Megalodon, the gigantic shark that makes Jaws look like a trout, no longer swims the seas.
Today, the extinct 50-foot-long predator remains a figure in works of fiction and the speculations of cryptozoologists and also, more helpfully, in the fossil record, along with the mineralized remains of lesser sharks, marine mammals, corals, and lots and lots of shellfish.
William & Mary paleontologist Rowan Lockwood is a member of a group that has examined 23 million years of the fossil record of marine organisms and mapped out a set of extinction patterns in a pre-human world. Their paper, “Paleontological baselines for evaluating extinction risk in the modern oceans,” was published in the May 1 issue of the journal Science.
Lockwood, an associate professor in the university’s Department of Geology, explained that the paper is essentially a description of the long-term loss of natural species in pre-human oceans, global extinctions tracked through the fossil record, then modeled mathematically. Lockwood said that the 23 million-year range of the study represents a relatively stable period of life on earth—no big meteorite strikes, continental rearrangements or other large-scale cataclysmic events known to be mass-extinction triggers. And no humans.
“The idea is to look at extinctions without us,” she said. “The goal is to use that examination of the fossil record to predict what organisms are likely to go extinct in the future, what ones are at higher risk.”
Lockwood points out that the Science piece describes the natural evolution-and-extinction cycle, forces that continue to play out in the oceans today. Anthropogenic forces such as overharvesting, climate change and point-source pollution make the circle of life today quite a bit different than in the past.
“In our paper we look at what we call ‘background extinction,’ the constant turnover of species,” she explained. Lockwood cited recent research by other scientists quantifying the effects of humans on the oceans. When it comes to the ability to damage the oceans, even near-legendary megapredators of ages past can’t match Homo sapiens.
“If you know what those background extinction rates are, you can compare what’s happening today with that,” she said. “You can say, OK, how does the human compare to the Megalodon shark? And the answer is that we’re much, much, much, much worse. We’re much more effective predators than anything that has come before.”
Lockwood noted that the examination of background extinction reveals that some of the life forms in today’s oceans are quite similar to the seas of the remote past.
“A lot of the organisms that are living in the ocean today have close cousins that went extinct 20-some million years ago,” she explained. So, as a species of clam went extinct, its niche in the ecosystem was usually filled by another, closely related, species of clam that had evolved in a way to outcompete the extinct bivalve. Snails that evolved an adaptive advantage replaced other snails, and so on.
There were exceptions, though. For example, Lockwood said that orcas and other toothed whales largely filled the niche vacated by Megalodon, which went extinct some three million years ago, “and we’re all safer for it,” she adds.
Some species persisted, too. Lockwood and her collaborators who were working with shellfish had to sort through several instances in which they found the same species that had been assigned different names, one to describe modern specimens and another for those in the fossil record.
The record of background extinction, compared with studies of modern oceans, also reveals some especially troubling aspects of the effects of human activity.
“What we’ve found, ironically, is that a lot of the areas where we’re overharvesting are naturally extinction-prone,” Lockwood said. Comparison of modern oceans with the background extinction data shows similar concentrations, she explained, essentially an unfortunate magnification of anthropogenic effects that increases the extinction rate in fragile areas such as tropical oceans.
“We wouldn’t be seeing this level of extinctions without humans. Humans are having a disproportionate impact on extinctions,” she said. “It’s useful to have this background data, because it’s telling us that humans are not just another part of nature. Our effect is much greater than you would expect a single species to have on an ecosystem.”
The accelerated, human-driven, extinctions matter. Lockwood said that the effects of overharvesting are generally concentrated near the top of the food chain, as humans have made severe dents in the populations of whales, sharks and other fish.
“Ecosystems need these top predators to function,” she said, going on to explain that human self-interest in slowing excess extinctions extends beyond academic concerns about ecosystems.
“There are all these animals out there in the oceans that we don’t know much about. They could be valuable to us in the fight against cancer or in some other medical aspect,” Lockwood said. “We haven’t really evaluated any of them. And we’re losing them.”
Lockwood is a 2015 recipient of a Plumeri Award, a stipend-bearing honor recognizing innovation and creativity among William & Mary’s faculty. She and her co-authors are members of the Determinants of Extinction in Ancient and Modern Seas Working Group supported by the National Evolutionary Synthesis Center, funded by the National Science Foundation.Lead author on the Science article is S. Finnegan of the University of California, Berkeley. Other collaborators are S.C. Anderson, Simon Fraser University; P.G. Harnik, Franklin and Marshall College; C. Simpson, National Museum of Natural History; D.P. Tittensor and H.K. Lotze, Dalhousie University; J.E. Byrnes, University of Massachusetts; Z.V. Finkel, Mount Allison University; D.R. Lindberg, University of California, Berkeley; L.H. Liow, University of Oslo; C.M. McClain, National Evolutionary Synthesis Center; C.L. McGuire, University of Washington; A. O'Dea, Smithsonian Tropical Research Institute; and J.M. Pandolfi, University of Queensland.