THE ATLAS RIB IN ARCHAEOPTERYX AND ITS EVOLUTIONARY IMPLICATIONS

The presence of the atlas rib in Archaeopteryx is reported for the first time. The morphology and position of this bone in Archaeopteryx generally retain the plesiomorphic conditions for archosaurs. The reduction in robustness of the atlas rib is an evolutionary trend apparent in Theropoda and may be associated with the change in the site of origin of the subvertebral muscle from the atlas rib to the anterior cervical centra. The ansa on the atlas in some extant birds, widely regarded as a remnant of the atlas rib, is morphologically and topologically different from the atlas rib in other archosaurians, including Archaeopteryx, suggesting that the former structure might not be homologous with the atlas rib.