Diving with Sharks Through Deep Time

As the summer heats up and people flock to the beach, one of Earth’s oldest predators swims back into the spotlight. Whether they’re wreaking havoc on the big screen or gliding through an aquarium tank, we can’t get enough of sharks. Few images in nature spark more awe or dread than a dorsal fin slicing through the water.

Yet despite our fascination, these predators remain misunderstood, wrongly vilified and widely hunted—which is detrimental to the entire ocean. Sharks, who have been around longer than trees, have kept our oceans healthy for hundreds of millions of years. These horror movie staples are actually oceanic heroes (well, to everyone but their prey).

The National Museum of Natural History is one of the best places to learn how these iconic fish have changed through the eons. Visitors can marvel at remarkably-preserved prehistoric sharks in Deep Time or step inside a fossil shark’s tooth-studded maw and gawk at a thresher shark's whip-like tail in the Sant Ocean Hall. In the museum’s food court, visitors can even eat beneath a life-size model of Megalodon, a prehistoric shark three times bigger than the largest great whites. Behind the scenes, an assortment of sharks are preserved in alcohol or stored on ice for future research into these toothy denizens of the deep.

Whether you’re wading into the museum’s exhibits or taking a deep dive into its sprawling paleobiology collection, celebrate Shark Week with a quick swim through the 450-million-year evolutionary odyssey of the world’s most iconic fish. 

Humble beginnings

While fossil scales hint that small sharks likely evolved during the late Ordivician period, the oldest shark teeth date back to the Devonian, when a group of fish called spiny sharks were patrolling shallow waters. But these pug-nosed fish looked more like snaggletooth trout than sharks—they had bulky, diamond-shaped scales and spines sticking out near their fins. 

But what set these spiny sharks apart from the bony, armored fish of their day was cartilage—a flexible, lightweight structure that formed most of their bodies. While the material is great for swimming, it rarely passes the test of time. After the animal dies, cartilage rots away, which is why tough material like teeth are often the only part of the shark that fossilizes. 

In the middle of the Devonian around 415 million years ago, a group of eel-like sharks called Antarctilamna slithered into the fossil record. Slightly later came Cladoselache, a medium-sized predator whose streamlined body and forked tail offered an early glimpse of the body type sharks would eventually perfect. But these early sharks could only get so big as larger predators called placoderms, a group of giant, armored fish, ruled the seas and preyed on the pint-sized sharks.

Sharks bulk up (and get weird)

When the armored placoderms died out 359 million years ago, prehistoric sharks finally stopped swimming for their lives. With few predators and ample prey, sharks and their close relatives, chimaeras (another group of cartilaginous fish still found in the deep sea), rose up to fill a variety of ecological roles, including that of apex predator. 

During the Carboniferous, between 359 and 299 million years ago, there were more distinct types of sharks and chimeras patrolling the ocean than at any other time in Earth’s history. And with so many sharks in the ocean, many of these fish adopted strange traits to gain an ecological edge.

Odd cartilaginous fishes included Falcatus, a group of tiny sharks that had curving spines poking out of their heads, and Stethacanthus, a chimaera whose dorsal fin was flattened like an anvil. Some of the stranger sharks from the time are found in the Edestus genus. These sharks, whose larger members rivaled great white sharks in size, sported jaws studded with continually growing curved blades of teeth called whorls. In Edestus' case, the tooth whorls sprouted from their mouth like a pair of scissors. Later chimaeras known as the Helicoprions, had buzzsaw-like whorls of teeth. For decades researchers were stumped by how these spiral sets of teeth fit inside the animal’s mouth because no living animal sports any dentition remotely close to these tooth whorls.

Dawn of modern sharks

The end of the Permian period 250 million years ago was just short of an ocean-wide apocalypse. Seething volcanoes in Siberia made the Earth’s climate inhospitable. Oxygen was siphoned off from the oceans, causing around 96% of all marine species to suffocate into extinction. 

Sharks were part of the few animals to emerge from this calamity. But they soon found themselves swimming through strange seas dominated by reptiles. While dinosaurs reigned on land, marine reptiles like dolphin-shaped Ichthyosaurs, burly Pliosaurs, long-necked Plesiosaurs and ferocious sea lizards called Mosasaurs all dominated the oceans at various times during the Mesozoic era, which stretched from 250 to 66 million years ago. Even some fish, like the bulldog-faced Xiphactinus, were large enough to bully most sharks.

