The 500 Million Year Rise of ‘Bullies’
COLUMBIA, Mo. --- The drill holes left in fossil shells by hunters such as snails and slugs show marine predators have grown steadily bigger and more powerful over time but stuck to picking off small prey, rather than using their added heft to pursue larger quarry, new research shows.
The study, published today in Science, found the difference in size ratios between marine predators and tough-shelled prey such as scallops and clams increased 67-fold over the past 500 million years. The study’s authors, including John Huntley from the University of Missouri, say the widening gap could be caused by greater numbers and better nutritional value of prey species and perhaps to minimize predators’ vulnerability to their own enemies.
Drilling predators such as snails, slugs and beetles penetrate their prey’s protective skeleton and eat the soft flesh inside, leaving behind a telltale hole in the shell. Trillions of these drill holes exist in the fossil record, providing valuable information about predation, or the action of attacking ones’ prey, over millions of years. But while drill holes have been used extensively to explore questions about the intensity of predation the authors realized they also could shed light on predator-prey size ratios.
“As paleobiologists, we are fortunate to have this interaction between predator and prey preserved as drill holes in shells,” said Huntley, an assistant professor of geological sciences in the MU College of Arts and Science. “Overall, it is exceedingly rare for direct evidence of predator-prey interactions to be preserved in the fossil record, but here, as my colleagues and I have demonstrated, not only do we see this interaction preserved in shells, but we can extract key information like body size, of both predators and prey, from drilled shells.”
Predation is a major ecological process in modern ecosystems, but its role in shaping animal evolution has been contentious. This study sheds light on predation’s ability to drive evolutionary changes by supporting a critical tenet of the escalation hypothesis: the idea that top-down pressure from increasingly larger and stronger predators helped trigger key evolutionary developments in prey species such as defensive armor, better mobility and stealth tactics like burrowing into the sea floor.
The team used data compiled from 6,943 drilled animals representing many fossil species to examine trends in the size of drill holes, prey size and predator-prey size ratios, starting in the Cambrian Period—when most marine organisms appeared—and running to the present.
Despite growing bigger, predators may not have needed to switch to larger targets because prey became more nutritious through time, the researchers said. In the Paleozoic Era, about 541 million to 252 million years ago, clam-like organisms known as brachiopods were the most common prey available. But predators gained few nutrients from brachiopods and gradually transitioned to mollusks, similarly-sized but meatier prey that became abundant in oceans after the Paleozoic.
“It is particularly interesting that the predators increased in size through time but the prey did not,” Huntley said. “The identities of the dominant prey items changed through time and we know that individuals of a given size became more nutritious over that time span. This work confirms a central tenet of the Escalation hypothesis, that predators have become larger and more powerful through time. Our work gives new insight into the question of whether or how ecological processes, the daily struggle for life, influence macroevolutionary processes in deep time, which is a significant question still being examined.”
The study, “Increase in predator-prey size ratios throughout the Phanerozoic history of marine ecosystems,” was published in the journal, Science. Co-authors on the study include Adiël A. Klompmaker, a postdoctoral fellow at the University of California, Berkeley; Michal Kowalewski, a professor and Jon L. and Beverly A. Thompson Chair of Invertebrate Paleontology at the University of Florida; and Seth Finnegan, an assistant professor of integrative biology at the University of California, Berkeley. Funding for the research came from the National Science Foundation, the Packard Foundation and the Jon L. and Beverly A. Thompson Endowment Fund. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agency.