Craniidae

Craniidae Temporal range: Ordovician–Recent PreЄ Є O S D C P T J K Pg N
Petrocrania brachiopods attached to a strophomenid brachiopod; Upper Ordovician of southeastern Indiana.
Scientific classification
Kingdom:	Animalia
Phylum:	Brachiopoda
Class:	Craniata
Order:	Craniida Waagen, 1885
Suborder:	Craniidina Waagen, 1885
Superfamily:	Cranioidea Menke, 1828
Family:	Craniidae Menke, 1828
Genera
Crania Retzius, 1781 Acanthocrania Williams, 1943 Ancistrocrania Dall, 1877 = Cranopsis Craniscus Dall, 1871 Conocrania Smirnova, 1996 Danocrania Rozenkrantz, 1964 Isocrania Jäkel, 1902 Lepidocrania Cooper and Grant, 1974 Nematocrania Grant, 1976 Neoancistrocrania Laurin, 1992 Novocrania Lee and Bruton, 2001 Orthisocrania Rowell, 1963 Petrocrania Raymond, 1911 Philhedracrania Koken, 1889 Pseudocrania McCoy, 1851 Valdiviathyris Helmcke, 1940[1]
Synonyms
Valdiviathyrididae

The Craniidae are a family of brachiopods (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 valve. All other brachiopods are supposed to have a stalk or pedicle, at least as an adolescent, but in craniids a pedicle is not known from any development stage.^[2]

They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often. Valdiviathyris and Neoancistrocrania were sometimes separated in a family Valdiviathyrididae but this has turned out to be unjustified.^[3]

Most Craniidae are long extinct forms known only from fossils like all other Craniforma. 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 would obviously have taken place in the meantime, this was essentially silent mutations and marginal adaptations to cooler habitat. Present-day Valdiviathyris are all but inseparable from those of the Late Eocene and the genus cannot even be divided into chronospecies. Thus, V. quenstedti is a true living fossil and one of the oldest and most long-lived species known to science.^[3]

References

• Robinson, Jeffrey H. & Lee, Daphne E. (2007): The Recent and Paleogene craniid brachiopod, Valdiviathyris quenstedti Helmcke, 1940. Systematics and Biodiversity 5(1): 123–131. doi:10.1017/S1477200006002179 (HTML abstract)