Fishes can be divided into three major groups: jawless, cartilaginous, and bony. Cartilaginous fishes (class Chondrichthyes) can be split into two groups: sharks, rays, and skates (subclass Elasmobranchii) and chimaeras (subclass Holocephali). Sharks are a relatively small group, with only about 370 species compared with approximately 28,000 kinds of bony fishes. Sharks therefore make up less than 2% of all living fish species. General characteristics of sharks include: • five to seven external gill slits on each side of the head (with no protective gill cover as in bony fishes), • skin covered with placoid scales known as denticles • teeth that are regularly shed and replaced • an upper jaw that is not firmly attached to the skull, and • usually a heterocercal tail in which the upper lobe is longer than the lower one. Because sharks don’t have bony skeletons, fossils of complete sharks are rare. Paleontologists rely mostly on fossilized teeth, scales and more calcified vertebrae to piece together their history. The body shape of most shark groups has changed little from those found in the fossil record. One intriguing fossil shark is Carcharodon megalodon, one of the largest sharks to have ever lived. Known only from its teeth, C. megalodon is believed to have been a close relative of the white shark. It is estimated to have reached a length of at least 40 feet, and may have weighed around 20 tons. C. megalodon is thought to have fed on large fishes and mammals.