Characterization of the dipnoi, a monophyletic group

Fossil and extant dipnoans form a well‐defined group of osteichthyans. Tooth plates, a feature in common for extant and the majority of fossil dipnoans, are not found in all dipnoans. Nevertheless primitive dipnoans can be defined by 21 characters of the head skeleton: bone arrangement in the posterior part of the skull roof, relation between supraorbital sensory line and bones, five extrascapulars, ossified (soft in post‐Devonian dipnoans) upper lip, lack of premaxilla and maxilla, number and arrangement of bones in cheek region, lack of coronoids, the presence of an adsymphysial plate, ossified (soft in post‐Devonian dipnoans) lower lip, relationship of oral and mandibular sensory canal to “infradentaries,” course of neurovascular system in lower jaw, symphysial tubuli in lower jaw, gular‐shaped submandibulars, anterior naris at the edge and posterior naris within the mouth, median contact of pterygoids back to jaw articulation, posterior position of parasphenoid (buccohypophysial foramen very anterior in parasphenoid), unpaired “vomer,” autostyly, neurocranial support of posterior dermal skull roof, no isolated hypobranchials, and pharyngobranchials reduced or lacking. These 21 features distinguish the dipnoans from all other sarcopterygians. The Lower Devonian genus Diabolepis , which is said by some authors to have a closer relationship to dipnoans than to any other sarcopterygian, is considered to be inadequately known at present for definite statements about its relationship.