Caption For Photo: This is a photo of a Great White Shark(Carcharodon carcharias)! This shark is found in the Atlantic Ocean. As you can see the teeth of the shark is very sharp. Shark teeth usually shed approximately, 35,000 teeth in a lifetime!
Shark Evolution:If you went back in time and looked at the first, unremarkable prehistoric sharks of the Ordavician 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 plipsaurs and mosarsaurs 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.)
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 Cladoseche, 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 Stanthecasus, Oranthencasus and Xenthencasus. Stenencasus 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 itchysaures 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 Crestorses period, about 100 million years ago. Both Cretoxvwina(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.
Shark Evolution:If you went back in time and looked at the first, unremarkable prehistoric sharks of the Ordavician 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 plipsaurs and mosarsaurs 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.)
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 Cladoseche, 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 Stanthecasus, Oranthencasus and Xenthencasus. Stenencasus 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 itchysaures 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 Crestorses period, about 100 million years ago. Both Cretoxvwina(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.