THE INFLUENCE OF SPINES ON DUROPHAGOUS PREDATION AND LIFE HISTORY OF ATRYPID BRACHIOPODS FROM THE LIME CREEK FORMATION, IOWA

An understanding of the selective pressures that lead to the innovation of evolutionary novelties for defense is critical for studying predator-prey relationships through time. While such structures are observable in the fossil record, morphological data alone is often insufficient for investigating community interactions. One method for discerning these interactions is to examine preserved injury markings due to attempted predation. Using brachiopods, researchers have investigated how the presence of valve spines may have affected the frequency of attempted durophagous predation. However, most of this work only observed individual specimens or have compared distantly-related species. In this study, I compared the frequency of predator-induced injuries on the spiny brachiopod Atrypa rockfordensis and the spineless Atrypa devoniana collected from the Late Devonian (Frasnian) Lime Creek Formation in north-central Iowa. Since predators attacked from the edge of the valve surface, I measured the distance between injuries and the posterior foramen to determine the size of the specimen at the time of injury. Results demonstrate that A. rockfordensis was only attacked at smaller body sizes (length < 20 mm) whereas A. devoniana experienced attacks regardless of size, suggesting that spines may have been an efficient defense against predators. Furthermore, survivorship analysis based on size cohorts suggests that A. rockfordensis had a relatively high juvenile mortality, possibly due to insufficient spine development early in life, while A. devoniana had a constant mortality rate, reflective of its lack of defense with maturity. These findings provide evidence of attempted predation as a means of discerning interactions among organisms and are useful in understanding the evolution of spines in brachiopods at the resolution of species within the same genus.