A LUNGFISH (SARCOPTERYGII, DIPNOMORPHA) TOOTH PLATE FROM THE LOWER DEVONIAN OF VIETNAM AND THE ONSET OF MODERN DIPNOAN DENTAL ORGANIZATION

An isolated pterygoid tooth plate of a lungfish from the Lower Devonian (Lochkovian–Pragian) of Vietnam is described. The material is negatively preserved and lacks associated jawbones to assign a reliable taxonomic status and phylogenetic position. The tooth plate is composed of five main tooth rows radially arranged, separated by several interrow denticles and flanked medially and laterally by smaller accessory rows and by a denticulated posterolateral field. The general structure of the tooth plate displays predominantly primitive traits similar to those known in the contemporaneous Diabolepis from China. However, as opposed to Diabolepis, the tooth plate possesses fewer and more organized rows of teeth, a condition typical of more derived dipnoans. Comparisons with other primitive forms indicate that the arrangement of teeth into distinct rows with a lack of interrow denticles is typical of juvenile specimens of Diabolepis, which may imply that the presence of discrete tooth rows in more derived lungfish taxa is a product of paedomorphosis. Given its puzzling mix of primitive and derived features, this new pterygoid tooth plate can be tentatively assigned to a previously known lungfish prearticular from northern Vietnam, although with caution. Pending the finding of more complete remains, the condition shown by this new form might indicate that the establishment of the modern lungfish tooth plate organization occurred no later than the Pragian.