Introduction

The general shape, function and development of vertebrae tend to be highly conserved among mammals (Vaughan, 1970; Simmons and Geisler, 1998; Hildebrand and Goslow, 2001; Buchholtz, 2007), where the vertebral column is divided into five distinct regions: cervical, thoracic, lumbar, sacral and caudal. Typical mammalian vertebrae consist of a centrum, a neural arch and two pairs of zygopophyses. On the centrum, a pair of dorsally directed pedicles fuse with the lamina to form the neural arch for protection of the spinal cord. An intervertebral disc separates each centrum; it facilitates multiaxial motion and acts as a cushion between adjacent centra (Hildebrand and Goslow, 2001). Although individual vertebrae separated by intervertebral discs typically remain distinctly separate bones throughout life, vertebral bodies may fuse into multibone units.

Characteristic fusions of vertebrae are well known in turtles and birds, but also occur to varying degrees in some mammals. Fusion of three or more vertebrae into a sacrum that articulates to the ilium is a primitive characteristic in mammals; its loss is considered a derived trait (e.g., Flower, 1885; Vaughan, 1970) and is usually seen only in obligate aquatic mammals. In contrast to the loss of sacral fusion, the cervical vertebrae of many cetaceans are cranio-caudally compressed and often fuse into units of two to seven vertebrae, presumably to provide rigidity of the neck (Flower, 1885). In addition, some rodents, such as jerboas (Dipus sagitta), have fused cervical vertebrae. Jerboas use ricochetal locomotion (using only the hind feet for forward propulsion) and fused cervical vertebrae may provide increased surface area for muscle attachment and vertebral column strength to avoid whiplash injury (Hatt, 1932).