NEW INFORMATION ON THE MORPHOLOGY OF YUNNANOLEPID ANTIARCHS (PLACODERM FISHES) FROM THE EARLY DEVONIAN OF SOUTH CHINA

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 t he nasal openings bounded laterally by the sclerotic ring. Yunnanolepi s parvus is reinterpreted to have the same structure, assumed primitiv e for antiarchs. The suborbital plates of Phymolepis meet in the midli ne, are crossed by a single sensory groove, and also carry possible cu taneous sensory pits as in arthrodires. Three bones (submarginal, prel ateral, and infraprelateral) are identified in the cheek complex, and comparisons with Yunnanolepis suggest that they were also present in o ther primitive antiarchs. Traces of an opercular cartilage are describ ed for the first time in an antiarch. The sensory canal pattern of Phy molepis differs from that of Yunnanolepis, with the supraorbital canal entering the preorbital depression from the lateral prate, an ethmoid commissure on, or just beneath, the rostral margin of the premedian a nd two branches to the cheek originating on the lateral plate of the s kull. 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 Yunnanolepiform es 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 infraprelat eral bones of antiarchs have homologs in the cheek of arthrodires, but the attachment of the postsuborbital to the quadrate is regarded as a n arthrodiran synapomorphy. Contrary to previous claims, the quadrate has no dermal bone attachment in Bothriolepis. The absence of small ch eek bones in asterolepidoids may be secondary, but the midline suture between suborbitals is considered to be a symplesiomorphy within the a ntiarchs.