The Craniidae are a

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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, wh ...

, 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 consist of calcium carbonate. Other special characteristics of this family are that no outgrowths are developed to form a hinge between both valves, nor is there any support for the lophophore. As adults, craniids either lived free on the ocean floor or, more commonly, were attached to a hard object with all or part of the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

valve. In craniids, a pedicle is not known from any development stage. They are the only members of the order Craniida and the

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...

suborder Craniidina and superfamily Cranioidea; consequently, the latter two

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

are presently redundant and not used very often. ''

Valdiviathyris ''Valdiviathyris'' is a genus of craniate brachiopods that has changed little since the Silurian, from when fossils are known. The extant species ''V. quenstedti'' is known from the late Eocene. It was initially known only from the holotype coll ...

'' and ''

Neoancistrocrania ''Neoancistrocrania'' is a monotypic genus of brachiopod 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 bivalv ...

'' were sometimes separated in a family Valdiviathyrididae but this has turned out to be unjustified. Most Craniidae are long extinct forms known only from

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s like all other

Craniforma Craniata is a class of brachiopods originating in the Cambrian period and still extant today. It is the only class within the subphylum Craniiformea, one of three major subphyla of brachiopods alongside linguliforms and rhynchonelliforms. Cran ...

. However, some 20 species of this 470-million-year-old lineage are surviving today. They include '' Valdiviathyris quenstedti'' which has remained essentially unchanged for the last 35 million years or so. Although some minimal

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

would obviously have taken place in the meantime, this was essentially

silent mutation Silent mutations are mutations in DNA that do not have an observable effect on the organism's phenotype. They are a specific type of neutral mutation. The phrase ''silent mutation'' is often used interchangeably with the phrase '' synonymous muta ...

s and marginal

adaptations In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the ...

to cooler

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

. Present-day ''Valdiviathyris'' are all but inseparable from those of the Late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

and the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

cannot even be divided into

chronospecies A chronospecies is a species derived from a sequential development pattern that involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. The sequence of alterations eventually produces a population that is p ...

. Thus, ''V. quenstedti'' is a true

living fossil A living fossil is an extant taxon that cosmetically resembles related species known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living foss ...

and one of the oldest and most long-lived

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

known to science.

Gallery

Image:Isocrania costata Sowerby 1823.jpg, '' Isocrania costata'', from the Upper Cretaceous of

The Netherlands ) , anthem = ( en, "William of Nassau") , image_map = , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of the Netherlands , established_title = Before independence , established_date = Spanish Netherl ...

Image:EncrustedStroph.JPG, '' Petrocrania'' (small round shells) encrusting an Ordovician strophomenid brachiopod

References

Brachiopod families Extant Ordovician first appearances Craniata {{brachiopod-stub