The anterior-most vertebrae and occiput of Eusthenopteron : implications in the origin of the tetrapod atlas-axis complex

The anterior-most elements of the vertebral column of Eusthenopteron foordi are examined by direct observation of articulated material. Large blade-like supraneural spines are present above neural arches 1, 3, and 5, but not 2 or 4. The supraneural spines support a muscular complex involved in feeding, not only through raising the snout, but also in depressing the lower jaws by the forward swing of the quadrate. The similarity in skulls of osteolepiforms, and between osteolepiforms and Paleozoic tetrapods implies that a cranio-vertebral joint of similar function existed in the ancestor of tetrapods, forming the precursor of the tetrapod atlas-axis complex.The distinct nature of the basioccipital and exocippitals of tetrapods is the result of reduced ossification of the occiput, and no the incorporation of vertebral elements onto the back of the braincase relative to the rhipidistian condition. The similarity between the occiput and succeeding vertebrae of some primitive tetrapods is due to analogous ossification of notochordal tissue. Bony occipital condyles arose separately in many lineages of Paleozoic tetrapods. The atlas-axis complex of tetrapods is constrained by the pattern of occipital articulation. It is an excellent character for diagnosis of particular groups, but a poor character for determining relationships between separate lineages