Molecular phylogeny of rhynchonellide articulate brachiopods (Brachiopoda, Rhynchonellida)

Abstract

We present here the first report based on phylogenetic analyses of small subunit (SSU/18S) and large subunit (LSU/28S) ribosomal DNA (rDNA) sequences from a wider-than-token sample of rhynchonellide articulate brachiopods, with data from 11 of ∼20 extant genera (12 species) belonging to all four extant superfamilies. Data exploration by network and saturation analyses shows that the molecular sequence data are free from major aberrations and are suitable for phylogenetic reconstruction despite the presence of large deletions in four SSU rDNA sequences. Although molecular sequence analyses cannot directly illuminate the systematics of fossils, the independent, genealogical evidence and phylogenetic inferences about extant forms that they make possible are highly relevant to paleontological systematics because they highlight the limitations of evolutionary inference from morphology. Parsimony, distance, maximum likelihood (no clock) and Bayesian (relaxed-clock) analyses all find a tree topology that disagrees strongly with the existing superfamily classification. All tested phylogenetic reconstructions agree that the taxa analyzed fall into three clades designated A1, A2, and B that reflect two major divergence events. The relaxed-clock analysis indicates that clades A and B diverged in the Paleozoic, while clades A1 and A2 reflect Permo-Triassic (and later) events. Morphological homoplasy and possible gene co-option are suggested as the main sources for the discord between the morpho-classification, the results of cladistic analyses of morphology, and the relationships reconstructed from molecular sequences. The origin, function and evolutionary implications of the deletion-bearing rhynchonellide SSU rDNA sequences are briefly discussed in relation to pseudogenes and concerted evolution in the rDNA genomic region.

Embryonic chirality and the evolution of spiralian left–right asymmetries

“…The expression of nodal in the brachiopod N. anomala demonstrated a similar timing and location to that in T. transversa (figure 2 b ), albeit these two species differ significantly in the mode of gastrulation and mesoderm development [47,63]. Since the last common ancestor of T. transversa and N. anomala corresponds to the last common ancestor to all brachiopods [64], our findings indicate that the most probable ancestral expression of nodal in brachiopods was in the anterior right, mature mesoderm. This contrasts with the expression in gastropod molluscs, where nodal is already expressed at relatively early stages (32-cells) and in ectodermal derivatives of the shell and head region [31].…”

The Evolution of Brachiopoda

“…Of course, such a diagram, with orders as terminal taxa, implies the monophyly of each order, which we know does not obtain for many orders. For some orders (Rhynchonellida), the degree of incongruity of morphological results (Schreiber et al 2013) and molecular results (Cohen & Bitner 2013), even among only the few species extant, is worrisome. For others (Terebratulida), the agreement appears to be greater (Cohen 2007;S.J.…”

Craniid brachiopods: aspects of clade structure and distribution reflect continental drift (Brachiopoda: Craniiformea)

“…These analyses (Cohen, ) also show that the root of the craniid radiation is on the branch between Novocrania and Neoancistrocrania , meaning that the latter can be used to polarize the former. For dating analysis, more distant outgroups (ideally with known divergence times, but see below) are required, and these are provided by sequences from selected terebratulide and rhynchonellide articulate brachiopods (Cohen & Bitner, ).…”

“…Progress towards the second aim has been affected by changes in the personnel responsible for the morphological work and by the contributing author's growing appreciation of the importance of developmental constraint and morphological homoplasy in brachiopod classification and phylogeny (Gould, ; Scotland, Olmstead & Bennett, ; Cohen et al ., ; Bitner & Cohen, in press; Cohen & Bitner, ).…”