by Aaron G. Filler, MD, PhD
Figure 8-5 Rib Articulations in the Amniotes
A – In the synapsid ancestors of the mammals, the ventral rib head articulated on a pararthrum on a small intercentrum between the adjacent main vertebral bodies, while in diapsids, the pararthrum moved onto the body of the main pleurocentrum.
B – Ribs occurred on all cervical thoracic and lumbar vertebrae in Thrinaxodon – a cynodont synapsid close to the origin point of the mammals.
C – In a marsupial, the upper rib articulation (diarthrum) descends to help form the lumbar transverse process (LTP).
D – In most primates, the posterior half of the lower rib articulation forms the base of the LTP.
A1 - Modified, from: Williston, S. W. and W. K. Gregory (1925). The Osteology of the Reptiles. Cambridge, Harvard University Press.
A2 - Modified, from: Hoffstetter, R. and J.-P. Gasc (1969). Vertebrae and ribs of modern reptiles. Biology of the Reptilia - Morphology A. C. Gans, A. d. A. Bellairs and T. S. Parsons. London, Academic Press. 1: 201-310. Underlying drawing reproduced by permission of Jean-Pierre Gasc.
B - From: Jenkins, F. A. (1971). The Postcranial Skeleton of African Cynodonts; Problems in the Early Evolution of the Mammalian Postcranial Skeleton. New Haven, Peabody Museum of Natural History Yale University. Drawing reproduced courtesy of Yale Peabody Museum of Natural History.
C/D - Modified, from: Filler, A. G. (1986). Axial Character Seriation in Mammals: An Historical and Morphological Exploration of the Origin, Development, Use and Current Collapse of the Homology Paradigm - PhD Thesis. Cambridge, Massachusetts - (Digital 2nd Printing: BrownWalker Press, Boca Raton, Florida 2007, 349 p), Harvard University.