The braincase of the chondrichthyan Doliodus from the Lower Devonian Campbellton Formation of New Brunswick, Canada

The braincase of the late Lower Devonian (Emsian) chondrichthyan Doliodus is described for the first time. Its postorbital process is extended ventrally and probably enclosed part of the infraorbital sensory canal, as in some placoderms. Doliodus has a shark‐like dentition, but its upper anterior tooth files were supported by the internasal cartilage of the braincase, not by the palatoquadrates. Modern selachian jaws and dentitions are not representative of primitive crown‐group gnathostomes because they display a mixture of conserved and derived character states. Separation of the palatoquadrates by the internasal cartilage is probably a primitive condition for crown‐group gnathostomes. Continuity of the upper dental arcade across the ethmoid region may represent a synapomorphy of chondrichthyans and some acanthodians (the condition is not found in placoderms or osteichthyans). Exclusion of the arcade from the ethmoid region is probably apomorphic within elasmobranchs. Doliodus has curious bar‐like, paired subcranial ridges ending posteriorly at the hyomandibular articulation. These superficially resemble visceral arch infrapharyngohyals fused to the floor of the braincase, adding circumstantial palaeontological support to the old proposal that parts of visceral arches may be incorporated into the gnathostome braincase, although it seems more plausible that they formed in the lateral margins of the embryonic parachordal or hypotic lamina.