A study of stasis and change in two species lineages from the Middle Devonian of New York state

More than 5000 measurements were taken on over 1000 specimens of two species of brachiopods, Mediospirifer audaculus and Athyris spiriferoides, from the Middle Devonian Hamilton Group of New York state. Statistical analyses were performed on these data, with specimens partitioned by their occurrence in one of many paleoenvironments and stratigraphic horizons. Neither species showed substantial morphological departures between first appearance and extinction (the range of the Hamilton Group, roughly 5 m.y.). However, oscillations in morphology were discovered in both taxa. For the two species we studied, groups of organisms occurring in a single paleoenvironment undergo moderate morphological change through time; however, the net sum of changes through time in all paleoenvironments in which these species occur is essentially zero. Therefore, stasis may be partly a property of the organization of species into different environmental populations. Different “environmental populations” may evolve, but they will typically do so in several different “directions,” generally producing no net change. The difference between the morphology of species in different environments over the whole interval of the Hamilton Group is also nil, thereby ruling out any major role that ecophenotypic effects could play in the patterns recognized herein.