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http://upload.wikimedia.org/wikipedia/commons/d/de/Orthacanthus_senckenbergianus_1.JPG

Orthacanthus Senckenbergianus

PALAEOZOIC 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).


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