Ventilation and the origin of jawed vertebrates: a new mouth

TLDR

Jaws are traditionally viewed as the most anterior arches of the branchial basket, and their evolution is modeled as a progression from unjointed arches to ventilatory jaws and finally to feeding jaws driven by increased activity. The study re‑examines oropharyngeal homologies between Agnatha and Chondrichthyes to argue that jaws initially evolved for ventilation. During jaw enlargement, the anterior pharynx was displaced forward to form a new buccal mouth while the premandibular cheeks and lips of the old mouth persisted in early gnathostomes. Jaw enlargement first prevented water reflux during expiration, then enabled prey capture, and ultimately tilted forward to squeeze the ancestral oral cavity into a slit between jaws and lips.

Abstract

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).