Homeotic Evolution in the Mammalia: Diversification of Therian Axial Seriation and the Morphogenetic Basis of Human Origins
Evidence for independent formation of a parallel segmental identity gradient for ribs that may differ from the vertebral gradient is demonstrated by frame shifting. The synapsid (sy) primitive condition has a principal (capitular) rib head articulating on a pararthrum on the intercentrum (ic) which seems to serve as a morphogenetic “target”. In basal therians (th), there is no intercentrum, but the rib head still articulates between the two centra (pleurocentra) as if the lost intercentral morphogenetic target were still present. The articulation is divided into a pre-pararthrum (red) on the anterior end of the following vertebra (iso-segmental) and a post-pararthrum (orange) on the posterior end of leading vertebra. In the posterior thorax of many eutherians (e.g. Euarchontoglires, the Xenarthran Order Pilosa) and some metatherians, the post-pararthral articulation is lost (post1)-“pre-pararthral dominance”-and the diarthral (blue) articulation is also suppressed in many groups (post2). However in metatherians, the Xenarthran Order Cingulata, Hippopotamidae and Cetacea, it is the pre-pararthrum that is lost-“post-pararthral dominance”-in the posterior thorax so that the capitulum articulates only with the post-pararthrum (ant1). The post-pararthrum may move away from the intervertebral space (ant2). In some groups, the diarthrum is also lost so that the rib (e.g. r9) articulates only with the leading vertebra (T8)-this is seen sporadically in the posterior thorax in myomorph, hystricomorph and anomaluromorph rodents and perissodactyls.