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Why is a cartilaginous skeleton not necessarily a primitive trait in sharks?
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chondrichthyan
Southern stingrays (Dasyatis americana or Hypanus americanus) are found in the western Atlantic Ocean.
A cartilaginous skeleton in sharks and other chondrichthyans is not considered a primitive trait, but rather one that is likely degenerate.1
While it was once believed that the cartilaginous skeleton represented an evolutionary stage preceding the bony fishes (Osteichthyes), evidence suggests that the two groups evolved independently. The ancestors of modern sharks may have been placoderms—specifically a group known as petalichthyids—which possessed ossified skeletons and well-developed fins. Consequently, the cartilaginous nature of the shark skeleton is viewed as a secondary loss of bone rather than a primitive, ancestral condition.1