Thecideide phylogeny, heterochrony, and the gradual acquisition of characters

Thecideide phylogeny has been investigated by constructing a matrix of 54 characters (some multistate, all unordered, equal weight) and up to 46 ingroup taxa (at least one per genus). Terebratulide, strophomenate, and spire-bearing outgroups were used, together and separately, to polarize the characters and root the trees. Most terebratulide-rooted trees have the Jurassic genus Eudesella basal, and many distal clades were stratigraphically inverted, with the oldest members in derived positions. Trees constructed using other outgroups placed many Triassic taxa, for instance Thecospira, at or near the root and showed better agreement with the stratigraphic record. One common feature of all trees, supported by the presence of a broad median septum and ramuli, was the appearance of a derived Lacazella clade that includes many of the Thecideidae. Characters often associated with Theideida appear gradually over time and are often homoplastic: the relatively large-bodied Thecospira is often basal, suggesting a gradual rather than abrupt decrease in thecideide size. Likewise, Thecospira does not have a dorsal median septum as an adult, suggesting the acquisition of this character later in the group's history. The expansion of the shell primary layer (and/or reduction of the secondary layer), occurs in derived taxa, and may occur more than once.