The Mysterious Decline of Sharks: A 19 Million-Year-Old Puzzle
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Chapter 1: The Ancient Survivors
Sharks have been a part of Earth's ecosystem for over 420 million years, enduring numerous catastrophic events that led to the demise of many species. However, recent research suggests that they faced a near-extinction event approximately 19 million years ago, as detailed in a study published in the journal Science.
The researchers, Elizabeth Sibert from Yale University and Leah Rubin from the State University of New York College of Environmental Science and Forestry, uncovered evidence of a significant mass extinction that decimated many shark species inhabiting the open oceans.
The team analyzed microfossils of shark scales, known as ichthyolith denticles, collected from mud samples taken from the seafloor in the North and South Pacific. These samples, retrieved from the upper 15 meters of sediment deposited over the last 40 million years, revealed a dramatic decline in both the abundance and diversity of shark scales during the Miocene epoch.
Initially, the ratio of shark fossils to fish fossils was one to five, indicating that sharks made up about 20% of the fish population. However, after 19 million years ago, this ratio shifted drastically, with only one shark denticle found for every 100 fish teeth. Given that these core samples originated from locations separated by over 2,000 miles, it appears this catastrophic event was global in nature.
“There seems to have been a major extinction event in the early Miocene, which knocked out about 90 percent of sharks in the open ocean,” states Sibert. This level of extinction is more than double that experienced by sharks during the Cretaceous-Paleogene extinction event, which occurred 66 million years ago and resulted in the extinction of non-avian dinosaurs.
Video Description: Sharks nearly went extinct 19 million years ago, and researchers are uncovering the evidence behind this mystery.
Further examination of the denticles revealed which shark species survived and which did not. Sharks like the great white, known for their long migrations along coastlines, possess denticles with parallel ridges that help reduce drag in the water. In contrast, species like cookiecutter sharks, which prefer to stay in specific ocean patches to ambush prey, have denticles with geometric patterns that resemble abstract art.
Notably, fewer species with geometric denticles survived this extinction, leading to the current scenario where only 53 open-ocean shark species remain, compared to the hundreds of coastal shark species.
Section 1.1: The Miocene Enigma
The reasons behind this mass extinction remain elusive. As Rubin notes, “There are no significant climate events during the early Miocene.” This period is characterized as a transition phase, with Earth gradually evolving into an “ice house” state, marked by the formation of a permanent ice sheet over Antarctica. Although the climate was warmer than today, the oceans were not drastically different, with the absence of whales and dolphins, which would not evolve for another two million years.
Could an environmental factor have triggered the decline of sharks? It's difficult to ascertain, as carbon and oxygen levels did not exhibit unusual fluctuations. More data is necessary to draw accurate conclusions.
“It seems that the extinction here is highly selective, as only sharks appear to be impacted, rather than pelagic groups more generally,” says Matt Friedman from the University of Michigan in Ann Arbor. Selective extinctions are documented throughout geological history, and it is still premature to speculate, but the event may have predominantly affected sharks, according to Friedman. Sibert even suggested that a pathogen, such as a virus, might have contributed to the population decline, although confirming this theory would be challenging.
The researchers propose that this extinction took place over a span of approximately 100,000 years, which is considered abrupt in geological terms. Intriguingly, the abundance and diversity of shark scales in the sediment have remained consistent from 19 million years ago to the present, suggesting that sharks have not fully recovered from this catastrophic event.
Section 1.2: Ongoing Mysteries
Given that sharks have persisted for 420 million years, overcoming multiple extinction events, Sibert believes that something significant must have transpired during the early Miocene. The fate of sharks may provide insights into broader changes in Earth’s systems during this time.
Nonetheless, some scientists remain skeptical about the extinction event theory. Charles Underwood from Birkbeck, University of London, posits that the changes in shark denticle abundance and diversity could reflect a shift in denticle type rather than a mass extinction. This perspective suggests that the fossil evidence may indicate a change in the preservation potential of shark remains instead of an actual extinction event.
Rubin acknowledges that their recent study raises more questions than it answers. Future research should investigate whether this extinction event was widespread across the world's oceans and if it affected other marine life. Analyzing conditions in lakes and terrestrial ecosystems will aid in constructing a clearer picture of the Miocene Earth. “There’s definitely more data waiting to be found,” adds Rubin.
Chapter 2: Reflections on Extinction
Video Description: Discover how shark populations drastically declined 19 million years ago and the implications of this event.
By examining remnants of shark skin, shed throughout their lives and buried in seafloor sediments, scientists have unearthed evidence of a previously unknown extinction event. This dramatic decrease in shark biodiversity approximately 19 million years ago may have led to the loss of nearly 90% of their population. This historical insight underscores the fragility of marine ecosystems and highlights the challenges species face in recovering from near-extinction.
Today, there are over 400 species of sharks inhabiting the world's oceans. However, oceanic sharks and rays have experienced a staggering decline of over 71% in the last fifty years. The remaining species are increasingly threatened by human activities, including overfishing, plastic pollution, and illegal shark finning. The parallels between this contemporary crisis and the Miocene extinction serve as a cautionary tale, emphasizing that we know the causes of today’s threats and possess the means to avert them.