Sharks Began Roaming the Oceans 400 Million Years Ago as Deep-Sea Bottom-Dwellers
From an evolutionary perspective, sharks are among the oldest animal species still alive today. Their persistence speaks to an extraordinary ability to survive multiple mass extinctions and adapt to vastly different ocean environments.
But what the very first sharks looked like has long been a mystery, since fossil records of these cartilaginous fish have been scarce. Now, researchers from institutions in Australia, Austria, and the U.S. have pieced together clues by comparing the body shapes of living shark species with reconstructions of ancient ocean conditions.
Their results, published in Ecology and Evolution, suggest that sharks first evolved near the sea floor as slow-moving bottom-dwellers, only later taking on the fast, streamlined forms we associate with today’s great white and hammerhead predators with changes likely driven by shifting climates and ocean habitats.
A Plethora of Shark Species
Sharks play a vital role in maintaining healthy marine ecosystems. They come in a stunning variety of shapes and sizes, adapted to different environmental niches. While most of us picture great whites or hammerheads, these familiar giants represent only a handful of the roughly 500 species alive today.
There’s the six-inch lantern shark (Etmopterus perryi), the 40-foot whale shark (Rhincodon typus), and oddballs like the long-finned thresher sharks or the slob-shaped wobbegongs. Each has a deep evolutionary story behind its appearance.
Today, scientists broadly group sharks into two body plans: one flat and flounder-like, suited for a benthic (bottom-dwelling) lifestyle, and another more deep-bodied and streamlined, adapted for fast swimming in open water (pelagic). Which of these forms came first, however, has been unclear.
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The First Sharks Were Bottom-Dwellers
Sharks are older than dinosaurs, first appearing more than 400 million years ago. Most of what we know comes from fossilized teeth, since their skeletons are made of cartilage rather than bone, and rarely fossilize. That gap has made it difficult to trace how their body shapes evolved.
To tackle the question, researchers analyzed body shape and lifestyle data from 452 living shark species, alongside reconstructions of sea levels, continental positions, and surface temperatures spanning the past 250 million years.
Using a method called ancestral state reconstruction, which estimates traits of extinct species by studying modern relatives, the team found that the earliest sharks were benthic, hugging the ocean floor. Later, during the Jurassic and Cretaceous periods (200 million years to 66 million years ago), some lineages adapted to a pelagic lifestyle, a shift reflected in changes to their body design.
Climate Change Shaped Shark Evolution
Environmental records from these eras line up with the evolutionary shift. Rising sea levels, warmer waters, and changing tectonic plates created new habitats, opening opportunities for sharks to expand into them.
This highlights again how climate change (past and present) can reshape ecosystems and drive adaptations in the species that inhabit them. For sharks, these changes enabled the incredible diversity we see today, from bottom-hugging ambush hunters to sleek, open-ocean cruisers.
Studies like this remind us of the richness of shark species beyond the stereotypical man-eater image. Protecting the full spectrum of sharks, not just the big and fearsome ones, is key to safeguarding ocean health.
Read More: The Demon Shark: A New Shark Discovered Deep Off the Australian Coast
Article Sources
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