A,B - A key feature of mammalian anatomy is the Laminapophysis (NLM) that carries the major muscle attachments of the thoracic spine. In most mammals, this structure “splits” at the thoraco-lumbar transition to yield a superiorly directed mamillary process and an inferiorly directed styloid process. The LTP (lumbar transverse process) of a monkey is in series with and homologous to the rib and not to any NLM-related structure.

C - In the chimpanzees and humans, the styloid is transformed into a laterally directed LTP (Lumbar transverse process) so that the styloid appears to be missing. The “splitting sequence” shows that the human/chimp LTP is serially homologous to the styloid of the monkey and not to the rib.  In this young juvenile chimp, the growth timing of the LTP is shown by the dark cartilage plate at its tip.