Phylogeny and Classification: Linguliformea and Craniiformea

Abstract

Brachiopods within the subphyla Linguliformea Williams et al., 1996 and Craniiformea Popov et al., 1993 comprise most, but not all, of the taxa previously grouped together in the Class Inarticulata Huxley, as defined in the first edition of the brachiopod volume of theTreatise on Invertebrate Paleontology(Rowell, 1965). The phylogeny and classification of the inarticulated organophosphatic-shelled linguliforms (typified by the modern lingulides; Fig. 1) and the inarticulated and partly articulated organocarbonate-shelled craniiforms (typified by the modern craniides; Fig. 1) have been debated during the last decade, in connection with attempts to analyze the phylogeny of the entire phylum (for summaries see Holmer et al., 1995; Carlson, 1995; Williams et al., 1996, 2000a). Much of the discussion has been centered around the conflicting results of phylogenetic analyses, either based exclusively on the anatomy and morphology of the Recent taxa (e.g., Popov et al., 1993; Carlson, 1991, 1995), or including also the more numerous, mainly Cambro-Ordovician, fossil taxa (e.g., Carlson, 1991; Holmer et al., 1995; Williams et al., 1996, 2000a). The cladistic analyses have either tended to support the monophyly of the Class Inarticulata (e.g., Carlson, 1991, 1995), or indicated that this group is paraphyletic (e.g., Popov et al., 1993, Holmer et al., 1995; Fig. 1). The resulting cladograms in the comprehensive studies of fossil and modern taxa by Williams et al. (1996, 2000a), which were the basis for the classification adopted in the latest edition of the brachiopod volume of theTreatise on Invertebrate Paleontology, largely support a monophyletic Subphylum Linguliformea; however, the phylogenetic position of the Craniiformea remains uncertain, although it may represent a monophyletic group (Fig. 1). Recent molecular phylogenies have given some support to a monophyletic Linguliformea, but also more surprisingly have indicated that the phoronids might form a clade (formally recognized as the Subphylum Phoroniformea Cohen 2000) within the Brachiopoda, possibly together with the inarticulated modern forms (Cohen and Gawthrop, 1996, 1997; Cohen, 2000).