The orthotetides (Orthotetida) are an extinct order of

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s in the class

Strophomenata Strophomenata is an extinct class of brachiopods in the subphylum Rhynchonelliformea. They originated in the Cambrian period, hugely diversified during the Ordovician, and faced near extinction from the Permian-Triassic extinction. Only a few l ...

. Though not particularly diverse or abundant relative to strophomenides (

Strophomenida Strophomenida is an extinct order of articulate brachiopods which lived from the lower Ordovician period to the mid Carboniferous period. Strophomenida is part of the extinct class Strophomenata, and was the largest known order of brachiopods, ...

) or productides (

Productida Productida is an extinct order of brachiopods in the extinct class Strophomenata. Members of Productida first appeared during the Silurian. They represented the most abundant group of brachiopods during the Permian The Permian ( ) is a geolog ...

), orthotetides were nevertheless the longest-lasting order of strophomenates, surviving from the

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period Ma (milli ...

(“

Llanvirn The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period Ma (millio ...

”) up until the

Late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

. Externally, many orthotetides are difficult to distinguish from strophomenides. Most fundamental differences between the two orders are internal: orthotetides have more elaborate

cardinal processes Rhynchonelliformea is a major subphylum and clade of brachiopods. It is roughly equivalent to the former class Articulata, which was used previously in brachiopod taxonomy up until the 1990s. These so-called articulated brachiopods have many ana ...

and a greater diversity of shell microstructure.

Anatomy

All orthotetides have a strophic (straight) hinge line, and a shell profile ranging from biconvex (both valves convex) to concavoconvex (concave dorsal valve, convex ventral valve). In most other regards, the shell profile, ornamentation, and microstructure are strongly variable between orthotetide subgroups.

Shell form

Internally, the ventral valve has a pair of deltidiodont (blunt) hinge teeth. Each tooth is preceded by a dental plate, with the form of a sharp straight crest. Externally, the ventral valve has a large interarea (broad triangular depression) at its rear edge along the hinge line. The

pseudodeltidium Rhynchonelliformea is a major subphylum and clade of brachiopods. It is roughly equivalent to the former class Articulata, which was used previously in brachiopod taxonomy up until the 1990s. These so-called articulated brachiopods have many ana ...

(plate-like middle portion of the interarea) may be flat or smoothly arched. A few species bear a

pedicle Pedicle or pedicel may refer to: Human anatomy *Pedicle of vertebral arch, the segment between the transverse process and the vertebral body, and is often used as a radiographic marker and entry point in vertebroplasty and kyphoplasty procedures ...

opening near the apex of the pseudodeltidium, but many orthotetides close up the opening. The dorsal valve has well-developed internal

cardinalia Rhynchonelliformea is a major subphylum and clade of brachiopods. It is roughly equivalent to the former class Articulata, which was used previously in brachiopod taxonomy up until the 1990s. These so-called articulated brachiopods have many ana ...

(muscle/hinge/organ attachment structures). The cardinal processes (inner crests of the cardinalia) are modified into a pair of prominent lobes or prongs extending from the shell’s inner surface. They may be flanked by an additional projection (

brachiophore Rhynchonelliformea is a major subphylum and clade of brachiopods. It is roughly equivalent to the former class Articulata, which was used previously in brachiopod taxonomy up until the 1990s. These so-called articulated brachiopods have many ana ...

) on the front rim of each tooth socket. The dorsal interarea and its associated structures are small or absent, unlike their equivalents on the ventral valve.

Microstructure

The thick internal (secondary) shell layer has a microstructure of stacked laminar blades. Some othotetides are impunctate, with perfectly flat laminae in their secondary layer. Others develop tiny tubercles derived from stacked deflections oriented externally (the extropunctate condition) or internally (pseudopunctate). The extropunctate condition is unique to orthotetides. The pseudopunctate condition is characteristic of their relatives the strophomenides, with one major difference: orthotetide pseudopunctae always lack taleolae (internal calcite supporting rods). Despite their similarities, the laminar microstructure of orthotetide and strophomenide shells is most likely a case of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

, with both groups originating independently from non-laminar brachiopods.

Subgroups

Orthotetida is divided into two suborders: the Triplesiidina and Orthotetidina. The Triplesiidina are probably derived from older brachiopods in the order Billingsellida, while the Orthotetidina are most likely descendants of early triplesiidines.

Triplesiidina

The Triplesiidina have strongly biconvex shells which are often uniplicate (with a drooping rim and a broad central sulcus on the ventral valve). The ventral interarea is enlarged, with a small pedicle opening at the apex of the pseudodeltidium. The pseudodeltidium is mostly flat, apart from a narrow vertical fold (monticulus) along its midline. The cardinal processes are horn-like, elongated and recurved prongs which are joined at the base but otherwise free-standing. The shell microstructure is typically impunctate.

Orthotetidina

The Orthotetidina tend to be slightly concavoconvex, with an ornamentation of costellae (thin ridges radiating from the back of the shell to the rim). Most orthotetidines lack a pedicle foramen, with the ventral valve cemented directly onto the substrate. Orthotetidines trend towards the development of a pseudopunctate or extropunctate microstructure to varying degrees, relative to their impunctate ancestors. In addition, small pits are often abundant near the hinge area of the ventral valve. Other trends in orthotetidine evolution include the deepening of the ventral valve, the elongation of the cardinal processes, and the development of a perideltidium (a raised surface adjacent to the pseudodeltidium).

List of families

* Suborder Orthotetidina Waagen, 1884 ** Superfamily Orthotetoidea *** Family Orthotetidae ower Carboniferous – Middle Permian*** Family Pulsiidae iddle Devonian – Upper Carboniferous*** Family Orthotetellidae ower Permian*** Family Derbyiidae ower Carboniferous – Upper Permian*** Family Meekellidae ower Carboniferous – Upper Permian*** Family Schuchertellidae iddle Devonian – Permian** Superfamily

Chilidiopsoidea Strophomenata is an extinct class of brachiopods in the subphylum Rhynchonelliformea. They originated in the Cambrian period, hugely diversified during the Ordovician, and faced near extinction from the Permian-Triassic extinction. Only a few l ...

*** Family

Chilidiopsidae Strophomenata is an extinct class of brachiopods in the subphylum Rhynchonelliformea. They originated in the Cambrian period, hugely diversified during the Ordovician, and faced near extinction from the Permian-Triassic extinction. Only a few l ...

pper Ordovician – Middle Devonian*** Family Areostrophiidae

Wenlock Wenlock may refer to: Places United Kingdom * Little Wenlock, a village in Shropshire * Much Wenlock, a town in Shropshire ** (Much) Wenlock (UK Parliament constituency) ** Wenlock Priory, a 7th/12th-century monastery * Wenlock Basin, a canal basi ...

) – Lower Carboniferous (Serpukhovian)] * Suborder Triplesiidina Moore, 1952 ** Superfamily Triplesioidea *** Family Triplesiidae [Middle Ordovician ("Llanvirn") – mid-late Silurian (Ludlow Epoch, Ludlow)]

References

{{Taxonbar, from=Q48971355 Strophomenata Brachiopod orders Middle Ordovician first appearances Lopingian extinctions