(CN) --- More than 70% of the shark population mysteriously died 19 million years ago according to new research released Thursday, and researchers have been investigating what may have caused this dramatic plummet in the ancient shark population since the discovery.
In a study published in the journal Science, lead author Elizabeth Sibert, Hutchinson postdoctoral associate in Yale's Department of Earth and Planetary Sciences and the Yale Institute for Biospheric Studies, and co-author Leah Rubin, an incoming doctoral student at the State University of New York College of Environmental Science and Forestry, discussed their findings on this ominous moment in the timeline of sharks' history.
"We happened upon this extinction almost by accident," Sibert said. "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."
Sharks have taken many forms over the course of their history, descendents of relatives who existed alongside the dinosaurs, and as such, they are often referred to as living fossils. They have an immense family tree chock full of diversity. They belong to a group called cartilaginous fishes, which means their bodies are made up of mostly cartilage, except for their teeth.
Their origins have been long disputed, but they are said to have begun evolving around 450 million years ago. They are among some of the most resilient species, living through each of the five mass extinctions. Despite this, they are not free from their vulnerabilities, as seen in their current declining numbers due to human activity.
During the event in question, Sibert says more than 70% of all sharks mysteriously died, with the most loss out in the open waters. This decline was surprisingly double the loss they suffered during the most devastating mass extinction on record, the Cretaceous-Paleogene, which famously eliminated the dinosaurs, along with 80% of all living things. The cause of this mass death is still unknown, as the authors add that there was no significant change in climate or ecosystem at the time that would’ve prompted a response this drastic.
"This interval isn't known for any major changes in Earth's history," said Sibert, "yet it completely transformed the nature of what it means to be a predator living in the open ocean."
Today the shark population is facing a similar decline, only this time the cause is quite clear. Human-caused climate change has dramatically impacted living conditions in the ocean, bringing warmer and more acidic waters, which in turn affects the availability of food, habitat and the survivability of young, vulnerable sharks.
The other biggest threat sharks currently face from human activity is overfishing. Unregulated fisheries capturing sharks for their fins are responsible for the death of over 100 million sharks every year, and many of these sharks are becoming trapped in nets that may not have even been targeting them. Overfishing of sharks is pushing many species closer and closer to extinction, which would be a big problem for the world’s oceans.
"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 Rubin. "This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times."
The extinction of sharks would be devastating for the ocean and its inhabitants. They are known as apex predators, which makes them absolutely invaluable in maintaining ocean health. Without them, their prey would skyrocket in numbers, upsetting the balance of the food chain and making resources scarce, ultimately resulting in the destruction of ecosystems.
This phenomenon has already been seen in the bay area of the United States, where the loss of a keystone predator made way for the sea urchin population to explode, which in turn resulted in the loss of the once beautiful and unique kelp forests.
The authors add that as extinction events continue to be brought to light, more research has ensued to understand the cause and effect of each disturbance. Moving forward, they hope to investigate why the number of sharks never fully recovered, as there were ten times more sharks in the water before this event than there exists today. They are also curious as to whether this die-off caused surviving shark populations to avoid the open ocean.
"This work could tip-off a race to understand this time period and its implications for not only the rise of modern ecosystems, but the causes of major collapses in shark diversity," said Pincelli Hull, assistant professor of Earth and planetary science at Yale. "It represents a major change in ocean ecosystems at a time that was previously thought to be unremarkable."
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