Based on these experimental results from extant out-group clades, ten key features of the tetrapod feeding mechanism can be hypothesized as having been present in the terrestrial anamniotic ancestors of amniotes. (1) Prior to the onset of the mouth opening, gape was held constant with the jaws closed or nearly so prior to initial prey capture, or held at a constant low value before mouth opening begins during the prey transport cycle. (2) Mouth opening occurred as a result of both cranial elevation (the product of epaxial muscle activity) and lower jaw depression (caused by activity in the rectus cervicis and depressor mandibulae muscles). (3) The presence of a fleshy tongue permitted lingual-based prey capture and transport. (4) During prey capture, protraction of the hyoid apparatus was used to project the tongue, and occurred as the mouth was opening, or during a period of relatively stable gape. (5) Retraction of the hyoid apparatus during prey capture returned the tongue to the mouth and continued even after the mouth was closed. (6) The prey capture gape cycle was characterized by four phases-a period of mouth opening, a period of stable or slowly increasing gape, a second period of further mouth opening, and a period of rapid mouth closing. (7) A forward lunge occurred concomitant with the prey capture gape cycle. (8) Terrestrial prey transports exhibited a bell-shaped gape profile and were distinct kinematically and electromyographically from the initial capture of prey. (9) Transport of prey within the mouth occurred by posteroventral movements of the hyoid apparatus during mouth opening (in a manner similar to aquatic prey transport). (10) During transport, the hyoid apparatus was protracted (moved anterodorsally) during a Recovery Phase, after the mouth was closed.
Several of these plesiomorphic patterns are modified significantly within amniotes, and yet without an understanding of the historical origin of amniote functional traits we would be unable to identify sequences of historical transformation in feeding function or to identify homologous functional attributes of feeding systems among tetrapods.
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