New York, Jun 6 (IANS): Nineteen million years ago, sharks nearly disappeared from Earth's oceans, according to a new study, which provides evidence for a previously unknown mass ocean extinction event.

Sharks as a species never recovered from this, said Earth scientists from Yale University and the College of the Atlantic.

The shark today represents only a fraction of what it once was, according to the study published in the journal Science.

The findings showed that sharks vanished from the record during the early Miocene roughly 19 million years ago, declining in abundance by more than 90 per cent and in morphological diversity by more than 70 per cent.

This puzzling extinction event appears to have occurred independently of any known global climate event or terrestrial mass extinction. While the drivers remain unknown, the team suggest that this event fundamentally altered pelagic predator ecology and subsequently set the stage for the large, migratory shark lineages that now dominate Earth's oceans.

"We happened upon this extinction almost by accident," said Elizabeth Sibert, lead author and postdoctoral associate in Yale's Department of Earth and Planetary Sciences.

"I study microfossil fish teeth and shark scales in deep-sea sediments, and we decided to generate an 85-million-year-long record of fish and shark abundance, just to get a sense of what the normal variability of that population looked like in the long term.

"What we found, though, was this sudden drop-off in shark abundance around 19 million years ago, and we knew we had to investigate further," Sibert said.

For the study, the team used microfossils in the sediment cores called ichthyoliths -- scales and teeth shed from sharks and other bony fishes that naturally accumulate on the seafloor. They constructed a record of shark diversity and abundance spanning nearly the last 40 million years.

"The current state of declining shark populations is certainly cause for concern and this paper helps put these declines in the context of shark populations through the last 40 million years," said Leah Rubin, a student at the College of the Atlantic at the time of the research.

"This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times," Rubin stated.