Craniata is a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, whi ...

originating in the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

period and still extant today. It is the only

within the

subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of "subdivision (rank), subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists ...

Craniiformea, one of three major subphyla of brachiopods alongside linguliforms and rhynchonelliforms. Craniata is divided into three

orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

: the extinct

Craniopsida Craniopsidae is an extinct family of craniiform brachiopods which lived from the Lower Cambrian (Botomian) to the Lower Carboniferous (Tournaisian). It is the only family in the monotypic In biology, a monotypic taxon is a taxonomic group (t ...

and

Trimerellida Trimerellida is an extinct order of craniate brachiopods, containing the superfamily Trimerelloidea and the families Adensuidae, Trimerellidae, and Ussuniidae. Trimerellidae is a small but widespread family of warm-water brachiopods ranging f ...

, and the living

Craniida The Craniidae are a family of brachiopods, commonly known as lamp shells. Although it belongs to a subdivision called the inarticulata which have shells where the mineral content consist of calcium phosphate, the Craniidae have shells that consis ...

, which provides most information on their biology. Living members of the class have shells which are composed of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

, though some extinct forms my have

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...

shells. The shells are inarticulate (lack a hinge with distinct tooth-and-socket connections) and are usually rounded in outline. There is no pedicle, with the rear edge of the body cavity having the form of a smooth and flat wall perforated by the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

This class of brachiopods has an unsupported

lophophore The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata. Living craniiforms have a distinctive muscle system, which can be reconstructed from muscle scars in extinct forms as well. There are two pairs of column-like

adductor muscles The adductor muscles of the hip are a group of muscles mostly used for bringing the thighs together (called adduction). Structure The adductor group is made up of: *Adductor brevis *Adductor longus *Adductor magnus * Adductor minimus This is o ...

, which extend vertically and function to close the shell. There are also two pairs of oblique muscles, which lie at a shallow angle and help to slide and rotate the valves against each other. The internal oblique muscles are closely spaced, extending up and back from the middle of the ventral valve to the back of the dorsal valve. The lateral oblique muscles are widely spaced, extending forwards from the back of the ventral shell to the front of the soft body wall. Craniiforms share some similarities with both linguliforms and rhynchonelliforms, though they are distinct from either group. Like linguliforms, they have an anus, inarticulate shells, and a muscle system mainly based on adductor and oblique muscles. Like rhynchonelliforms, they have a calcareous shell and a modified lophophore.

References

Protostome classes {{brachiopod-stub