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From late Devonian to late Jurassic there were two shark groups, the Hybodonts
and the Xenacanths
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The Xenacanths were freshwater sharks and were very abundant
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They are identified most easily from their characteristic teeth, which
have two pointed, elongate lateral cusps with one smaller cusp in the middle
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Long “head to tail” dorsal fin and very often a large spine protruded from
behind the braincase (Fenton and Fenton)
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Some got larger than 2 m
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