Each year, the discovery of new shark species underlines how little we know about ocean biodiversity. In a recent report conducted by Sibert and Rubin, they reported an unexpected finding: a large-scale extinction of sharks in the pelagic ocean, the largest ecosystem on Earth, about 19 million years ago. Their discovery suggests that some extinctions in the open sea of the past may have been mysterious.
The study of Sibert and Rubin takes advantage of a system by using tiny, hard bits of shark skin and bony fish teeth that naturally fall from their bodies to the seafloor. These extremely small fossils provide a rich record of ancient oceanic sharks and their abundance for thousands of years. By studying fossils from many regions, the diversity patterns can give major insights into evolution (进化) of the open sea that would be otherwise unknown.
Sibert and Rubin quantified(量化)a past extinction of sharks, reporting a 90% decline in number and >70% drop in diversity. They found that nearer-shore sharks appear to survive, but migratory ones go extinct. The finding of this study is that sharks had undergone a widespread extinction that reorganized their communities from 16 million to 20 million years ago.
Sibert and Rubin narrowed the disappearance of sharks to a window of time under 100,000 years around 19 million years ago, but the causes of this event remain unclear. Because this time period does not stand out as a period of major climatic change, the authors do not attribute(归因于) environmental factors as an extinction driver. As for other causes, the loss of shark diversity is directly linked to overfishing.
The loss of sharks from the oceans has profound, complex, and unavoidable ecological consequences because their presence reflects the stability of marine ecosystems. Yet, one-quarter of the global diversity of sharks is currently threatened with extinction. Despite recent improvements in conservation actions, shark communities never recovered from a mysterious extinction event 19 million years ago; the ecological fate of what remains is now in our hands.
