A STUDY OF STASIS AND CHANGE IN 2 SPECIES LINEAGES FROM THE MIDDLE DEVONIAN OF NEW-YORK-STATE

More than 5000 measurements were taken on over 1000 specimens of two s pecies of brachiopods, Mediospirifer audaculus and Athyris spiriferoid es, from the Middle Devonian Hamilton Group of New York state. Statist ical analyses were performed on these data, with specimens partitioned by their occurrence in one of many paleoenvironments and stratigraphi c horizons. Neither species showed substantial morphological departure s between first appearance and extinction (the range of the Hamilton G roup, roughly 5 m.y.). However, oscillations in morphology were discov ered 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 sp ecies into different environmental populations. Different ''environmen tal populations'' may evolve, but they will typically do so in several different ''directions,'' generally producing no net change. The diff erence between the morphology of species in different environments ove r the whole interval of the Hamilton Group is also nil, thereby ruling out any major role that ecophenotypic effects could play in the patte rns recognized herein.