Chondrichthyes
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The class of cartilaginous fish including sharks, rays, and skates. Unlike bony fish, their skeleton is made of cartilage rather than bone.
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Chondrichthyes originated ~450 million years ago and today comprises ~1,100 species. Their cartilaginous skeleton is reinforced with calcium phosphate (tesserae) in calcified cartilage, providing rigidity without bone. Unlike bony fish, they lack a gas bladder — sharks control buoyancy via a large, oil-rich liver (squalene lowers specific gravity). Teeth are continuously shed and replaced (polyphyodonty) — large sharks shed and replace tens of thousands of teeth in a lifetime. Reproductive strategies range from egg-laying (oviparous, sharks) to placental viviparity in some carcharhinid sharks. Over a third of chondrichthyans are threatened with extinction due to overfishing and shark-finning.