prehistoric sharks
If you went back in time and looked at the first, unremarkable prehistoric sharks of the Ordovicianperiod--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.)
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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 SharksWe 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.
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Pearls from plants and animals! Fossils/artifacts from sacred sites
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80 million free references Biology - Physics - Humanities
Buried Treasure Fossils
www.buriedtreasurefossils.com
Finest quality fossil shark teeth. Widest selection - anywhere!
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 SharksWe 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.
Megalodon
Megalodon (/ˈmɛɡələdɒn/ meg-ə-lə-don; meaning "big tooth", from Ancient Greek: μέγας (megas) “big, mighty” + ὀδόν (odon) (from ὀδούς (odous) "tooth")),[1] is an extinct species of shark that lived approximately 28 to 1.5 million years ago, during the Cenozoic Era (late Oligocene to early Pleistocene).
The taxonomic assignment of C. megalodon has been debated for nearly a century, and is still under dispute. The two major interpretations are Carcharodon megalodon (under family Lamnidae) or Carcharocles megalodon (under the family Otodontidae).[2] Consequently, the scientific name of this species is commonly abbreviated C. megalodon in the literature.
C. megalodon is regarded as one of the largest and most powerful predators in vertebrate history,[3] and likely had a profound impact on the structure of marine communities.[4] Fossil remains suggest that this giant shark reached a maximum length of 14–18 metres (46–59 ft),[3] and also affirm that it had a cosmopolitan distribution.[2] Scientists suggest that C. megalodon looked like a stockier version of the great white shark, Carcharodon carcharias.[5]
The taxonomic assignment of C. megalodon has been debated for nearly a century, and is still under dispute. The two major interpretations are Carcharodon megalodon (under family Lamnidae) or Carcharocles megalodon (under the family Otodontidae).[2] Consequently, the scientific name of this species is commonly abbreviated C. megalodon in the literature.
C. megalodon is regarded as one of the largest and most powerful predators in vertebrate history,[3] and likely had a profound impact on the structure of marine communities.[4] Fossil remains suggest that this giant shark reached a maximum length of 14–18 metres (46–59 ft),[3] and also affirm that it had a cosmopolitan distribution.[2] Scientists suggest that C. megalodon looked like a stockier version of the great white shark, Carcharodon carcharias.[5]
megamouth
The megamouth shark (Megachasma pelagios) is an extremely rare species of deepwater shark, and the smallest of the three planktivorous sharks besides the whale shark and basking shark. Since its discovery in 1976, few megamouth sharks have been seen, with 59 specimens known to have been caught or sighted as of May 2014, including three recordings on film. Like the other two filter feeders, it swims with its enormous mouth wide open, filtering water for plankton and jellyfish. It is distinctive for its large head with rubbery lips. It is so unlike any other type of shark that it is usually considered to be the sole extant species in the distinct family Megachasmidae, though suggestion has been made that it may belong in the family Cetorhinidae, of which the basking shark is currently the sole extant member.[citation needed] In addition to the living M. pelagios, however, two extinct megamouth species — the Cretaceous M. comanchensis and the Oligocene-Miocene M. applegatei — have also recently been proposed on the basis of fossilised tooth remains.