TEMPO AND MODE OF MIDDLE PENNSYLVANIAN MARINE GASTROPODS FROM THE APPALACHIAN BASIN USING GEOMETRIC MORPHOMETRICS

Tempo and mode within macroevolution have been of great interest to paleontologists for decades. Punctuated equilibrium argues that species tend to remain in morphological stasis through most of their stratigraphic ranges, though causal mechanisms for stasis vs. evolutionary change remain unclear. How does the magnitude and nature of environmental variation relate to the relative frequency of stasis vs. directional change within species? Marine Pennsylvanian gastropods of the Appalachian Basin are a good case study to explore tempo and mode due to their abundance and occurrence within a constantly changing environment. The Pennsylvanian Appalachian Basin has a rich fossil record through an interval of pronounced transgression and regression cycles. Geometric morphometric analysis of marine gastropods’ shell variation was used to evaluate morphological changes in species across sea level cycles in order to test for stasis versus environmentally driven trends through the Middle Pennsylvanian. Four species of Pennsylvanian marine gastropods were examined across three consecutive sea level cycles. A total of 250 specimens were photographed and landmarks capturing shell shape were defined and digitized. After Procrustes superimposition, morphospaces were defined with principal component analyses in MorphoJ. Their resulting morphospaces were compared within and between species and between stratigraphic levels. Stasis was observed for each of the species. Two of the species show a pronounced reduction in morphological variation in the youngest sample, which represents an interval of relative stability in sea level. Lack of morphological change across several transgression-regression cycles indicates that environmental variations were not significant causal drivers of anatomical change in these gastropod species.