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COMPARATIVE ANATOMY HOME	COMPARATIVE ANATOMY TABLE OF CONTENTS		OBL HOME	OBL REFERENCES
	ATLAS AND AXIS
The first two vertebrae of the vertebral column are given individual names. The first vertebra which articulates with the skull is known as the atlas and the second vertebra is called the axis. In mammals, modifications of the atlas/axis complex allow special movements of the head (e.g. shaking the head “yes” or “no” in humans). Fish lack any real specialization of the atlas/axis like that seen in tetrapods. This modification of the first two vertebrae occured in tetrapods after the most primitive members (such as Ichthyostega). In amniotes, the axis and atlas became more consolidated. The neural arch and centrum of the axis fused (Carroll). In cynodonts, the atlas and axis decreased in size and the centrum of the atlas attached to the axis (Kemp, 1982, p. 181). Intermediate cynodonts stabilized the number of cervical vertebrae at seven, as in mammals. This means that even though a giraffe has a longer neck than a bat, both mammals have seven homologous vertebrae in their necks (Carroll). Cervical ribs fused to the centrum in early mammals. These ribs were not fused in the most primitive mammals nor are they fused in modern montremes, although they had fused in some advanced cynodonts (Kemp, 1982, p.305; Carroll). Because the transverse processes of cervical and lumbar vertebrae are actually composed of ancestral transverse processes plus vestiges of ribs, they are referred to as pleurapophyses (Romer, p. 190). The early mammals lost the proatlas and the zygapophyses between the altas and axis, allowing for free rotation between the first two vertebrae. In the protoeutherian Asioryctes, the intercentrum of the atlas had not yet fused (Carroll, p. 447). Large intervertebral foramina developed between vertebrae in early mammals, perhaps to accommodate the nerves of a larger brachial plexus (Carroll). During the course of evolution, the pleurocentrum of the atlas became attached to the axis (rather than the atlas) and the atlas retained its intercentrum. In humans, the atlas rotates around this projection of the axis when we shake our heads “no”.


platypus atlas		platypus axis

cat atlas

mink		mink
		mink

human atlas		human axis

OTHER CERVICAL VERTEBRAE In mammals, the cervical ribs (which are present as distinct ribs in amphibians, reptiles, and mammal ancestors) fuse to the centra during development, forming a transverse foramen for the vertebral artery.

alligator		turtle

emu

platypus

cat

human