The Tooth Of The Matter: How Sharks Lost Half Their Ecological Roles
Analyzing over 9,000 fossil shark teeth reveals that today's sharks occupy less ecological space ... More than their ancient counterparts. (Maggie Martorell/Miami Herald/Tribune News Service via Getty Images)
Sharks are one of the ocean's oldest survivors, with a fossil record stretching back at least 250 million years. But their story isn't just merely about survival… it's about transformation. A global study by scientists Dr. Jack A. Cooper and Dr. Catalina Pimiento, analyzing more than 9,000 fossilized shark teeth from 537 species across the Cenozoic era (i.e. the last 66 million years), has shed light on how shark diversity, particularly in terms of function, has changed over time — and what that means for our oceans today.
Instead of focusing on just species numbers, the duo turned to shark teeth to understand functional diversity, or the variety of ecological roles sharks have played, based on differences in size, feeding strategy and tooth shape. By examining six specific dental traits known to link closely with function, the two researchers mapped out how shark ecosystems evolved, and shrank, throughout deep time. The teeth came from museums around the world and all the scientific literature they could get their hands on, spanning a global range and capturing an evolutionary story told in enamel.
For about 60 million years, from the Paleocene through the Miocene (roughly 66 to 10 million years ago), sharks thrived in a wide variety of ecological niches. During this time, 66% to 87% of the potential ecological 'functional space' was occupied. That means sharks weren't just numerous — they were doing lots of different things. Some were small suction feeders, others were massive predators, and many filled in-between roles that helped balance marine food webs. Notably, this period featured high levels of functional redundancy, where multiple species performed similar roles. That kind of overlap isn't wasteful. Think of it like a buffer against extinction: if one species disappears, another can step in to keep the ecosystem running.
But this balance took a major hit about 30 million years ago during the Oligocene. Functional redundancy dropped by about 45%, making the ecosystem more fragile. The loss of overlap meant that each remaining species held a more unique, irreplaceable role. This left the whole system more vulnerable, and the trend didn't improve. From the late Miocene onward, about 10 million years ago, shark functional diversity began a steady decline. Today, sharks have lost 44% of the functional richness they once had. Many of the roles once filled by mid-sized suction feeders and large-bodied predators are simply… gone.
What's striking is that extinct sharks were doing more than their modern relatives in terms of ecological variety. They spanned a broader range of functional roles than the sharks we see in today's oceans. The loss of those species — and their roles — means our current shark populations are not only smaller in number, but also less functionally diverse. This matters, Cooper and Pimiento explain, because functional diversity is closely tied to ecosystem health. The fewer roles sharks fill, the less resilient our oceans become.
For about 60 million years, from the Paleocene through the Miocene (roughly 66 to 10 million years ... More ago), sharks thrived in a wide variety of ecological niches. During this time, 66% to 87% of the potential ecological 'functional space' was occupied. That means sharks weren't just numerous — they were doing lots of different things. Some, like Haimirichia amonensis, were small suction feeders likely adapted to snapping up soft-bodied prey like squid. Others, such as the massive Otodus megalodon, were apex predators capable of preying on large marine mammals. In between were species like Hemipristis serra, with serrated teeth suited for slicing through flesh, indicating a role as a powerful mid-sized predator. There were also bottom-dwellers like Galeorhinus cuvieri, which probably hunted crustaceans and small fish, and long-snouted filter-feeders like Pseudomegachasma, which may have filtered plankton from the water. Together, these species helped maintain a balance in the marine food web by occupying a broad spectrum of feeding strategies, sizes, and ecological roles.
Importantly, this decline didn't happen overnight. It's (thankfully?) been unfolding for millions of years. But the pressure modern sharks face from overfishing, habitat destruction and climate change is piling on top of an already weakened system. It's like kicking a structure that's already crumbling at the foundation. What's left today is a thinner slice of the rich ecological tapestry that sharks once wove through the oceans. And if we continue to lose the few remaining threads, the consequences could ripple far beyond sharks themselves.
Cooper and Pimiento hope their work acts as both a warning and a guide. By understanding how shark functional diversity has changed through time, scientists can better predict which roles are most at risk and which species are keystones worth prioritizing for conservation. Protecting the sharks of today isn't just about saving individual species, they argue, but about preserving the roles they play and the balance they bring to marine ecosystems.
Sharks have weathered mass extinctions, sea-level shifts and global climate swings. But their long history on our blue marble planet also shows us that their current decline isn't part of a natural cycle, but a breaking point. The oceans of the past were richer, not just in life but in function. If we want to keep the ocean ecosystems we rely on, we'll need to make sure sharks can keep doing the jobs they've done for millions of years.
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