Several of the caudal vertebrae in birds are also fused, forming the pygostyle which supports the rectrices. In tailless amphibians, the caudal vertebrae are fused into a single bone, the rodlike urostyle. The loss of the locomotor role of the tail in mammals led to its contraction. Among reptiles it is well developed, consists of many vertebrae, and is important in locomotion. The caudal segment, usually very mobile, performs a variety of functions. In mammals, the sacrum also consists of one to ten (generally two to four) fused vertebrae, of which only the first and second are true sacral vertebrae. The two sacral vertebrae of birds fused with the lumbar, posterior thoracic, and anterior caudal segments to form a single bone, the complex sacrum that supports the pelvis. Amphibians have one sacral vertebra and reptiles usually have two. The vertebrae of this segment are fixed to the pelvic bones by modified ribs joined with the transverse processes to form the sacrum.

The sacral segment is well developed in all terrestrial vertebrates because of the special role of the rear extremities in locomotion. Among amphibians and reptiles there are no clear-cut distinctions between the segments all the vertebrae situated between the cervical and sacral segments are usually called spinal. The lumbar segment has rudimentary ribs and is generally more mobile than the thoracic segment. In birds, some of the thoracic vertebrae fused to form a single structure permitting flight. The thoracic segment has well-developed ribs, most of which articulate with the sternum to form the thorax amphibians do not have a thorax. The first two cervical vertebrae of amniotes, the atlas and epistropheus, have a shape permitting both tilting and rotating of the head. Amphibians have only one cervical vertabra (the head can only bend down), reptiles have an average of eight (four to nine or more), birds have 11 to 25, and mammals have seven only the manatees and didactyl sloths have six, whereas tridactyl sloths have nine or ten. The ribs of this segment are poorly developed or absent, while the vertebrae generally have well-developed condyles that permit great flexibility of the neck. The need to move the head gave rise to the cervical segment. Terrestrial vertebrates have a spine consisting of four or five segments: cervical, thoracic, lumbar (absent in some groups), sacral, and caudal. Fishes have two segments, a truncal segment with strong ribs and a caudal segment whose vertebrae bear the hemal arches that protect the caudal artery and vein. Functionally, the spine is divided into segments whose number increases from the lower vertebrates to the higher. The secondary function of the spine is to protect the spinal cord, which is contained in a special canal formed by the vertebral bodies from below and by the widened arches from above.

The need to strengthen the spine gave rise to articular processes of the vertebrae, which formerly had been jointed only by the vertebral bodies and connected by ligaments. The supportive role of the spine increased markedly when vertebrates began living on land. The loss of the flexibility that the chorda had provided was compensated for by the greater elasticity and strength of the spine, properties important for the functioning of the truncal musculature. Historically, the appearance of the spine was related to increased mobility and speed in water. Instead, paired arches-superior in cyclostomes and superior and inferior in the others-rest freely on a well-developed chorda. However, cyclostomes, dip-noans, holocephalans, and acipenserids do not have vertebral bodies even when adult. In ontogeny, the development of the cartilaginous or, more commonly, the bony vertebrae making up the spine results in the contraction of the chorda found in embryos and subsequent elimination of the chorda. In phylogeny the spine replaces the chorda of lower chordates. The main part of the axial skeleton of vertebrate animals and of man.

Spine of the scapula