The giant 15-kilometer-wide (9.3-mile) asteroid that smashed into Earth 66 million years ago may have been terrible news for the (non-avian) dinosaurs, but it was good news for sharks, at least as far as shark diversity is concerned. The astronomical event that marked the dinosaurs' final death knell also triggered an explosion in (some) shark species.
Palaeontologists from Uppsala University in Sweden and the University of New England in Australia studied the size and shape of almost 600 shark tooth fossils from around the time of the end-Cretaceous mass extinction. The results, published in the journal Current Biology, indicate that post-asteroid, an order of shark called Carcharhiniformes, or ground sharks, supplanted the Lamniformes (aka mackerel sharks), which had dominated the waters during the reign of the dinosaurs.
Shark skeletons are made from cartilage, a material that is notoriously hard to preserve. Therefore, palaeontologists often focus their research on fossilized shark teeth, which are discarded over an individual shark's lifetime and tend to last much longer.
Those used in this particular study were between 72 million and 56 million years old, from both before and after the asteroid struck. To determine what particular order of shark a tooth belonged to, the palaeontologists examined its breadth and the height of the crown – a triangular-shaped tooth, for example, is suggestive of a large-prey-eating animal, whereas a long, thin tooth is much better for eating fish.
The results suggest that flightless dinosaurs weren't the only animals to face mass extinction. Sharks, too, experienced huge losses, in particular, a group of mollusk-munching (now-extinct) fish called anacoracids. But unlike relatives of the Brontosaurus and Diplodocus, certain types of sharks – namely Carcharhiniformes – flourished in their new environment. Carcharhiniformes are the largest order of shark alive today, with more than 250 species including the tiger shark, blacktip reef shark, and utterly bizarre-looking hammerhead shark. Lamniformes like the great white, goblin shark, and incredibly rare megamouth shark were once the dominant group, but now number just 15 species.
So, why is this?
There is no conclusive explanation yet, but the study authors suspect it has something to do with the food supply. The climate change sparked by the asteroid wiped out species across the board, including marine reptiles and cephalopods, which may have been the ancient Lamniformes' preferred food types. Carcharhiniformes instead fed on small bony fish, a type of animal that saw populations increase after the end-Cretaceous mass extinction. It is also possible that the loss of apex predators (the Lamniformes and marine reptiles) benefited the smaller Carcharhiniformes.
"Going into this study, we knew that sharks underwent important losses in species richness across the extinction," said Nicolás Campione of the University of New England in a statement.
"But to our surprise, we found virtually no change in disparity across this major transition. This suggests to us that species richness and disparity may have been decoupled across this interval."
Further research is needed to understand what happened to other shark orders, such as the Squaliformes and Pristiophoriformes. But Campione and his colleagues hope this research on sharks' evolutionary history will help aid shark conservation efforts today, when roughly 50 percent of species are listed by the IUCN as endangered, threatened, or near-threatened due to issues like overfishing.
