total in life. The cortical bone (the outermost layer) is thickest at the neck of the rib (between the joint and the costal cartilage), at max , and was filled with spongy bone. That is, unlike many other aquatic mammals, the ribs did not exhibit osteosclerosis. They did exhibit pachyostosis, and were made thicker and heavier with additional layers of lamellar bone. The ribs' shape indicates Ambulocetus had a narrow and heart-shaped thorax looking at it head-on. Ribs are thickest at the T8–T10 level. Ribs are broadest at the sternum, which suggests strong sternocostal joints. The ribs have a slight S-curve in side view, with the rib heads angled headwards, and the sternocostal joints angled tailwards. The holotype preserves a central and a tailward sternum bone which are both exceedingly thick, about on the outer margins and decreasing towards the centre. The central sternum bone is longer and wider than the tailward one. The eight preserved lumbar vertebrae at the lower back are much longer than the thoracic, and the centra and transverse processes, from L1–L7, continually increase in length and height. The short transverse processes on L8 are probably due to its proximity to the ilium on the hip. The undersides are concave. The spinous processes are long and tall, and project headward from L1–L5, and straight-up from L6–L8. The spinous processes are bulbous on the tailward side to support epaxial muscles. The vertebral laminae are excavated headward to support the interspinous ligaments which connect the spinous processes. The vertebrae are about as robust as those of modern pinniped such as sow leopard seals and bull walruses. The postzygapophyses (the surface where the vertebrae join with each other) is flat rather than revolute, which would have made the series more flexible than that of terrestrial relatives. For the four preserved sacral vertebrae (at the sacrum, between the pelvic bones), the transverse processes of S1 are smaller than those of L8. There is a robust sacroiliac joint with the hip. For the spinous processes, those of S1–S3 are fused. Metapophyses jut straight up from each lamina near the joint, progressively getting smaller with each vertebra. Only five of the tail (caudal) vertebrae are preserved: a possible C1 or C2, a possible C3, a possible C4, a possible C7, and a possible C8. The more headward tail vertebrae have thick transverse processes, whereas those of the middle tail vertebrae are longer