THECIDEID PHYLOGENY, HETEROCHRONY, AND THE GRADUAL ACQUISITION OF CHARACTERS.

Glenn Jaecks

"The classification of thecideid has been contentious: they have been included in the strophomenates, spire-bearers and terebratulides, all within the last three decades. Each of the evolutionary scenarios implied by these alternative classifications have been supported by emphasizing different character suites, shell microstructure and median septum morphology in particular. Most recently they have been afforded order status because they share few characters with any one group. While phylogenetic analysis has so far been little help in resolving this issue, it has provided a clearer picture of gradual character evolution within Thecideida ...
"Characters often associated with Thecideida appear piece-meal and are often homoplastic: the often basal thecideid, Thecospira, is smaller than most outgroup members, yet larger than most other thecideids, suggesting a gradual decrease in 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 other prominent feature in thecideid evolution, the expansion of the shell primary layer (or reduction of the secondary layer), occurs in derived taxa, and may occur more than once. Both shell microstructure and median septum characters played were needed to resolve thecideid relationships, and both character suits are less homoplastic than average thecideid characters. "