Orthotetida

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. == 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 '' (Upper Ordovician of Oklahoma, family Triplesiidae). From the front, with the brachial valve on top.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 [Lower Carboniferous – Middle Permian] • Family Pulsiidae [Middle Devonian – Upper Carboniferous] • Family Orthotetellidae [Lower Permian] • Family Derbyiidae [Lower Carboniferous – Upper Permian] • Family Meekellidae [Lower Carboniferous – Upper Permian] • Family Schuchertellidae [Middle Devonian – Permian] • Superfamily Chilidiopsoidea • Family Chilidiopsidae [Upper Ordovician – Middle Devonian] • Family Areostrophiidae [mid-Silurian (Wenlock) – Lower Carboniferous (Serpukhovian)] • Suborder Triplesiidina Moore, 1952 • Superfamily Triplesioidea • Family Triplesiidae [Middle Ordovician ("Llanvirn") – mid-late Silurian (Ludlow)] ==References==