Over 19 million years ago, the world's open oceans were completely teeming with sharks, approximately ten times more than today.All of a sudden these large marine predators nearly all disappeared.
Ancient Shark Denticles
This devastating and strange mass extinction event was discovered not too long through a series of accidental investigations, and from the proof gotten so far, what led to the sudden end to so many species is still unclear .
Elizabeth Sibert, a paleoceanographer from Yale University explained: "I study the teeth of microfossil fish and scales of sharks in deep-sea sediments, and we made a decision to generate an 85-million-year-long record of fish and abundance of shark, just to get a sense of what the typical variability of that population looked like in the long term."
When the team made a comparison of the ratio of ancient shark denticles - little V-shaped skin coverings that looks like teeth more than scales - to other fish teeth embedded up to 5,700 meters deep in the seafloor, they identified a clear shift in ocean life taking place about 19 million years ago.
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Cretaceous-Paleogene Extinction Event
Prior to this time, sediment samples held an abundance of denticles and teeth, which fall off fish bodies naturally and alight on the seafloor. Following this point, however, just a third of the samples have any evidence of shark denticles. In the early Miocene, between about 16 and 20 million years ago, open ocean sediments went from containing one shark fossil for each five fish fossils to one shark fossil for each 100 fish fossils.
This unanticipated drop-off in shark abundance is times two as large as what was discovered for the Cretaceous-Paleogene extinction incident, which saw the death of three-quarters of all animal and plant life about 66 million years ago. The existence of ancient denticles in ocean sediment is just a proxy for the number of sharks that previously swam in our oceans, but the same unexpected shift in pattern was noticed in multiple regions all over the world.
Decrease in Shark Abundance
In sediment hearts from both the North Pacific and the South Pacific, researchers discovered evidence for a sharp decrease in shark abundance, evaluated to be over 90 percent. During this time the assortment of sharks swimming in the world's oceans also took a dive, declining by more than 70 percent.
After this unexpected transformation, which probably took place in less than 100,000 years, the diversity of sharks in Earth's oceans never remained the same. While many species of open ocean sharks vanished during this extinction event, coastal sharks were quite luckier. Lineages of today are mainly derived from those survivors. In sediment samples deposited following the extinction event, researchers discovered no new types of shark denticles, which proposes few shark species have emerged since.
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