It was amongst all these giants that the modern shark lineage first emerged. During the early Jurassic around 195 million years, a group of blunt-nosed sharks called the Hexanchiformes entered the frame. While their modern descendents like the sixgill shark are now referred to as living fossils, these sharks were on the cutting edge of evolution back in the day. Their streamlined bodies and flexible, protruding jaws allowed them to chase and scarf down larger prey. The Cretaceous saw the arrival of the Squalicorax, or crow sharks, who patrolled the coasts and fed on the remains of dinosaurs that were washed out to sea.

Reign of the mega sharks

Just as sharks were finding their groove, another cataclysmic extinction rocked the planet. The asteroid impact that closed the Mesozoic decimated many larger shark species, but small and deep water species were able to persist. These survivors found themselves at another ecological crossroads as the Mesozoic’s giant fish and marine reptiles had been wiped out.

These relatively empty seas allowed the scrawny sharks to bulk back up as they began mingling with an entirely new group of prey and foe—marine mammals. Some mackerel sharks, the precursors to modern species like makos and great white sharks, ballooned up to previously unimaginable sizes to hunt early whales.

Around 23 million years, shark evolution reached its zenith with the rise of mega-toothed sharks like Megalodon. With a scientific name (Carcharocles megalodon) that means “glorious shark, big tooth,” you know Megalodon was a terrifying predator. But even that name does not do the giant fish justice—at around 60 feet long and armed with the strongest predicted bite force of any animal ever, Megalodon makes ‘Jaws’ look like a minnow. Just about anything was on the Megalodon’s menu, including baleen whales.

For years, scientists have grouped this mega-shark in the lineage of modern great white sharks, but recent research has found Megalodon is actually more closely related to speedy mako sharks. Which is interesting considering that ancient great whites likely took a sizable bite out of Megalodon’s prey. The super-sized shark disappeared around 3 million years ago as a cooling climate and smaller, more efficient predators brought about the reign of modern sharks.

A fishy future

Over nearly half a billion years on Earth, sharks have seen it all. They have weathered five mass extinctions that wiped out competitors like placoderms and giant sea reptiles and swam through hothouse worlds and ice ages without missing much of a beat. But this ancient lineage of predators may have finally met a foe it can’t outlast—humans.

Despite the hysteria sparked by the handful of annual shark attacks, the relationship between humans and sharks is infinitely more dangerous for the fish. An estimated 100 million sharks are killed by humans each year. Some, like the quick-twitched mako shark, are hunted for sport while others are killed for their fins, which are the main ingredient of shark fin soup, a delicacy in parts of Asia. But many of these breathtaking predators are killed by negligence. They are ensnared in deadly fishing lines or hauled aboard as bycatch.

This slaughter is a loss to us all. Sharks are trendsetters—their sleek, tooth-encrusted skin has inspired products like olympic swimsuits—and their hardy immune systems have made them darlings of medical research. But their most important impact is on a balanced ecosystem. Thanks to their hunting prowess, sharks keep populations of prey like seals and fish in check, which sends beneficial ripples throughout the entire marine environment. Oceans have flourished for hundreds of millions of years under their watch. With today’s oceans rapidly changing, we need sharks now more than ever. 

Related Stories:
Megalodon May Be Extinct, but There’s a Life-size One at the Smithsonian
Rare Megamouth Shark Arrives at the Smithsonian
The Experts Behind the New Fossil Hall Wrap Their Minds Around ‘Deep Time’
​​What’s Next for the 1.2 Million Prehistoric Fossils Now at Smithsonian

Jack Tamisiea | | READ MORE

Jack Tamisiea is a Science Communications Specialist at the Smithsonian's National Museum of Natural History. In addition to covering all things natural history for the museum's blog, Smithsonian Voices, he tracks media coverage and coordinates filming activities for the museum's Office of Communications and Public Affairs. Jack recently completed his masters in science writing at Johns Hopkins University and his writing has appeared in the New York Times, Scientific American, National Geographic and other science-focused publications. In his free time, he loves exploring the outdoors with a sketchbook and camera. You can read more of Jack's work at https://jacktamisiea.com.