The largest sharks ever to hunt in Earth’s oceans may have gotten so big thanks to their predatory behavior in the womb, scientists report October 5 in Historical Biology.
The idea emerged from a study that first analyzed the sizes and shapes of modern and ancient shark teeth, using those data to estimate body sizes of the fish. Paleobiologist Kenshu Shimada of DePaul University in Chicago and colleagues focused on an order of sharks called lamniformes, of which only about 15 species still exist today, including fierce, fast great white and mako sharks as well as filter-feeding basking sharks (SN: 8/2/18).
Well over 200 lamniform species existed in the past, some of them quite large, Shimada says. But none is thought to have rivaled Otodus megalodon, commonly called megalodon, which lived between about 23 million and 2.5 million years ago. Determining just how giant these creatures were is challenging, though, because sharks’ skeletons are made of cartilage, not bone, and little remains of now-extinct species but their teeth. However, those teeth are abundant in the fossil record: A single shark can shed tens of thousands of teeth in its lifetime (SN: 8/10/18).
Shimada and his colleagues found that the height of megalodon’s tooth crowns was an extreme outlier among their data, suggesting a total body length of at least 14 meters, twice as long as any other shark that isn’t a filter feeder. But four other extinct species of lamniforms exhibited “gigantism,” growing to over six meters long — not megalodon-scale, but still quite large, Shimada says. Gigantism also occurs in several modern species, including great white, mako and thresher sharks.
The study “gives a broad overview on the relationships between tooth, jaw and body size in an important number of lamniform fossil lineages,” says Humberto Férron, a paleobiologist at the University of Bristol in England.
Yet why megalodon and its relatives could get so big remains unclear. The extinct and modern lamniform species that can grow to these sizes also all happen to be warm-blooded. Regulating body temperature enables them to swim faster and catch more energetic prey. So warm-bloodedness, or endothermy, may be one key to their gigantism.
But Shimada and colleagues felt that explanation was incomplete, because it didn’t address why this group of sharks in particular might have developed the endothermy that led to gigantism. So, in the new study, the team suggests that a behavior unique to this order may also play a role — a kind of cannibalism that occurs in the womb.
Sharks in general have a reproductive strategy known as ovoviviparity: The embryos develop inside eggs that stay inside their mothers until they are ready to hatch. But ovoviviparity turns extreme among all lamniform sharks — from the fierce predators to the gentle filter-feeders. The first shark pup to hatch inside the mother proceeds to eat the rest of the eggs, a behavior called intrauterine cannibalism. By the time the pup emerges from its mother, it’s already quite large and ready to defend itself against predators.
That behavior, combined with the right environmental conditions such as favorable water temperatures and the availability of food, may give some lamniforms the green light to grow gigantic, the researchers say.
It’s an interesting, out-of-the-box idea, says Stephen Godfrey, a paleontologist at Calvert Marine Museum in Solomons, Md. It’s possible that intrauterine cannibalism is linked to some lamniforms becoming warm-blooded in the first place, he says. It could help these sharks grow big enough to take on bigger prey — and, in turn, require more energy, such as that provided by an evolutionary adaptation like warm-bloodedness, to maintain such an active lifestyle.
But it still doesn’t quite explain the unique super-gigantism of megalodon, Godfrey adds. For that, you also would need a food source. “If there had been no large prey, I very much doubt that there would have been macro-predatory giant sharks,” he says.
Férron agrees. “The idea is novel,” he says. “In my opinion, the evolution of gigantism in megalodon was the result of a combination of factors,” he says. The reproductive strategy may have helped them grow big, endothermy would have kept them active, and abundant large prey would have kept them fed.
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