Stepwise formation of autostyly in lungfishes revealed by comparative analysis of palatoquadrate

Autostyly, defined by fusion of the palatoquadrate to the neurocranium and loss of the hyomandibula as a suspensory element, is a hallmark of lungfish cranial organization among sarcopterygians, yet its evolutionary origin remains incompletely understood. This study elucidates this transition through a detailed comparative analysis of the palatoquadrate and its articulations in three sequentially diverging stem dipnoans: Youngolepis, Diabolepis, and the earliest eudipnoan Paleolophus. Youngolepis exhibits a reoriented palatoquadrate, indicating functional reorganization of jaw mechanics prior to autostyly. In Diabolepis, reduction of the pars autopalatina and retention of a primitive suborbital articulation suggest further modification of jaw suspension. Paleolophus documents the earliest unambiguous autostyly, with direct palatoquadrate-neurocranial integration preceding complete fusion with the lateral commissure, along with an enlarged adductor fossa. Comparison with Middle and Late Devonian lungfishes indicates that full autostyly was achieved only after the development of a robust, integrated otic process. These results demonstrate that lungfish autostyly evolved stepwise, driven by increasing biomechanical demands associated with durophagous feeding.