Introducing Sharks, Skates, Rays, and Chimaeras 3
What are sharks and how are they classified?
What then is a shark? The taxonomic class Chondrichthyes includes
sharks, skates, rays, and chimaeras because they share a number of com-
mon traits. They are all fishes (aquatic animals with a vertebral column that
breathe using gills and have scales and fins). All Chondrichthyes have a car-
tilaginous skeleton, which is a skeletal support system softer than bone be-
cause it is not as hardened with calcium as is true bone, although the skel-
eton is moderately hardened with calcium crystals. All groups also breathe
via gills by pushing or pulling water in through their mouths and out their
gill slits (with some minor exceptions). Because of anatomical differences,
sharks, skates, and rays are grouped in one subclass, the Elasmobranchii
(“plate gills”), and chimaeras are grouped in a different subclass, the Holo-
cephali (“whole heads”). (The taxonomic names are shortened when used
as adjectives or nouns—e.g., subclass Elasmobranchii, the elasmobranch
sharks.)
Taxonomists and systematists, scientists who study and classify animals
into groups on the basis of their characteristics and evolutionary relation-
ships, have organized sharks into a hierarchy of groups that reflects those
relationships. With over a thousand animals plus fossil forms to consider,
not all systematists agree 100% of the time on the best classification, and
new discoveries require modifications of older ideas.
In this book we generally follow the classification used by Joseph Nel-
son in the fourth (2006) edition of his internationally accepted Fishes of the
World. Nelson has put together a classification from various authorities,
chiefly from the work of Leonard J. V. Compagno. (The background pub-
lications are listed in the bibliography of Nelson’s book for anyone who
wants to explore the details.)
In this classification, elasmobranchs are divided into standard taxo-
nomic groups and subgroups, reflecting their evolutionary relationships.
The major groups, going from larger, more inclusive groups (infraclass, di-
vision, subdivision) to smaller, more exclusive taxonomic groups (superor-
der, order, family) are as follows, listed in phylogenetic order, which means
that more primitive groups are listed first (additional details are given in
appendix A):
Class Chondrichthyes (cartilaginous fishes)
Subclass Elasmobranchii (sharklike fishes)
Infraclass Euselachii (sharks and rays)
Division Neoselachii (modern sharks, to distinguish them from
the many, very different, extinct taxa)