Dinosaurs, birds, and pterosaurs were not the only species to suffer extinctions at the hands of an asteroid that plummeted through the surface of the Earth 66 million years ago. Researchers have found that a mass extinction of sharks followed, wiping out most of what had been the dominant group of these ocean-going predators during the Cretaceous period.
While dinosaurs terrorized the land, giant marine reptiles and a great diversity of sharks patrolled the seas. Some of these sharks, in a group known as the anacoracids, fed on marine molluscs and reptiles, and, as scientists report today in the journal Current Biology, the loss of these prey to the asteroid may have been a contributing factor in their extinction.
“Subtle but important shifts may have laid the foundation for what later became the radiation of carcharhiniforms, or ground sharks, which are the most diverse lineage of sharks today,” says study co-author Nicolás Campione, a paleontologist at the University of New England in Armidale, Australia. “After the extinction, carcharhiniforms proliferated… and we hypothesize that changes in food availability played an important role in this shift.”
Shark skeletons are predominantly cartilage, which fossilizes poorly, making the study of ancient sharks difficult. Instead, the international group of authors behind the study looked to something more durable. They analyzed hundreds of shark teeth from fossil deposits left before and after the extinction event to see how the number and shape of teeth changed over time. In very general terms, wide, triangular teeth indicate a cutting function useful for eating large prey, while thinner, longer teeth suggest a grasping function important when eating fish.
“This paper makes a strong case for studying sharks and shark fossils as part of the end-Cretaceous extinction,” says William E. Bemis, curator of ichthyology at the Cornell Museum of Vertebrates in Ithaca, New York, who was not one of the study authors.
“Sharks offer a glimpse into changes in marine systems in particular, as groups of marine reptiles went extinct, some to be eventually replaced by marine mammals, while bony fishes explosively radiated,” he says.
There are two major groups of predatory sharks today. Carcharhiniform or ground sharks—including bull, tiger and hammerhead sharks—are the most numerous, represented by more than 250 species. Lamniform or mackerel sharks—such as great white, mako and grey nurse sharks—number only 15 species today.
But during the Cretaceous period this dominance was reversed, with mackerel sharks—in particular, a diverse group of great-white-like species called the anacoracids—being far more numerous than ground sharks. One fearsome and stocky species, Squalicorax, which reached up to 16 feet in length, may have relied on giant marine reptiles for sustenance.
Relatively smaller plesiosaurs and mosasaurs, including Plioplatecarpus and Prognathodon, were likely easy pickings for sharks such as Squalicorax. But one of the most common marine reptiles still present at the end of the Cretaceous period was Mosasaurus. At 56 feet long, a full grown Mosasaurus would likely have been too big to be threatened by a Squalicorax. In fact, this apex predator may itself have preyed upon sharks. But Campione says Squalicorax would likely have scavenged dead mosasaurs and hunted juveniles comparable to or smaller than itself in size. Their diet may also have included many large species of squid in coiled shells, known as ammonites.
But when the large prey these mackerel sharks relied upon abruptly vanished after the impact, so too did the sharks that fed upon them. As many as 34 percent of all shark species alive at that time are thought to have died out. Many of the sharks that eventually replaced them were fish-eating species that came to be the ancestors of most of the sharks in our oceans today.
“The end-Cretaceous extinction saw major losses in marine reptiles and cephalopods that would have been an important food source for anacoracids such as Squalicorax,” says Camipone. “The post-extinction world, however, saw the rise of bony fishes. So anacoracids, which would have relied on large marine organisms for sustenance, did poorly across the extinction, whereas predominantly fish-eating forms, such as smaller-bodied houndsharks, did well.”
It's interesting that the overall variety of tooth shapes is similar both before and following recovery from the end-Cretaceous event, comments Michael Coates, an expert on the evolution of fish at the University of Chicago. “As a loose metaphor, the shows goes on, but with a reshuffled cast…. the post-extinction replacements moving into, apparently, much the same eco-space formerly occupied by the extinction victims.”
“It's a nice study that adds detail to the emerging picture of how major groups of vertebrates suffered and recovered from the end-Cretaceous mass extinction,” he adds. “Inevitably, this study is limited to the record of shed teeth, shark body fossils being extremely rare. But teeth provide a useful marker of what kinds were present and persisted.”
Through the study of teeth, the researchers were “able to get a glimpse at the lives of extinct sharks,” says Campione. At least 50 percent of living shark species are threatened or declining, so understanding what led to shark extinctions in the past might offer insights into preventing them suffering the same fate, he adds.
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