Sharks, rays, and chimaeras belong to Class Chondrichthyes, fishes without true bones. Instead, their skeletons (including their jaws and vertebrae) are comprised of cartilage. This cartilage is often calcified, making it hard, like bone, but not truly ossified. The only bony parts of Chondrichthyes fishes are their teeth and scales called dermal denticles.
The first shark fossils consist of tiny dermal denticles. These scales are structurally similar to teeth. These first known sharks are thought to have been active swimmers with paired fins and a torpedo-shaped body.
Most sharks in the Paleozoic era resembled modern-day actively swimming sharks but had more primitive skeletons. Their teeth and fin support structures varied widely. They all had large triangular paired pectoral fins, and many also had paired pelvic fins. Most had tails in which the upper and lower lobes were the same size, unlike some modern sharks, like great whites, in which the top lobe is significantly larger than the bottom lobe. One odd fossil species from this time, Xenacanthus, had a tail without separate lobes, like a modern sea snake.
These early sharks had no calcified vertebrae, though they may have had uncalcified (and unfossilized) cartilage segments. The notochord, a stiff but flexible rod where the modern backbone would be expected, was probably present instead. (In most modern vertebrates, thenotochord is present only in the embryo, but its original purpose was to assist the first fish with swimming.)
