Crossopterygian fishes from the Devonian of Antarctica: systematics, relationships and biogeographic significance

Abstract

Four new genera of osteolepifonns and a new rhizodontifonn genus are described from the Middle-Late Devonian Aztec Siltstone (Beacon Supergroup) of Antarctica. Other indetenninate osteolepid, eusthenopterid and rhizodontid remains, and a single indetenninate porolepifonn scale, are described from the same fonnation. Koharalepis jarviki n.gen., n.sp. is a large cosmine-covered osteolepifonn with a broad flat head, two pairs of dennosphenotics, large extratemporals, a large postorbital in the cheek which does not reach the orbit, an elongate jugal, lachrymal separating the maxilla from the premaxilla at the jaw margin, and large anteriorly pointed median and triangular lateral extrascapulars. It is placed with Canowindra Thomson and Beelarongia Long in the new family Canowindridae. Platyethmoidia antarctica n.gen., n.sp. is a poorly known form with a very broad, flat fronto-ethmoidal shield with dorsomesially oriented slit-like nares, and a lower jaw with a deep articular region. It may be closely related to Gyroptychius? australis Young & Gorter. Mahalalepis resima n.gen., n.sp. is interpreted on the morphology of the frontoethmoidal shield as an early megalichthyid, and a new definition of this family is presented. Vorobjevaia dolonodon n.gen., n.sp. is a poorly known osteolepid with a distinctive jaw morphology. The rhizodontifonn Notorhizodon mackelveyi n.gen., n.sp. was the largest fish in the Aztec fauna, attaining a length of over 3 m. It is characterised by elongate frontals and small dennosphenotics in the skull, Rhizodus-type tusks, and strong tooth flanges on the coronoid and dennopalatine series bones in the jaws. The braincase in a rhizodontifonn is described for the first time, and in general morphology closely resembles that of Osteolepifonnes. Notorhizodon resembles Screbinodus Andrews in dennal ornament and Barameda Long in skull pattern. All previously described crossopterygian material from the Aztec Siltstone is revised, and Gyroptychius antarctic us (Smith Woodward) is regarded as a nomen nudum. The stratigraphic distribution of rhipidistians in the Aztec Siltstone is summarised, and the biogeography and phylogenetic relationships of the