Review of the tetrapod skull-neck boundary: implications for the evolution of the atlas-axis complex

This review describes variation in modern and fossil occiput-atlas-axis complex anatomy of total group Tetrapoda with the aim of documenting the range of structural variation throughout their evolutionary history to establish grounds for comparison of the complex between tetrapod clades. This review reveals that every modern tetrapod has an atlas with morphology unique to its vertebral column that articulates to the skull, composed, typically, of paired neural arch halves and an intercentrum. Maximally complex tetrapod atlantes articulate to paired proatlas halves and are composed of paired neural arch halves, an intercentrum, and a pleurocentrum. The centra may occur as left and right halves but are most often singular elements in adults. Lissamphibians often have an interglenoid tubercle extending anteriorly from their atlas centrum. Stem tetrapods develop a specialised second cervical vertebra, the axis, most often distinguished from its posterior neighbours by an anteriorly oriented odontoid process contributing to the skull-neck joint. An axis is retained in nearly all subsequently diverging tetrapod clades, except for lissamphibians and their closest relatives. Exemplar fossil taxa reveal patterns of atlas-axis evolution throughout the tetrapod lineage. Here, synthesis of osteological data from extinct and extant taxa provides a basis for hypotheses of skull-neck boundary evolution in tetrapods. For example, convergent trends towards fewer separate components in adult tetrapod atlas-axis complexes, except in crocodylians and rhyncocephalians, are illuminated. Further insights into the development of the atlas and axis may help support or refute these hypotheses and will contribute to a more complete understanding of the origin of observed variation.