• jaws;
  • lampreys;
  • vertebrate palaeontology;
  • gnamostomes;
  • agnathans;
  • elasmobranchs;
  • pharynx;
  • gills;
  • branchial arches;
  • holocephalians;
  • ostracoderms

This study investigates the origin of jaws by re-assessing homologies between the oropharyngeal regions of Agnatha and Chondrichthyes. In accordance with classical theory, jaws are interpreted as the most anterior arches of the ventilatory branchial basket. It is proposed that jaws first enlarged for a ventilatory function, i.e. closing the jaws prevented reflux of water through the mouth during forceful expiration. Next, they enlarged further to grasp prey in feeding. As they enlarged, the jaws tilted forward, squeezing the ancestral oral cavity in front of them (‘old mouth’) into a slit between the jaws and lips. Simultaneously, the anterior part of the pharynx behind the jaws was pulled forward and became a ‘new mouth’ (the buccal part of the buccopharyngeal cavity of gnathostomes). During the transition to gnamostomes, the premandibular cheeks and lips of the old mouth remained in place, and are represented in ammocoete lampreys, chimaeroids, and sharks. The stages in the evolution of gnathostomes, driven by selection for increasing activity, are modelled as: ancestral vertebrate (with unjointed branchial arches) to early pre-gnathostome (jointed internal arches and stronger ventilation) to late pre-gnadiostome (with mouth-closing, ventilatory ‘jaws’) to early gnathostome (feeding jaws).