Last week, I introduced the topic of studying predation in the fossil record, and some of the kinds of traces, such as drill holes, we can look at to give us clues as to how predators and prey interacted in the past.
Another kind of predation trace that we use in invertebrate palaeontology is called a repair scar. Repair scars are generated when a durophagous (shell-crushing) predator, such as a crab or fish, attack a shelled animal, but for some reason, fail to kill the prey. If the shelled animal is able to avoid being eaten, it will start to repair the damage to its shell, resulting in a visible scar where its shell was damaged. It is essentially the same as the callus that develops on a broken bone after it heals. Here is a picture of a snail with a large repair scar caused by a crab inserting the tip of its claw into the shell and peeling it back (like how you would peel an orange). I’ve outlined the scar on the right image.
Wait a second. Why would you study failed predation in the fossil record? Why wouldn’t you study successful predation? If you think about the nature of crushing predation, a successful attack is likely going to destroy most or all of the shell. It is much less likely for shell fragments to be preserved, and good luck trying to find or fit all of the shell bits back together. That kind of preservation just doesn’t happen (Stafford and Leighton, 2011). Although, check out a recent paper by members of our lab on a creative way to deal with all of the fragments, at least in the modern (Leighton et al. 2016).
So how can we know that repair scars, or failed attacks, are actually an accurate proxy for how much predation actually occurred? This is where studies of modern ecosystems come in very handy, and one of the reasons that our lab spends half of its time studying live ecosystems, even though we are palaeontologists.
When I was a Master’s student, I took a research class out at Bamfield Marine Sciences Centre where we looked at the rate of repair scars on populations of a common snail (Tegula funebralis – we called it Chlorostoma funebrale in the paper, but the name has since changed) (Molinaro et al. 2014). We collected the Tegula from eight different sites that we knew had different levels of predation based on previous studies, which had shown that the rate of crab predation strongly correlates with the environmental energy (quiet water settings have high levels of crab predation, whereas wave-dominated water settings have very low rates of crab predation) (Robels 1987; Robels et al. 1989). We found that there was a strong inverse relationship between repair scar frequency and the energy in the system. In other words, we found the highest rates of predation in quite water settings, where there were more crabs, which corroborated our hypothesis that repair scars accurately reflect the rate of predation in a system.
Knowing that repair scars are an accurate proxy for the rate of predation allows us to ask all sorts of interesting questions, both in the modern, and in the fossil record. We can answer simple, but important questions about how many predators there were in a system (Stafford et al. 2014), or how and why the rates of predation differ between places or time (Leighton, 2002). We can also look at more specific problems, such as whether reaching a certain size can prevent a predator from being able to crush you (Richards and Leighton 2012), or how the development of spines can make it impossible for crushing predators to grab prey (Linge-Johnsen et al. 2013), and what it all might mean for the evolution of both predator and prey (Vermeij 1987).
I am actually going to be looking at predation and repair scars on a couple of different snails and their ancestors for much of my dissertation, and will be using things like repair frequency to help me determine predation pressure along the modern and recent coast of the Pacific Northwest, and throughout the Plio-Pleistocene. My goal is to see how ocean acidification affects interactions between predators and prey through time, so tracking changes in repair scars will be helpful might indicate if there are changes in predation rates, or at least failed predation.
Leighton, L. R. 2002. Inferring predation intensity in the marine the fossil record. Paleobiology. 28:328-342.
Leighton, L. R., Chojnacki, N. C., Stafford, E. S., Tyler, C. L., and Schneider, C. L. 2016. Categorization of shell fragments provides a proxy for environmental energy and predation intensity. Journal of the Geological Society. 173:711–715.
Linge-Johnsen, S. A. Ahmed, M., and Leighton, L., R. 2013. The effect of spines of a Devonian productide brachiopod on durophagous predation. Palaeogeography, Palaeoclimatology, Palaeoecology. 375:30–37.
Molinaro, D. J., Stafford, E. S., Collins, B. M. J., Barclay, K. M., Tyler, C. L., Leighton, L. R., 2014. Peeling out predation intensity in the fossil record: A test of repair scar frequency as a suitable proxy for predation pressure along a modern predation gradient. Palaeogeography, Palaeoclimatology, Palaeoecology. 412:141-147.
Richards, E. J., and Leighton, L. R. 2012. Size refugia from predation through time: a case-study of two Middle Devonian brachiopod genera. Palaeogeography, Palaeoclimatology, Palaeoecology. 363 – 364:163-171.
Robels, C. 1987. Predator Foraging Characteristics and Prey Population Structure on a Sheltered Shore. Ecology. 68:1502 – 1514.
Robels, C. Sweetnam, D. A., Dittman, D. 1989. Diel variation of intertidal foraging by Cancer productus L. in British Columbia. Journal of Natural History. 23:1041 – 1049.
Stafford, E. S., and Leighton, L. R. 2011. Vermeij Crushing Analysis: A new old technique for estimating crushing predation in gastropod assemblages. Palaeogeography, Palaeoclimatology, Palaeoecology. 305:123 – 137.
Stafford, E. S., Tyler, C. L., and Leighton, L. R. 2014. Gastropod shell repair tracks predator abundance. Marine Ecology. 36:1176 – 1184.
Vermeij, G. J. 1987. Evolution and Escalation. Princeton University Press. Princeton, New Jersey.