Article: Reconstruction of the jaws and braincase in the Devonian placoderm fish Bothriolepis

New material of the antiarch Bothriolepis, from the Gogo Formation (early Upper Devonian, Canning Basin, Western Australia), provides morphological details of the visceral jaw elements, which were not previously known in antiarchs. The palatoquadrate lacks a high orbital process, and was attached to the ventral part only of the suborbital (mental) plate. This shows that the ethmoidal region of the braincase must have been considerably deeper than previously thought. Detailed descriptions are given of the dermal elements in the jaws (suborbital and infragnathal) and cheek (submarginal, prelateral, infraprelateral). On the evidence of the palatoquadrate the mental plate of antiarchs is homologized with the suborbital in other placoderms. The absence of supragnathals may be secondary, and the differentiation of the infragnathal into biting and non-biting divisions probably evolved independently in antiarchs and euarthrodires. Reassembly shows that the submarginal and infraprelateral plates in Bothriolepis fitted closely against the anterior ventrolateral to close the operculum. A new restoration of the endocranium is presented, based on the identification of a posterior postorbital process and cucullaris fossa in Asterolepis. It is suggested that in Bothriolepis the palatoquadrate had an amphistylic connection to subocular and subnasal shelves, that the lateral pit was bounded posteriorly by the anterior postorbital process to form a mandibular muscle fossa, and that the preorbital recess housed the rhinocapsular bone. Comparison with Yunnanolepis indicates that the preorbital depression in this form contained a discrete rostral capsule with lateral nasal openings, and that the 'orbital fenestra' in those antiarchs with a preorbital depression is equivalent to the suborbital fenestra of Bothriolepis. Certain characters defining the relationships of antiarchs to other placoderm groups are summarized in cladogram form; prelaterals, infraprelaterals, a long obstantic margin, and prominent posterolateral corners on the skull are proposed as synapomorphies of antiarchs and euarthrodires. Fusion of the quadrate to the postsuborbital is a possible additional synapomorphy of actinolepids and phlyctaenioids.