Homeotic Evolution in the Mammalia: Diversification of Therian Axial Seriation and the Morphogenetic Basis of Human Origins
Figure 20
Approximation, inflexion, and transposition of horizontal body planes.
Early synapsid: The horizontal septum (hs) incorporates the ribs and the principal rib head which articulates on the intercentrum (ic) that is in a ventral location between pleurocentra (pc). This places the septum ventral to the neuraxis (nx) and the pleurocentra (Figure 6A1). Therian Septo-Neural Approximation: In therian mammals, the intercentra are lost and the horizontal septum is repositioned to be just ventral to the neuraxis (Figure 7A). Septo-Neural Inflexion Ventrad: In most groups in the Euarchontoglires, the horizontal septum shifts ventrad away from the neuraxis in the lumbar region (Figure 12B). Dorsad inflexions occur sporadically throughout the Theria. Septo-Neural Transposition Dorsad: The horizontal septum is actually transposed to be dorsal to the neuraxis in hominiform hominoids (Figure 18A), the Ferungulata, many groups in the Afrotheria and Xenartha and sporadically in other groups including some rodents (Figure 25B). Diarthro-Neural Transposition: The diarthral plane of tubercular rib heads transposes to be ventral to the neuraxis in many australodelphian metatherians (Figure 7C).