PREDATOR-PREY DYNAMICS IN PENNSYLVANIAN NEAR-SHORE SHALES: EFFECTS OF MOLLUSCAN INVASION INTO "NORMAL" MARINE HABITATS

Predation is a major agent of natural selection, especially for prey experiencing high rates of predation. Predators exposed to multiple prey types are expected to choose prey yielding the most nutritional value for the least energetic cost. Prey choice has been explored rarely in Paleozoic systems. The majority of Early Paleozoic gastropods and bivalves were restricted to nearshore habitats but during the Carboniferous both classes radiated into “normal” marine systems, at which time molluscs were exposed to a greater risk of predation. Previously, predators primarily attacked brachiopods. How did the molluscan invasion of these ecosystems affect predator-prey dynamics? We examine several exceptionally well-preserved assemblages from Pennsylvanian shales in Texas that include diverse molluscs and brachiopods. Most taxa are similar in size and shell thickness, so predators might be expected to prefer fleshier molluscs over brachiopods.

Drilling was stereotyped and formed small, cylindrical holes. Drilling was restricted entirely to sedentary, epifaunal taxa, regardless of taxonomic affinity. Drilling predators preferred brachiopod prey (chi-square, p=0.01) but rarely sedentary gastropods were also attacked. Unlike their modern analogs, Pennsylvanian drillers may have preferred or were forced to consume sessile organisms.

Crushing predators were capable of consuming mobile prey, and left scars on gastropods (21% of specimens) much more frequently than bivalves (3%) or brachiopods (7%, chi-square, p<<0.01). This difference is either due to greater attach rates on gastropods or greater success against other taxa. However, both drilling and crushing frequency may be low for bivalves because most were infaunal. Some brachiopods are significantly larger than gastropods and therefore we expect large brachiopod taxa to have higher repair frequencies. This is not the pattern observed. Thus, we favor a greater attack rate on gastropods as an explanation, supporting the implication that predators may have changed their preferred prey choice from brachiopod to gastropod prey as molluscs invaded normal marine systems. The higher frequency of scars on gastropods suggests an increase in ornament in many gastropod lineages during the Late Carboniferous may have been an adaptive response to predation.