Early Cambrian – Holocene
Rhynchonellata is an extant class of stationary, low-level, epifaunal suspension feeders
Characteristics of the Class
- Most common class of Brachiopoda in the Cincinnatian
- Includes all extant articulated brachiopods
- Wide variety of morphologies
- Most shell forms are biconvex
- Triangular or rounded pedicle foramen
- Pedicle developed from a rudiment
- Pedicle attachment evidenced by imprints of adjustor muscle scars on the inside of the brachiopod
- The ventral muscle field is shifted forward to make room for the pedicle capsule
- Cyrmatodont (knoblike) teeth, elaborated cardinalia, and developed crura (in some)
Treatise on Invertebrate Paleontology, Part H, Vol. 3 (2000):
- Rhynchonelliform brachiopods with fibrous, impunctate, punctate or endopunctate, biconvex, strophic or astrophic shells, articulated by deltidiodont or cyrtomatodont teeth and sockets buttressed by brachiophores or supported by parallel socket or hinge plates repeatedly converging to form septalium or cruralium; pedicle opening as delthyrium or rounded foramen; interareas and notothyrium commonly vestigial or absent, well developed in some derived groups; ventral muscle scars with posteromedial adductors flanked or enclosed by diductors and laterally placed adjustors; dental plates less commonly converging to form spondylium; dorsal adductor scars petaloid or grouped and quadripartite; crura present in later groups and commonly extended as spiralia or loops; mantle canal systems variable as saccate, digitate, pinnate, or lemniscate impressions.
- Extended Information:
- The Rhynchonellata is the largest class of brachiopods. It is composed of about 3,000 genera assigned to 75 superfamilies and grouped into 10 orders. The diagnostic synapomorphy is a pedicle developed from a rudiment. In such extinct groups as the orthides and syntrophiidines, the presence such a pedicle is indicated by the imprint of adjustor scars on the shell interior. A pedicle such as this was variably developed throughout the class, being vestigial in some free-lying groups (pentameridines). But in the thecideides and some atrypides the pedicle rudiment developed into an adhesive pad attended by cementation of the ventral valve, a homoplasy that has caused great difficulties in past classifications of the brachiopods.
- The rhynchonellides and related spire-bearing and loop-bearing orders are further characterized by the reversal of the mantle rudiment during larval development. This transformation is indicated on the shells by a forward shift of the attachment area of the ventral muscle field to make way for the pedicle capsule. It is, therefore, well documented in extinct groups.
- Other synapopmorphies evident in the shell and used in classification at the ordinal level are transformations of articulatory devices. In particular, the development of cyrtomatodont teeth and the elaboration of cardinalia, giving rise to crura supporting the mouth segment of the lohphophore, characterize the rhynchonellides and all derived ordinal groups. Such orders, in turn, are well founded on the nature and disposition of spiralia and loops extending from the crura in support of the variously oriented brachia of the lophophore.
- Among other features used in the suprafamilial classification of the rhynchonellata, the strophic shells of spriferides (s.l), thecideides, and some terebratulides were derived from astrophic stem groups and, therefore, separably taxonomically from those of orthides and protoorthides. Punctuation is also homoplastic; but puntative homologues of the endopunctae characterizing terebratulies and thecideides occur in Jurassic species of spiriferides (s.l).