科研之友
微信
新浪微博
Facebook
分享链接
摘要
Predation scars offer insight into ancient predator-prey relationships and evolutionary processes. One approach to studying predation in fossils is through repair scars, which indicate a failed attempt at predation as the prey survives the attack and repairs its shell. A change in repair frequency is ambiguous as it may indicate either a change in the number of attacks on the prey or a change in the success of the predator. Size refugia, the size of the prey at which the predator will no longer take the prey, can potentially be used to differentiate between these two possibilities. The goal of this study is to examine predatory traces and size refugia in lineages of atrypide and strophomenide brachiopods to determine the relative performance of predators to prey through time in the Middle Devonian of Michigan. We reviewed 1197 specimens of Pseudoatrypa and 896 specimens of Strophodonta for crushing predation through multiple shales in the Middle Devonian Traverse Group of Alpena and Presque Isle Counties of Michigan, USA. The sampled units, in stratigraphic order, are Bell Shale, Ferron Point Fm., Genshaw Fm., Dock Street Clay, Norway Point, and Potter Farm Fm.; these shales were deposited in broadly similar environments. Repair frequency (R%), the percent of shells with repairs, was determined for each unit. Body size was measured for all specimens and the body size at the time of attack was measured for the specimens that had repair scars. The maximum size-at-attack for each unit was used as a proxy for size refuge. The results for the atrypides showed a U-shaped trend in R% through time, with the lowest R% in the Genshaw, and a size refuge that was roughly inverse to R%, showing an N-shaped trend, with the greatest size refuge in the Genshaw. Atrypides in the Genshaw likely never truly reach a size refuge (the predators could take any size of prey). There were no strongly significant results with respect to changes between R% and size refuge from unit-to-unit but this is likely due to the poor sample sizes for the Ferron Pt. and Dock St. Clay. The general trends, however, are statistically significant and suggest that the adaptive gap (the relative effectiveness) between predators and prey is fluctuating through time: during Genshaw time, the predators grew more effective relative to their prey, as indicated by the predator's ability to take larger prey, whereas pre- and post-Genshaw, the predators were less effective. This may imply that an evolutionary arms race occurred between predators and prey. No significant changes to the morphology of the prey through time were found but further research is needed to determine whether there was a change in morphology in the predators through time. The results for the strophomenides indicated that R% and size refuge almost completely track each other through time, which is in contrast to the atrypides in which these variables change in opposition to each other in almost every unit. The general trend suggests that the change in repair frequency of strophomenides from unit to unit is due to a change in the predator attack frequency instead of a change in the success of the predator. Further research is needed to determine if the presence of one brachiopod genus had an effect on the other, as there is no hard evidence that they lived in any of the same localities at the same time.
- 出版日期2012-11-20
