New information on the morphology of yunnanolepid antiarchs (Placoderm fishes) from the Early Devonian of South China

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Abstract
Skull morphology is described from acid-prepared articulated specimens of the antiarch Phymolepis cuifenshanensis from the Early Devonian of Yunnan Province, South China. The nasal and circum-orbital bones have a similar structure to the advanced antiarch Bothriolepis, but with the nasal openings bounded laterally by the sclerotic ring. Yunnanolepis parvus is reinterpreted to have the same structure, assumed primitive for antiarchs. The suborbital plates of Phymolepis meet in the midline, are crossed by a single sensory groove, and also carry possible cutaneous sensory pits as in arthrodires. Three bones (submarginal, prelateral, and infraprelateral) are identified in the cheek complex, and comparisons with Yunnanolepis suggest that they were also present in other primitive antiarchs. Traces of an opercular cartilage are described for the first time in an antiarch. The sensory canal pattern of Phymolepis differs from that of Yunnanolepis, with the supraorbital canal entering the preorbital depression from the lateral plate, an ethmoid commissure on, or just beneath, the rostral margin of the premedian, and two branches to the cheek originating on the lateral plate of the skull. A diversity of sensory canal patterns is predicted for the Early Devonian antiarchs of South China. The trunk armor of Phymolepis has two additional small bones previously unknown in antiarchs, which are proposed as homologs of the anterior lateral plate of arthrodires. The new morphological evidence supports the view that the Yunnanolepiformes is a paraphyletic group, that antiarchs and arthrodires are sister groups, and that Phymolepis may be more advanced than Yunnanolepis in aspects of the sensory canal pattern, but more primitive in retaining vestiges of the anterior lateral plate. The prelateral and infraprelateral bones of antiarchs have homologs in the cheek of arthrodires, but the attachment of the postsuborbital to the quadrate is regarded as an arthrodiran syna-pomorphy. Contrary to previous claims, the quadrate has no dermal bone attachment in Bothriolepis. The absence of small cheek bones in asterolepidoids may be secondary, but the midline suture between suborbitals is considered to be a symplesiomorphy within the antiarchs.

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