Sharks belong to the Phylum Chordata, the Subphylum Vertebrata, the Class Chondrichthyes, the Subclass Elasomobranchii and the Superorder Selachimorpha. From there, sharks can be classified into eight orders:
Hexanchiformes (frilled and cow sharks)
Squaliformes (dogfish sharks)
Pristiophoriformes (saw sharks)
Squantiniformes (angel sharks)
Heterodontiformes (bullhead sharks)
Orectolobiformes (carpet sharks)
Lamniformes (mackerel sharks)
Carcharhiniformes (ground sharks)
These orders are further divided into 34 families. Families are further divided into genera and species. Some orders include dozens of species, while others include as little as one. While the number of individual species varies, more than 500 species are known to exist.
Sharks came into existence more than 400 million years ago, between the Silurian and early Devonian period, and have survived at least four mass extinctions. The first sharks looked very different from modern sharks. Sharks most likely evolved from placoderms, a group of extinct armoured bony fish. The majority of modern sharks can be traced back to around 100 million years ago, during the Cretaceous period. Sharks have not changed much during this time, suggesting that these species are a highly evolved group.
We know about sharks' evolutionary history by studying fossils; however, as cartilage disintegrates shortly after death, sharks' skeletons are seldom preserved. Instead, scientists rely on the harder parts, such as teeth, fin spines and vertebrae, because these parts are partly calcified and are more likely to be preserved in the fossil record.
Sharks are relatively long-lived fish with lifespans ranging from 12 to 100 years. They typically have a slow rate of growth and also a long sexual maturation time. Depending on the species, a shark may not mature for two to more than 20 years. For females to become impregnated, internal fertilization must take place. Male sharks have two organs known as claspers, located at the base of the pelvic fin, that are used to impregnate the female. In younger males the claspers are soft, but as they reach adulthood the claspers become calcified. During mating, "love bites" are often inflicted on the female as the male attempts to stimulate the female.
Three reproductive methods have been observed in reproducing females: oviparity (female lays egg case containing embryos nourished by their yolk), aplacental viviparity (female produces live young nourished in the uterus by a yolk) and placental viviparity (female produce live young nourished in the uterus by a placenta formed by a modified yolk sac attached to the uterine wall). Aplacental viviparity is the most common method.
Sharks have one of the longest gestation or maturation period of any living vertebrate. Average gestation period ranges between nine to 12 months in length; however, it has been hypothesized that basking sharks' gestation period could reach upwards of three years. When pups are born they are fully formed and are able to live and catch prey without assistance from their mother.