A primitive chondrichthyan braincase from the Early Devonian of South Africa

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Abstract
The earliest-known chondrichthyan braincase, from the Early Devonian (Emsian) Gydo Formation of South Africa, is described along with parts of the visceral skeleton. Only the ventral surface of the braincase is exposed, but CT-scanning permits the investigation of its unprepared regions and internal features. There is a persistent cranial fissure (as in osteichthyans) separating the trabecular and parachordal regions. The parachordal cartilage is identical to isolated endoskeletal elements referred to Pucapampella from the Middle Devonian of Bolivia. The semicircular canals are arranged as in osteichthyans and chimaeroids, with a crus commune connecting the anterior and posterior canals dorsally. The elongate palatoquadrate has an ethmoidal and palatobasal articulation (it is unknown if a postorbital articulation was present). The mandibular joint is positioned lateral to the widest part of the parachordal region, and the hyoid arch probably helped support the jaw. The South African specimen demonstrates unequivocally that some features previously known only in osteichthyans (e.g., ventral otic fissure, posterior dorsal fontanelle, palatobasal articulation) were also present in primitive chondrichthyans and are actually plesiomorphic hold-overs whose distribution was primitively more universal among gnathostomes. Fossils rarely overturn the phylogenetic status of morphological characters at such deep phylogenetic levels, perhaps because their wider original distribution pattern was short-lived and is rarely recovered from the fossil record.