by Ben S. Roesch
Like mammals, birds, reptiles, amphibians, and other fishes, sharks (and skates and rays) are vertebrates belonging to the phylum Chordata and the subphylum Vertebrata. Sharks, skates, and rays are separated from their vertebrate cousins into the class Chondrichthyes--the "cartilaginous fishes."
The class Chondrichthyes, with over 950 different species, is comprised of two subclasses, the Elasmobranchii (sharks, skates and rays) and the Holocephali (the chimaeras, such as the ratfish and elephant fish). Chimaeras are big-eyed, stomachless, deep-sea creatures that possess an upper jaw which is fused to its cranium (unlike in sharks).
The subclass Elasmobranchii consists of only one extant subcohort, the Neoselachii (modern rays and sharks - we will ignore the fossil elasmobranchs for the purpose of this discussion). The Neoselachii consists of four extant superorders and one extinct superorder (Palaeospinacomorphii). The four living superorders are: Squalomorphii, Squatinomorphii, Rajomorphii (or Batoidea - the rays), and Galeomorphii.
Within the Squalomorphii, there are four orders, the Hexanchiformes (six- and seven-gilled sharks and frilled sharks), the Echinorhiniformes (the bramble sharks, genus Echinorhinus), the Squaliformes (dogfish sharks), and the Pristiophoriformes (sawsharks).
The Squatinomorphii contains one order, the Squatiniformes (angel sharks).
The Rajomorphii contains five orders: the Pristiformes (sawfish), the Rhinobatiformes (guitarfishes), the Torpediniformes (electric rays), the Rajiformes (skates), and the Myliobatiformes (stingrays). Although skates and rays are not called sharks, phylogenetically speaking they are sharks--just ones that have evolved a flattened body.
The Galeomorphii contains four orders and most of the species that one thinks of when the word "shark" comes up. These orders are: the Heterodontiformes (bullhead sharks), the Orectolobiformes (carpet sharks), the Lamniformes (mackerel sharks) and the Carcharhiniformes (ground or requiem sharks).
In total, there are about 869+ extant species of elasmobranchs, with about 400+ of those being sharks and the rest skates and rays.
These numbers increase every year, as new elasmobranch species are trawled from the deep-sea or fished off coral reefs. Taxonomy is also perhaps infamously known for its constant, yet essential, revisions of the relationships and identitity of different organisms. Classification of elasmobranchs certainly does not evade this process, and species are sometimes lumped in with other species, or renamed, or assigned to different families and other taxonomic groupings. It is certain, however, that such revisions will clarify our view of the taxonomy and phylogeny (evolutionary relationships) of elasmobranchs, leading to a better understanding of how these creatures evolved.