The Influence of Spines on Predation and Survivorship of Devonian Brachiopods

Brachiopods are small animals that live on the seafloor and are abundant in the Devonian fossil record (420 to 360 million years ago), making them excellent subjects for studying evolution in ancient marine environments. Some species sported large spines that cover their exterior surfaces, although these spines are typically broken off during fossilization. Paleontologists have hypothesized that these spines developed as an evolutionary response to predation, yet few studies have tested this idea due to the scarcity of specimens with intact spines. Recent work has been able to get around this problem by examining injury markings caused by predators that brachiopods accumulate over their lifetime, which preserve well in the fossil record. In this study, I examined injury markings on fossils of the spiny brachiopod Atrypa rockfordensis and the spineless Atrypa devoniana. Results demonstrate that A. devoniana experienced attacks across all body sizes whereas A. rockfordensis was only attacked at smaller sizes, suggesting that spines were an efficient defense against predators. Furthermore, survivorship analysis based on the overall body size at the time of death suggests that A. rockfordensis has high juvenile mortality, possibly correlated to a higher rate of predation when spines are not fully developed. These findings provide evidence for predator-prey interactions as an evolutionary pressure for the origin of defensive structures.https://digitalcommons.morris.umn.edu/urs_2016/1000/thumbnail.jp