If you went back in time and looked at the first, unremarkable prehistoric sharks of the Ordovician period--about 420 million years ago--you might never guess that their descendants would become such dominant creatures, holding their own against vicious aquatic reptiles like pliosaurs and mosasaurs and going on to become the "apex predators" of the world's oceans. Today, few creatures in the world inspire as much fear as the Great White Shark, the closest nature has come to a pure killing machine. (See a gallery of prehistoric shark pictures and an article explaining why there are no giant sharks alive today.)
Before discussing shark evolution, though, it's important to define what we mean by "shark." Technically, sharks are a suborder of fish whose skeletons are made out of cartilage rather than bone; sharks are also distinguished by their streamlined, hydrodynamic shapes, sharp teeth, and sandpaper-like skin. Frustratingly for paleontologists, skeletons made of cartilage don't persist in the fossil record nearly as well as skeletons made of bone--which is why so many prehistoric sharks are known primarily (if not exclusively) by their fossilized teeth.
The First Sharks
We don't have much in the way of direct evidence, except for a handful of fossilized scales, but the first sharks are believed to have evolved during the Ordovician period, about 420 million years ago (to put this into perspective, the first tetrapods didn't crawl up out of the sea until 400 million years ago). The most important genus that has left significant fossil evidence is the difficult-to-pronounce Cladoselache, numerous specimens of which have been found in the American midwest. As you might expect in such an early shark, Cladoselache was fairly small, and it had some odd, non-shark-like characteristics--such as a paucity of scales (except for small areas around its mouth and eyes) and a complete lack of "claspers," the sexual organ by which male sharks attach themselves (and transfer sperm to) the females.
After Cladoselache, the most important prehistoric sharks of ancient times were Stethacanthus, Orthacanthus and Xenacanthus. Stethacanthus measured only six feet from snout to tail but already boasted the full panoply of shark features--scales, sharp teeth, distinctive fin structure, and a sleek, hydrodynamic build. What set this genus apart were the bizarre, ironing-board-like structures atop the backs of males, which were probably somehow used during mating. The comparably ancient Stethacanthus and Orthacanthus were both fresh-water sharks, distinguished by their small size, eel-like bodies, and odd spikes protruding from the tops of their heads (which may have delivered jabs of poison to bothersome predators).
The Sharks of the Mesozoic Era
Considering how common they were during the preceding geologic periods, sharks kept a relatively low profile during most of the Mesozoic Era, because of intense competition from aquatic reptiles like ichthyosaurs and plesiosaurs. By far the most successful genus was Hybodus, which was built for survival: this prehistoric shark had two types of teeth, sharp ones for eating fish and flat ones for grinding mollusks, as well as a sharp blade jutting out of its dorsal fin to keep other predators at bay. The cartilaginous skeleton of Hybodus was unusually tough and calcified, explaining this shark's persistence both in the fossil record and in the world's oceans, which it prowled from the Triassic to the early Cretaceous periods.
Prehistoric sharks really came into their own during the middle Cretaceous period, about 100 million years ago. Both Cretoxyrhina (about 25 feet long) and Squalicorax (about 15 feet long) would be recognizable as "true" sharks by a modern observer; in fact, there's direct tooth-mark evidence that Squalicorax preyed on dinosaurs that blundered into its habitat. Perhaps the most surprising shark from the Cretaceous period is the recently discovered Ptychodus, a 30-foot-long monster whose numerous, flat teeth were adapted to grinding up tiny mollusks, rather than large fish or aquatic reptiles.
After the Mesozoic - Introducing Megalodon
After the dinosaurs (and their aquatic cousins) went extinct 65 million years ago, prehistoric sharks were free to complete their slow evolution into the remorseless killing machines we know today. Frustratingly, though, the fossil evidence for the sharks of the Miocene epoch (for example) consists almost exclusively of teeth--thousands and thousands of teeth, so many that you can buy yourself one on the open market. The Great White-sized Otodus, for example, is known almost exclusively by its teeth, from which paleontologists have reconstructed this fearsome, 30-foot-long shark.
By far the most famous prehistoric shark of this time period was Megalodon, adult specimens of which measured 70 feet from head to tail and weighed as much as 50 tons. Megalodon was a true apex predator of the worlds' oceans, feasting on everything from whales, dolphins and seals to giant fish and (presumably) equally giant squids. No one knows why this monster went extinct about two million years ago; the most likely candidates include climate change and the resulting disappearance of its usual prey.
The following is a list of prehistoric sharks; just click on the links for more information.
Cladoselache A strange, scaleless shark of the late Devonian period.
Cretoxyrhina Better known as the "Ginsu Shark."
Diablodontus This "devil-toothed" shark was recently discovered in Arizona.
Edestus This shark could have used a visit to the orthodontist.
Falcatus A tiny, big-eyed shark of the Carboniferous period.
Helicoprion This bizarre shark had teeth like a fruit roll-up.
Hybodus This prehistoric shark prowled the seas for over 100 million years.
Ischyrhiza A saw-toothed shark of the Cretaceous period.
Megalodon The biggest (and scariest) shark that ever lived.
Orthacanthus This prehistoric shark had a distinctive spine on its head.
Otodus All that remains of this prehistoric shark are its huge, sharp teeth.
Ptychodus This giant shark preyed on tiny mollusks.
Squalicorax The only prehistoric shark known to have eaten dinosaurs.
Stethacanthus The males of this shark had "ironing boards" on their backs.
Xenacanthus A spike-headed shark of the early Permian period.