A bang in the history of sharks is emerging: around 19 million years ago, the world population of these predatory fish collapsed by 90 percent and more than half of the species disappeared, according to a study of microfossils in sediment cores. What caused the dramatic mass extinction remains a mystery, as there are so far no indications of major ecological changes from that time. A similar collapse in global shark populations can be seen again today, the researchers emphasize – but with the clear cause of humans.
Which population and species fluctuations have there been in the fish in the open sea over the course of millions of years? This fundamental question was at the beginning of the research project that ultimately led to the surprising finding in sharks, report Elizabeth Sibert from Harvard University in Cambridge and Leah Rubin from the College of the Atlantic in Bar Harbor. “We first decided to make an 85 million year record of fish and shark abundance just to get a feel for what normal population variability looked like over the long term,” says Sibert.
To do this, the researchers examined fish teeth and shark scales in drill cores made from deep-sea sediments from two distant locations in the Pacific. These are fossil remains that, in contrast to other body structures of the fish, are well preserved and, despite their small size, can be assigned to different species or groups based on structural peculiarities. “When we examined the microfossils from the different layers of the drill cores, we first came across indications that there was once a sudden and drastic decline in the sharks, and then we followed up this trail in a targeted manner,” says Sibert.
Microfossils document a break-in
The previously unknown mass extinction in the Miocene era finally took on ever clearer contours: The sudden drop in the frequency of shark scales in the sediments showed that the shark populations collapsed by around 90 percent around 19 million years ago. Apparently these predatory fish were particularly affected: the extinction is characterized by the traces of an excessive decrease in the ratio of sharks to other fish, the populations of which have apparently remained largely stable.
The comparisons of the structural features of the microfossils before and after the collapse of the shark populations showed that more than 70 percent of the shark species were extinct at that time. In this group of animals, the loss of species was twice as high as 66 million years ago, when, in addition to dinosaurs and other land animals, many marine animals also disappeared. The study results also show that the extinction wave 19 million years ago had a lasting impact on the diversity of sharks: Their diversity is now only a fraction of what it was before the incision, the data suggests.
Need for research and a critical look at today
As for the cause of the mass extinction, the study now puzzles science. Because so far no climatic catastrophe or disturbance of the ecosystem is known that occurred in the time of the sudden collapse of the shark populations. Sibert and Rubin therefore now hope that their results will provide the impetus for further research. As they emphasize, discoveries of extinction events have already led to insights into the background. In addition to the cause of shark dying, subsequent studies could shed light on other questions: “This work could provide the impetus for more detailed investigations into the cause of the lasting collapse in shark diversity, as well as the circumstances in the period in question and the implications for the development of modern ecosystems, ”says Pincelli Hull from Yale University, referring to the surprising results of his two colleagues.
Leah Rubin concludes by moving from the past to the present, as there are relevant parallels: “The current declines in shark populations are worrying and this work helps to put these developments into the context of the history of shark populations over the past 40 million years to deliver. This is important for assessing the effects of today’s losses, ”says the scientist. In a comment on the study, Catalina Pimiento from the University of Zurich and Nicholas Pyenson from the Smithsonian Institution in Washington DC also emphasize the significance of the study for the present: “The shark communities of the open sea have apparently never recovered from the mysterious extinction event 19 million years ago . The ecological fate of what was left is now in our hands, ”write the scientists.
Source: Yale University, Article: Science: 10.1126 / science.aaz3549