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Sharks are the living descendants of an ancient lineage of animals, known from fossils to have existed at least since the Upper Silurian period, about 420 million years ago. Though superficially they may resemble simple fishes, they are, in fact, complex creatures with amazing adaptations resulting from a long evolutionary history. The fossil record indicates that the shark phylogeny tree includes several dead ends, lineages which went extinct.
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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.)
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During the late Paleozoic and Mesozoic eras, the dominant sharks belonged to a group called Hybodontoidea. Their fin and skeletal structure was more advanced than that of earlier sharks. Males had one or two pairs of hook-shaped spines on their heads, which were used during mating to hold the male and female together.
In terms of phylogeny, Hybodontoidea was a sister lineage of modern shark ancestors. Its first representatives appeared around the Mississippian period (360-320 million years ago) and lasted until the end of the Cretaceous period (65 million years ago), when they went extinct along with the dinosaurs. Hybodontoidea, then, survived for a longer stretch of time than the dinosaurs did, so even though they are now extinct, they were a very successful group. Early naturalists thought them to be the ancestors of modern sharks, but they have a number of unique features; thus, even before the discipline of cladistics was formalized, researchers determined that the hybodonts have no living descendants.
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Modern shark diversity
Modern sharks, as well as their close cousins the rays and skates, are a diverse group. About 360 species of living sharks have been described, many of which are now in danger of extinction. Learn more about the threat of shark finning.
The amazing sensory systems of sharks are soft-tissue features which do not fossilize. Thus cladistics researchers are not able to use these features to trace shark phylogeny as well as they can use skeletal features and teeth.
This is Part 1 of a two-article series on shark phylogeny. Part 2 explains what cladistics researchers believe the shark phylogeny tree looks like.