the eye moves to a position below the first furrow of the glabella and the palpebral lobe becomes narrower. In T. beckii, the eye moves to a position above the first furrow by the holaspid stage. Before T. eatoni reaches the holaspid stage and the maximum position of the eye, it is morphometrically similar to T. beckii. This morphologic change in cranidia is observed through younger stratigraphic levels as T. beckii transitions into T. eatoni. The youngest and most advanced form of T. eatoni occurs in the C. manitoutinensis zone.