The brachiopod order Atrypida originated in the Middle Ordovician and went extinct in the Late Devonian. Few cladistic studies have been undertaken for this group. Here we investigate their early evolution through the Silurian time. We present a parsimony-based phylogenetic analysis of 41 characters and 70 genera representing all the early taxonomic groups within the order. The stratigraphic record of the fossil genera analysed here strongly supports the parsimony phylogeny. Most currently recognized subfamilies and families may be identified within clades, except for the Atrypinae, Idiospirinae, and the Septatrypidae. This warrants subdivision of the former and redefinition of the two latter. The subfamily Atrypinae is redefined and a new subfamily of the Atrypidae, the Protatrypinae is split off. Tracing character evolution across the tree indicates that calcified spiralia and a jugum or jugal processes characterize all members of the Atrypida, except the most basal clade, the Cyclospiridae, that lacks a calcified jugum. Ribs are a homoplasious character, occurring in numerous clades throughout the tree. Darriwillian through Sandbian was a time with rapid evolution when most new autapomorphies seen in the atrypides evolved. This interval is part of the Great Ordovician Biodiversification Event (GOBE). Solid teeth, dorsal direction of spiralia, elaborate ornamentation, and frills evolved in the latest Katian into early Silurian time. Solid teeth and dorsally directed spiralia, seen in distantly related clades from the latest Katian on, may be analogous features. The Late Ordovician Mass Extinction (LOME) is indicated in the stratigraphic tree together with a possible event at the end of Aeronian. The tree supports the long-held assumptions that the Plectatrypinae evolved from the Spirigerininae. It further suggests that the genus Tuvaella is included with the Davidsonioidea. Likewise, the redefined Atrypinae is a sister group of the Lissatrypidae and a derived group in the phylogenetic tree.

The Treatise (Copper 2002) lists 66 Ordovician and Silurian genera in the order Atrypida.  We coded these genera for 41 discrete, unweighted, unordered, binary, and multistate morphological characters (Table 1, Figs. 1, 2). Defined or redefined genera described after 2002 yielded 13 additional taxa, for a total of 79. Our final dataset had 70 genera after eliminating 12 genera with incomplete character sets and the addition of three supplementary Devonian genera (see below). We analysed the data with parsimony analysis with PAUP version 4.01 (Swofford & Sullivan 2003), using heuristic search methods with 1000 replications. The data were also analysed with the program TNT with both traditional search methods and a new method search.

Mesquite: Mesquite Project

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