19 million years ago the sharks had a bad time

19 million years ago the sharks had a bad time
19 million years ago the sharks had a bad time

In history it has happened several times that many animals died in a short time, in some cases leading to the extinction of most of the living species. The best known of these mass extinctions was the one that led to the disappearance of the dinosaurs 66 million years ago, but there were four others of similar magnitude and others of even smaller size. And a new one has just been discovered, announced by an article published in the authoritative magazine Science: 19 million years ago, during the period called Miocene, more than 90 percent of the sharks died and more than 70 percent of the species in which these fish were divided at the time disappeared.

This was discovered by Elizabeth C. Sibert, an oceanographer and paleontologist at Yale University, and a younger collaborator, the ichthyologist Leah D. Rubin, and they did it thanks to two large piles of mud.

In 2015, Sibert began studying two sediment samples – the mud piles – taken thousands of kilometers away from each other from the depths of the Pacific Ocean. Inside the samples were layers of fossil remains of all kinds, dating back to an interval of 40 million years. Among the others, there was a large amount of shark dermal denticles, that is the microscopic scales with which the bodies of sharks are entirely covered (eyeballs included): sharks change them constantly, so they are very common among fossils.

By isolating the fossil dermal denticles and tracing them back to different periods of hundreds of thousands of years, Sibert noticed at one point a sharp decrease in their number: in the sediment layers prior to 19 million years ago he had found about one dent for every five teeth. of fish, in the following layers much less.

This reduction in denticles indicates that at some point in the Miocene the entire shark population declined by more than 90 percent. The fact that the reduction was found in sediments extracted from two very distant points of the Pacific seabed further suggests that what caused it was a worldwide phenomenon.

In a later phase of his research Sibert tried to understand whether the large decrease in the number of sharks was also due to the extinction of part of the species existing at the time. To understand this, she and Rubin classified the dermal denticles found in sediments into 88 different groups, based on their shape and orientation: the groups do not each correspond to a species, but still allow us to get an idea of ​​the biodiversity between species. of sharks in a given geological period.

Of the 88 groups of denticles found in sediments prior to 19 million years ago, only 9 were also found in later layers: this data allowed Sibert and Rubin to hypothesize that about 70 percent of shark species became extinct there. period.

It is not known what caused the great shark death. Sibert could only rule out that the mass extinction occurred due to global climate change because there was none at that time. What is known is that after what happened 19 million years ago the sharks never really recovered: there have not been as many as there were before, judging by the fossil dermal denticles. And there may be fewer and fewer: since 1970 the number of sharks in the world has decreased by 71 percent due to overfishing.

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