How Cannibalism in the Womb May Have Made Megalodon a Titanic Terror

Science
یکشنبه 13 مهر 1399 ساعت 23:24
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Smith sonian mag
There’s never been a bigger carnivorous shark than Otodus megalodon . At a maximum body size of 50 feet long, this ancient mako relative was the largest shark ever to chomp its way through the seas. No other shark species, even among its close relatives, grew quite so large. But how did megalodon become so exceptional?
A new study, published today in Historical Biology by DePaul University paleontologist Kenshu Shimada and colleagues, suggests that cannibalism in utero may have helped set up the rise of the largest meat-eating shark of all time. The researchers suggest that a biological connection existed between having large, hungry babies, a metabolism that ran warm and increases in size—with the appetites of baby sharks driving their mothers to eat more and get bigger, which led the babies to get bigger themselves.
Shimada and colleagues focused on the size of existing lamniform sharks, using measurements of today’s makos and their relatives to estimate the size of prehistoric sharks. By figuring out how body size relates to tooth size, the researchers were able to look at the fossil teeth of various extinct sharks and come up with refined estimates of how big those prehistoric fish were.
Most of the sharks were comparable in size to lamniformes alive today. Only four lineages of ancient lamniform shark got to be more than 20 feet in length, with Otodus megalodon being an extreme outlier at more than twice that maximum length. “We expected megalodon to be gigantic,” Shimada says, “but what surprised us was actually seeing in our data a 23-foot-gap between the size of megalodon and the size of the next largest carnivorous lamniform sharks.”
Kenshu Shimada and team created a chart showing the maximum sizes of sharks, with megalodon being the largest. ( Kenshu Shimada)
Part of what allowed megalodon to get so big has to do with the fact that many lamniform sharks have warmer body temperatures than other sharks. A great white shark isn’t warm-blooded in the same way that we are, for example, but the great fish can maintain some parts of its body at higher temperatures than the surrounding water thanks to specialized blood vessels that help retain and distribute the heat created by the contractions of the shark’s muscles. Scientists know this as mesothermy, and it’s likely that sharks like megalodon were mesotherms, too.
Running warm may have given the ancestors of megalodon and other lamniform sharks a route towards body sizes impossible for other species. The physiological difference allowed lamniform sharks to swim faster and feed in colder waters than other species. “The evolution of mesothermy is related with enhancing prey intake,” says Swansea University paleontologist Catalina Pimiento, who was not part of the new study. A warm, energetic shark needs more food than a slow, cold-running one, and large prey like blubber-rich seals offer a more economical way to feed. The largest predatory shark alive today, the great white shark, frequently feeds on marine mammals because its body requires such rich food.
But how did lamniform sharks evolve warm metabolisms in the first place? Shimada and colleagues suggest that competition in the womb, even cannibalism, had an important role to play.
Today’s lamniform sharks reproduce in a particular way. “Lamniform sharks don’t lay their eggs outside of the body, but instead the eggs hatch inside the mother,” Shimada says. From there, the pups develop until they’re ready to leave the womb. And they’re hungry. The little sharks that hatch early often eat unhatched eggs, and sometimes even their hatched siblings. And being that megalodon was a lamniform shark, it’s likely that the prehistoric giant’s babies would have acted like their modern counterparts.
A megalodon tooth fossil from the National Museum of Natural History's collection was found in North Carolina. ( Smithsonian Institution under CCO 1.0 )
Drawing from the relationship between physiology and reproduction in modern sharks, Shimada and coauthors propose that cannibalism in utero may have pushed these sharks to turn up the internal heat. Giving birth to a small number of large offspring may have required that mother sharks consume a greater amount of food, which may have been an evolutionary nudge towards mesothermy, with the needs of the babies and mother sharks opening a new evolutionary pathway. “This new paper suggests that intrauterine cannibalism may have been another driving mechanism for the evolution of mesothermy,” Pimiento says.
The relationship between the two doesn’t always work in lockstep, though. Pimiento notes that some sharks, like the sand tiger shark, are not mesothermic but still have cannibalistic embryos. These sharks are not open-ocean cruisers that target seals and whales, like megalodon did, but instead live a slower life along the coast and mostly dine on fish. The difference for megalodon is that the shark lived during a time when marine mammals thrived in the seas, their blubbery bodies providing a surplus of high-energy food. The possibility for giant predatory sharks was set in place by the needs of embryos and their mothers, and a surfeit of marine mammals offered an unprecedented opportunity to megalodon to grow far larger than any carnivorous shark before or since.
The path to larger sizes may have been led by those larger offspring. While it’s energetically costly for a mother shark to raise large embryos, Shimada says, those big babies would already have an advantage of being born large enough to hunt and avoid the jaws of many other predators. Add to that the fact that the number and size of pups varied between individual sharks and natural selection had the raw materials for larger and larger sharks to make their mark on the ocean when there was enough food to support such predators.
The task at hand is to find the critical evidence. While paleontologists have yet to uncover direct evidence for how many pups megalodon had or how many were birthed at a time, some rare shark fossils have been found with embryos. It’s possible such a find could help provide just that much more context to how the largest meat-eating shark of all time came to be. As much as we’re fascinated with enormous, whale-crunching megalodon, the sought-after clues may lie with baby sharks that beat the odds before even being born.
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