LE3 .A278 2016

2016

Pufahl, Peir

Acadia University

Bachelor of Science

Honours

Biology

Biology

The ostracod Velatomorpha altilis thrived in brackish coastal environments of the Carboniferous. Especially well preserved examples occur in strata exposed at the Joggins Fossil Cliffs World Heritage Site, Cumberland Basin, Nova Scotia, providing an opportunity to investigate the link between evolutionary mode and temporal changes in depositional environment. The relationship between the evolutionary mode of V. altilis and its environment is especially important because few other studies have examined the evolutionary mode of organisms living in marginal environments. The evolutionary mode of V. altilis was analyzed by measuring the length, height, width, area, and perimeter of valves from 332 specimens from limestone at five stratigraphic levels in the Boss Point and overlying Joggins formations. Interpretations of depositional environment in this study were based in part on ostracod population structure with no statistical treatment of the data. After quantitative model-based analysis, an unbiased random walk was supported as the evolutionary mode when stracods from all five stratigraphic levels were included. Stasis was the suggested evolutionary mode only when ostracods from each formation were considered separately. This is consistent with previous research on V. altilis from the Joggins Formation. Stasis is expected in fluctuating, stressed environments, such as the brackish coastal paleoenvironments of the Boss Point and Joggins formations. This is because organisms that inhabit those environments are usually tolerant to changing conditions. It is possible that between the formations an environmental threshold was reached that triggered rapid change in shell size. Ostracod population structure analysis supports this interpretation and suggests shell size may record a difference in hydrologic regimeof limestone between the Boss Point and Joggins formations. This represents a classic example of punctuated equilibrium model of evolutionary change wherein long periods of stasis are punctuated by geologically short periods of drastic morphological change.

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https://scholar.acadiau.ca/islandora/object/theses:1424