It seems the Chicxulub asteroid that slammed into our planet some 66 million years ago killing all dinosaurs except the avian ones also changed shark evolution forever.
According to a new study, today’s sharks owe their incredibly rich diversity to a mass extinction event occurring at the end of the Cretaceous.
Published this week in the journal Cell Biology, the researchers examined hundreds of fossilized teeth belonging to two major shark lineages and uncovered that the rulers of the Cretaceous oceans are now virtually extinct.
Known as Lamniformes, this ancient shark order dominated the seas between 142 million and 66 million years ago, but saw an abrupt end that coincided with the Cretaceous-Paleogene extinction event, reports Phys.org.
Also known as Mackerel sharks, Lamniformes are barely present in today’s oceans, where they have been outnumbered by Carcharhiniformes — one of the most diverse groups of sharks in existence.
“Our study found that the shift from lamniform- to carcharhiniform-dominated assemblages may well have been the result of the end-Cretaceous mass extinction,” said study lead author Mohamad Bazzi, a paleontologist at Uppsala University in Sweden.
The same mass extinction event that shaped the future of sharks also triggered critical changes in mammalian evolution, turning on the lights for the predominantly-nocturnal mammals of the time and ushering them into a new dawn, the Inquisitr previously reported.
“..study found that the shift from lamniform- to carcharhiniform-dominated assemblages may well have been the result of the end-Cretaceous mass extinction”
The end-Cretaceous extinction unleashed modern shark diversity https://t.co/khbQvb5wHt via @UU_University @EurekAlert pic.twitter.com/Pk2NnKQXUj
— Michael Petter (@MikPetter) August 3, 2018
The team arrived at their conclusion after analyzing 597 teeth fossils dated between 72 million and 56 million years ago and belonging to ancient sharks from all across the globe, reports Gizmodo.
While complete shark fossils are a real challenge to come by, fossilized shark teeth are easier to find, notes Bazzi.
“Unlike other vertebrates, the cartilaginous skeletons of sharks do not easily fossilize and so our knowledge of these fishes is largely limited to the thousands of isolated teeth they shed throughout their lives,” he points out.
But teeth fossils are more than enough to reveal many important aspects of shark biology, including details about their diet, says Bazzi. All these precious details could help researchers figure out why some shark species die out while others survive and how to keep the currently endangered species from going extinct. More than 50 percent of shark species worldwide are currently either endangered, threatened, or near-threatened, show the researchers.
Study co-author Nicolás Campione, a paleontologist at the University of New England in Armidale, Australia, also chimed in on the matter.
“By studying their teeth, we are able to get a glimpse at the lives of extinct sharks, and by understanding the mechanisms that have shaped their evolution in the past, perhaps we can provide some insights into how to mitigate further losses in current ecosystems.”
After looking at the fossils’ shapes and sizes, the team discovered that Lamniformes with wide, triangle-shaped teeth became extinct. Meanwhile, the Carcharhiniforms with the exact same tooth type flourished and went on to have a booming evolution, giving rise to more than 200 species.
The Carcharhiniforms that prowl today’s oceans include hammerheads, tiger sharks, and blacktip reef sharks, while the modern descendants of the prehistoric Lamniformes include the great white shark and the mako shark are numbered among.
“Carcharhiniforms are the most common shark group today and it would seem that the initial steps towards this dominance started approximately 66 million years ago,” says Bazzi.
One possible explanation for that is the availability of food. The study reveals that the Chicxulub asteroid killed off a large number of marine reptiles and cephalopods, including the ones the Lamniformes fed on.
This freed up the oceans for small, bony fish species, which were the targeted prey of Carcharhiniform sharks and which also proliferated after the mass extinction and the end of the Cretaceous. Therefore, it’s entirely possible that Carcharhiniforms may have survived this mass extinction event because their prey also made it through, argues the new research.
“It is likely that the loss of apex predators (such as Lamniforms and marine reptiles) benefited mid-trophic sharks, a role fulfilled by many carcharhiniforms,” states a news release by Uppsala University.