a picture of extinct specie of a shark
According to recent research, a large number of sharks died suddenly 19 million years ago. Fossils from the Pacific Ocean’s strata show that 90 percent of them disappeared. So yet, scientists have no idea why.
Sharks Dying Off
“It’s a fantastic mystery,” Elizabeth Sibert adds. She was in charge of the new study. At Yale University she is a paleobiologist and oceanographer. That is in New Haven, Connecticut. “For 400 million years, sharks have lived on the earth. Nevertheless, the disaster wiped off 90% of them.
Sharks have been defeated in the past. It began during the Great Dying 250 million years ago. Most large ocean species perished as a result of this catastrophe. Much later, around 66 million years ago, a massive asteroid collided with Earth. It wiped off most dinosaurs, as well as 30 to 40% of shark species. Following then, sharks dominated the seas for around 45 million years. They even survived significant climatic disturbances, such as 56 million years ago when global carbon dioxide levels rose, and temperatures skyrocketed.
The freshly found fossils represent an unexpected twist in the shark’s narrative. Sibert combed over the silt for fish teeth and shark scales. She collaborated with Leah Rubin, a College of the Atlantic student in Bar Harbor, Maine. That material had been collected by scientists on numerous voyages to the North and South Pacific seas. “The study arose from a desire better to understand the natural background diversity of these fossils,” adds Sibert.
Sharks’ bodies are primarily made of cartilage. Unlike bone, cartilage is challenging to fossilize. Sharks, on the other hand, have skin coated with microscopic scales. Each scale is approximately the size of a human hair follicle. These scales provide a good record of previous shark abundance. They are made of the same rugged material as shark teeth. Both can get fossilized in sediments. “And we will find hundreds more [scales] than a tooth,” Sibert continues.
What her team uncovered surprised them. Between 66 million and 19 million years ago, the ratio of fish teeth to shark scales remained constant at around 5 to 1. The ratio then underwent a drastic swing, with 100 fish teeth appearing for each shark scale. According to the scientists, the shift occurred abruptly – within 100,000 years or so.
The rapid loss of shark scales coincided with a shift in the form of the scales. This reveals information on shark variety.
Most contemporary sharks have lined grooves in their scales, which may aid in their swimming speed. The scales of other sharks have geometric forms. The researchers examined the shift in the abundance of distinct scale forms before and after 19 million years ago. This indicated a significant decline in shark diversity. It indicates that seven out of every ten shark species have become extinct.
And, as Rubin points out, this extinction event was extremely “selective.” The geometric scales “were nearly gone” after the occurrence. She says that the prior variety in sharks was never observed again. She and Sibert published their findings in Science on June 4th.