Craniida

The Craniidae are a family of brachiopods, the only surviving members of the subphylum Craniiformea. They are the only members of the order Craniida, the monotypic suborder Craniidina, and the superfamily Cranioidea; consequently, the latter two taxa are at present redundant and rarely used. There are three living genera within Craniidae: '' Neoancistrocrania'', '' Novocrania'', and '' Valdiviathyris''. As adults, craniids either live freely on the ocean floor or, more commonly, cement themselves onto a hard object with all or part of the ventral valve.

Like the burrowing lingulids, Craniids are inarticulate brachiopods. There are no outgrowths forming a hinge between both valves, nor is there any support for the lophophore. Unlike lingulids, which have shells consisting of apatite and organic material, craniids have shells composed mainly of calcium carbonate. No craniids are known to bear a pedicle at any development stage.

Evolution

Extinct craniids

Most craniid genera are extinct, known only from fossils like other craniiforms. Craniids first appeared in the later part of the Tremadocian, the first stage of the Lower Ordovician. In the Lower Ordovician, they were mostly restricted to peri-Gondwanan terranes (modern central Europe) in the South Polar region. By the Middle Ordovician, they had spread northwards to Baltica. In the Late Ordovician, their range expanded eastwards to Avalonia before crossing the Iapetus Ocean to Laurentia.

Craniid diversity and abundance was respectable but still fairly low during the Late Ordovician, and even lower through the rest of the Paleozoic. The craniid fossil record is patchy, with fossils unknown from the Upper Carboniferous, Upper Permian, and the entire Triassic. They reappear in the Oxfordian stage of the Late Jurassic and briefly regain their Ordovician level of diversity in the Late Cretaceous.

Living craniids

Craniids are remarkable for their slow rate of evolution. Approximately 11 species of this 480-million-year-old lineage still survive today, with minimal differences relative to their fossil counterparts. One species, '' Valdiviathyris quenstedti,'' has remained essentially unchanged for the last 35 million years or so. Although some evolution would have taken place in the meantime, this was essentially silent mutations and marginal adaptations to cooler habitat. Present-day ''Valdiviathyris'' are indistinguishable from fossils of the Late Eocene, and the genus cannot even be divided into chronospecies. ''V. quenstedti'' can be considered a living fossil and one of the oldest and most long-lived species known to science.

''Valdiviathyris'' and ''Neoancistrocrania'' have occasionally been separated into their own family, Valdiviathyrididae, though few authors follow this suggestion. ''Craniscus'' has sometimes been cited as a fourth living craniid genus,'''' based on "''Craniscus japonica''", a putative species from waters off Japan. Genetic evidence has clarified this misconception, revealing that "''Craniscus japonica''" actually represents a misattributed species of ''Neoancistrocrania''.

List of genera

From the '' Treatise on Invertebrate Paleontology Part H, Revised'' (unless stated otherwise):
* †'' Acanthocrania'' Williams, 1943 Caradoc") – Lower Carboniferous">Caradoc_(age).html" ;"title="pper Ordovician ("Caradoc (age)">Caradoc") – Lower Carboniferous* †''Ancistrocrania'' Dall, 1877 Synonym (taxonomy), = ''Cranopsis'' [Upper Cretaceous (Coniacian) – Paleocene (Danian?)]
* †''Celidocrania'' Liu, Xhy, & Xue, 1985 iddle Ordovician(possible synonym of ''Acanthocrania'')
* † ''Crania'' Retzius, 1781 pper Cretaceous (Campanian – Maastrichtian">Campanian.html" ;"title="pper Cretaceous (Campanian">pper Cretaceous (Campanian – Maastrichtian)]
* †''Craniscus'' Dall, 1871 [Upper Jurassic ( Oxfordian)]
*†'' Conocrania'' Smirnova, 1996 ower Cretaceous (Berriasian – Valanginian">Berriasian.html" ;"title="ower Cretaceous (Berriasian">ower Cretaceous (Berriasian – Valanginian)]
*†''Danocrania'' Rozenkrantz, 1964 [Upper Cretaceous (Maastrichtian) – Paleocene (Danian?)]
*†''Deliella'' Halamski, 2004 [Lower Devonian ( Emsian) – Middle Devonian ( Givetian)]
*†'' Isocrania'' Jäkel, 1902 pper Cretaceous (Campanian) – Paleocene (Danian?)*†'' Lepidocrania'' Cooper and Grant, 1974 nowiki/>Lower Permian">Lower_Permian.html" ;"title="nowiki/>Lower Permian">nowiki/>Lower Permian*†''Mesocrania'' Smirnova, 1997 [Upper Jurassic (Oxfordian) – Lower Cretaceous (Berriasian)]
*†''Nematocrania'' Grant, 1976 [Lower Permian (Artinskian)]
*'' Neoancistrocrania'' Laurin, 1992
*'' Novocrania'' Lee and Bruton, 2001 = ''Neocrania''
*†'' Orthisocrania'' Rowell, 1963 rdovician*†'' Petrocrania'' Raymond, 1911 – Lower Carboniferous">ower Ordovician (Tremadocian) – Lower Carboniferous*†'' Philhedra'' Koken, 1889 rdovician*†'' Pseudocrania'' McCoy, 1851 ower Ordovician ("Arenig")*'' Valdiviathyris'' Helmcke, 1940

Gallery

File:EncrustedStroph.JPG, '' Petrocrania'' (small rounded shells) encrusting an Ordovician strophomenide brachiopod
File:Estonian Museum of Natural History Specimen No 193665 photo (g32 g32-3-2 jpg).jpg, '' Pseudocrania'' ''sp''., from the Ordovician of Estonia
File:Isocrania costata Sowerby 1823.jpg, '' Isocrania costata'', from the Upper Cretaceous of The Netherlands
File:Novocrania anomala 001.png, Illustration of '' Novocrania anomala'', a living craniid native to the North Atlantic

References

{{Authority control

Brachiopod families
Extant Ordovician first appearances
Craniata