A supra-ordinal classification of the Brachiopoda

Abstract

A new classification of the Brachiopoda is proposed to take into account recent advances in our understanding of the anatomy, shell morphology, ontogeny and phylogeny of the phylum. The use of phylogenetic analysis to help rationalize this new information did not obviate the dilemma facing all previous classifications of how best to reconcile fossil and living data. Over 95% of all recognized genera are founded on extinct species, with the greatest diversity occurring in Cambro-Ordovician times when all but two of the 26 major groups constituting the phylum first appeared. Only five of these groups survive to the present day, albeit as well dispersed representatives of the early diversity. To com pare phylogenies extrapolated from these data, phylogenetic analyses of Recent and Cambro-Ordovician groups were conducted independently by using 55 biological characters for the former group and 69 morphological (and inferred anatomical) features for the latter; only 12 characters were common to both exercises. The cladogram derived for seven Recent suprafamilial taxa, with Phoronis and cyclostome and ctenostome bryozoans as outgroups, is virtually the same as that being obtained by studies of the brachiopod genome. It is also largely compatible with the cladogram for 33 Cambro-Ordovician suprafamilial taxa with Phoronis as outgroup. This cladogram has, in turn, been subjected to stratocladistic tests and has been shown to be consistent with the stratigraphic records of the taxa analysed. A reconciliation of the genealogies derived from the Recent and Cambro-Ordovician data, represented by 14 taxa and clades (with Phoronis as outgroup), was effected by using the 19 synapomorphies characterizing these groups. The resultant cladogram shows living organophosphatic-shelled lingulids (and discinids) as a sister group to a clade of all other living brachiopods. This clade, however, includes the extinct organophosphatic-shelled paterinids and the organocalcitic-shelled craniids. The inclusion of the craniids, in particular, is a cladistic compromise that is inconsistent with genetic and some anatomical and morphological evidence. It was therefore decided to accommodate these inconsistencies by dividing the Brachiopoda into three subphyla, each typified by Recent species with early Palaeozoic ancestors and defined by easily identifiable synapomorphies. The inarticulated Linguliformea, consisting of two classes (Lingulata and Paterinata), is characterized by an organophosphatic shell with a stratiform secondary layer and by planktotrophic larvae. Its modern representatives are the lingulids and discinids. The inarticulated Craniiformea is primarily distinguished by an organocarbonate shell with a laminar secondary layer and the absence of a pedicle throughout ontogeny. The craniids are the sole Recent descendants. The mainly articulated Rhynchonelliformea is the largest subphylum as it embraces five Classes (Chileata, Obolellata, Kutorginata, Strophomenata and Rhynchonellata). Its synapomorphies include an organocarbonate shell with a fibrous secondary layer, the presence of a pedicle without a coelomic core and the development of a recognizable diductor muscle system controlling the opening of the valves about a hinge axis defined by interareas. All Recent brachiopod species articulating with cyrtomatodont teeth and sockets are rhynchonelliforms.