Dipnoans as sarcopterygians

Abstract

Dipnoans are osteichthyans, and are the sister group of crossopterygians (actinistians, onychodontiforms, porolepiforms, osteolepiforms, and including tetrapods). They share with crossopterygians the following derived features: anocleithrum, connection between the preopercular and infraorbital sensory lines, true enamel on teeth, cosmine, sclerotic ring with more than four plates, submandibular series, archipterygium, and process of endochondral bone formation. These features characterize the sarcopterygians (crossopterygians and dipnoans), whereas the intracranial joint, double-headed hyomandibula, and three extrascapulae are synapomorphies of crossoptery gians. Rhipidistians (onychodontiforms, porolepiforms, osteolepiforms, and including tetrapods) are characterized by two synapomorphies, the presence of an extratemporal and narrow submandibular bone(s). Plicidentine, four infradentaries, three coronoids, and a fenestra ventro-lateralis are synapomorphies of porolepiforms, osteolepiforms, and tetrapods. The tetrapods are most closely related to panderichthyid osteolepiforms (with which they share labyrinthodont plicidentine, three pairs of median skull roof bones, flat skull with high dorsally situated orbits, and marginal position of external naris). The common ancestor of dipnoans and tetrapods is also the common ancestor of crossopterygians (including tetrapods) and dipnoans; in other words, the hypothetical common ancestor of all sarcopterygians. The dipnoans are not the closest sister group of tetrapods, independently if living forms only are considered, or fossil forms included.