Following dissection, the spinal ligaments were observed in a selection of bipedal, pseudobipedal and quadrupedal animals during a search for an appropriate animal model for investigating the innervation of these ligaments. Midline spinal ligaments were found in all animals while lateral spinal ligaments could only be observed in bipedal (human) and pseudobipedal (avian) species. The presence of lateral spinal ligaments in these animals and their absence in quadrupeds suggests that the development of the lateral ligaments reflects a mechanical challenge unique to the erect spine and that these lateral spinal ligaments are fundamental to the stability of an erect posture.