Open-ocean sharks nearly vanished
New research provides evidence of a mysterious global shark extinction about 19 million years ago, and sharks never fully recovered.
Sharks were previously thought to be resilient to extinctions based on the available fossil record, deposits from relatively shallow water that lacked information on open-ocean species.
Now, researchers from the United States have found a new way to peer into the past using deep-sea sediment cores from the North and South Pacific oceans.
These cores contain microfossils from the last 40 million years, including fish teeth and shark dermal denticles, microscopic teeth-like scales that make up shark skin.
By counting the number of microfossils, Dr Elizabeth Sibert and Leah Rubin found one shark fossil for every five fish fossils.
Then around 19 million years ago an abrupt change took place, with one shark denticle found for every 100 fish teeth, or a 90% decrease.
“During this interval, sharks virtually disappeared from open-ocean sediments”, they write in their study published in the journal Science, “an event from which they never recovered”.
Sorting the dermal denticles by traits which vary between shark species, such as shape, Sibert and Rubin discovered that the number of shark species plummeted by over 70% at the same time.
“The transition was geologically abrupt, likely occurring in less than 100,000 years”, Sibert and Rubin continue, “followed by a permanent evolutionary and ecological suppression of the group to the present day”.
Although we do know that this extinction profoundly and permanently changed the structure of open-ocean fish and shark communities, we don’t know why.
The mystery deepens as there is no clear environmental reason for this abrupt decline, no evidence of an asteroid impact and strangely coastal species seemed to have survived.
Although we may not fully understand what happened in the past, with a 71% decrease in open-ocean shark numbers in the last 50 years and one-quarter of shark species currently threatened with extinction, we do know that history could repeat itself if we are not careful.