TIMING OF MORPHOLOGICAL CHANGE IN GASTROPODS DURING THE MID-PALEOZOIC PRECURSOR TO THE MESOZOIC MARINE REVOLUTION

The mid-Paleozoic Precursor (MPP) to the Mesozoic Marine Revolution is marked by contemporaneous changes in morphology of prey organisms with a radiation of durophagous (shell-crushing) predators (Signor and Brett, 1984). Changes in morphology of sessile organisms such as brachiopods and crinoids have suggested the MPP began in the mid-Devonian and Mississippian; however, the effects of the MPP on mobile prey organisms is not understood. The Signor-Brett hypothesis is tested further herein by examining the timing of the MPP in relation to mobile marine gastropods, and also by examining morphological changes in these taxa. Gastropods are ideal for exploring the effects of the MPP because they (1) are abundant in the Paleozoic, (2) are mobile, and (3) show a variety of morphologies inferred to be anti-predatory. Changes in global diversity and morphology of pleurotomariinid gastropods are analyzed using the Treatise on Invertebrate Paleontology as a representative sample.

Examining the diversity of pleurotomariinid gastropods through the Paleozoic reveals a rapid decline in the mid-Devonian, corresponding to the Frasnian-Famennian extinction. Pleurotomariinid gastropods through the Paleozoic manifest an overall increase in percent genera with potentially anti-predatory morphologies, particularly presence of nodes, from the Lower Devonian through the Lower Permian (Spearman's Ranked Correlation, r = 0.94, p < 0.001). While the diversity of pleurotomariinids declined in the Pennsylvanian, noded genera continued to increase in number. Pleurotomariinids with cords diversify from the lower to mid-Devonian while the overall diversity of pleurotomariinids rapidly declines. Average sphericity of pleurotomariinid genera also increases through the Paleozoic while the variance in sphericity decreases. This pattern could either be the result of selective pressures driving pleurotomariinids to more predator-resistant morphologies (spheres resist point-loading), or the result of functional constraints on pleurotomariniid morphology. These exploratory results need to be further examined through geographical analysis and predation traces.