Morphometric study of the middle Ordovician trilobite, Triarthrus becki, from the Mohawk River Valley, New York state

For the stratophenetic study in testing the Plus ça change model, the ultimate purpose of this research, a total of 259 specimens of Triarthrus becki were collected from the Mohawk River Valley, New York State. Additional collections of T. becki were made from three other geographic areas (Quebec, Kentucky, and Pennsylvania).; Evolutionary patterns may be obscured by the presence of non-evolutionary factors such as ontogeny/heterochrony and clinal geographic variation. In the study of ontogeny, I conclude that allometry is evident in meraspides and holaspides, but the degree of allometry in holaspides is very small relative to that in meraspides. The boundary between meraspis and holaspis in Triarthrus becki appears to correspond to a large change in the slope of the ontogenetic trajectory, but not to a complete cessation of allometric shape change.; It is important to eliminate allometric size effects when allometry is present in species studied before addressing the morphological evolutionary trend of the species. The ontogenetic effect on the study of morphological evolution in Triarthrus becki was mitigated by ontogenetic standardization.; Geographic variation in shape of Triarthrus becki is examined throughout the Appalachian Basin in order to test the possible effects of geographic variation on the morphological evolutionary trend of T. becki through the immigration and emigration of intraspecific variants. The result is that there is no evidence of immigration and emigration among the geographic areas.; The Plus ça change model can be tested with the Triarthrus becki lineage. According to this model, T. becki should show a gradual phyletic evolution pattern because Triarthrus was a cosmopolitan genus in late Ordovician deep water, basinal strata. Results indicate that the stratophenetic pattern exhibited by Triarthrus becki is much more similar to stasis than to the expected pattern of anagenesis predicted by the Plus ça change model.