ARE THE SPIRIFERINIDA A CLADE? TESTING HOMOLOGY AND HOMEOMORPHY IN EXTINCT SPIRE-BEARERS

Extinct brachiopod orders Spiriferida and Spiriferinida share many morphological features but are distinguished by a single feature: the presence of an impunctate or punctate shell structure. To test the monophyly of these higher taxa, we constructed a morphological matrix of 70 taxa and 116 characters sampled from the full stratigraphic range (Ordovician-Jurassic) of each. We analyzed the matrix with both parsimony and Bayesian methods to learn how results from the two methods compared. We conducted a third analysis utilizing a fossilized-birth-death model to test the role of stratigraphic position together with morphology in constructing a more robust hypothesis of evolutionary relationships.

Parsimony experiments recover a pattern largely consistent with the classification and evolutionary hypotheses of Carter et al. (2006) with the punctate Spiriferinida forming a clade within a paraphyletic Spiriferida. The impunctate delthyridoids form a grade basal to the spiriferinides. Jackknife analyses that remove shell structure characters position the spiriferinides as a grade basal to impunctate post-Silurian spiriferides. Upper Paleozoic spiriferides consistently form two subclades when internal and external character suites are separately removed.

Bayesian MK model analyses do not recover spiriferinides or delthyridines as monophyletic. The Suessioidea form a separate punctate clade with a long ghost lineage, and the delthyridines are dispersed polyphyletically in basal positions. Explicit stratigraphic information in the FBD model separates Devonian and Triassic spiriferinide cyrtiniform homeomorphs, indicating that Suessioidea is not monophyletic. Triassic spiriferinide monophyly with a Paleozoic root cannot be rejected.

Because we recovered significant differences in tree topology and character distribution, we advocate a multi-model approach to phylogenetic inference that acknowledges model assumptions. We can conclude that punctate spiriferinides descended from impunctate spiriferides. However, only parsimony recovers Spiriferinida as a clade, while Bayesian methods indicate two origins for punctae. Mesozoic spiriferinides re-radiated and evolved homeomorphs with then-extinct impunctate Paleozoic spiriferides, with important macroevolutionary implications.