Strophomenata

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Treatise on Invertebrate Paleontology, Part H, Vol. 2 (2000):

• Systematics:
Rhynchonelliform brachiopods with secondary shell composed preeminently of cross-bladed laminae but also of fibers or laminar laths in older groups; impunctate in earlier stocks but typically pseudopunctate with or without taleolae or rarely extropunctate; shell outline and profile variable, but essentially planar to weakly concavoconvex in strophomenides; strophic hinge line, commonly with high ventral interarea and reduced dorsal interarea; delthyrium and notothyrium variably covered by pseudoteltidium and chilidium; supra-apical foramen normally developed in larval shells but becoming lost in adults; some species cemented at the umbo; tubular spines uniquely developed in productides; deltidiodont teeth simple, transverse or peglike, lost in strophodontids and post-Famennian productidines, dental sockets commonly defined by low, flat-lying ridges parallel with hinge line; dorsal bases of diductor muscles inserted on notothyrial platform or normally on prominent cardinal process of varied morphology; ventral bases attached laterally of medially placed adductor scars; lophophore supports rare, posteriorly as brachiophores or medially as raised dorsal ridges; mantle canal systems saccate to pinnate, poorly known in later groups.

• Extended Information:
More than 1,500 genera, ranging throughout the Paleozoic era, have been assigned to the Strophomenata and are major constituents of many faunas, some in rock-forming quantities. They include some of the most bizarre species of the phylum so that the class embraces a number of groups characterized by autapomorphies meriting suprafamilial recognition as in the Lyttoniidina and Richthofenioidea.
The most inclusive morphological feature of the Strophomenata is a supra-apical or apical foramen, at least in juveniles, that became sealed in adult valves of many later groups. It is assumed that such a foramen accommodated an apically situated peduncular outgrowth of the ventral mantle that acted solely as an adhesive anchor and, in the absence of adjustor scars, never as an axis of rotation for the shell. The outgrowth is regarded as homologous with that inferred for the Obolellata and Kutoriginata.
Articulatory devices are also widely used in strophomenate classification; but the deltidiodont teeth and socket ridge apparatus, found in all older stocks, is not greatly different from that of early rhynchonellates (orthides) except in one respect. Early strophomenate socket ridges are weak and flat lying, a synapomophic condition precursory to taxonomically significant transformations like the loss of dentition in the later productides or its secondary elaboration in the later orthotetidines.
Shell structure and pseudopunctation have previously played a crucial taxonomic role in distinguishing strophomenates from other brachiopods. Cross-bladed lamination, however, is demonstrably homoplatic with, for example the chonetidine fabric of lathlike secondary fibers appearing some time after its development in Early Ordovician strophomenoids, triplesiidines and orthotetidines, all on contrast to the more orthodox fibrous secondary layers of plectambonitoids. This contrast is also true of the laminar-shelled billingselloids and the fibrous –shelled clitambonitidines, both of which have been provisionally assigned to the same order (Billingsellida). Similarly, pseudopunctation, so characteristic of plectambonitoids and strophomenoids, was not fully established in the orthotetidines until Devonian times, although sporadic traces of a form of pseudopunctation have been found in the otherwise impunctate triplesiidines and older orthotetidines (chilidiopsoids).
Such variability, which is matched in the elaboration of cardinalia, the distribution of spinose outgrowths on the shell and so on, suggests that homoplasy is a serious handicap to a phylogenetic classification of the Strophomenata.

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