The morphological revisions of freshwater fish from Late Jurassic - Early Cretaceous sites in Australia and other Gondwanan continents leads to new phylogenetic hypotheses of relationships among stem teleosts

Abstract

Australia has two important sites for freshwater Mesozoic fishes. The Talbragar Fossil Fish Bed in New South Wales is a Tithonian (Upper Jurassic) freshwater lake deposit containing the iconic form Cavenderichthys talbragarensis, first described by Woodward at the end of the 1800s. The Koonwarra Fossil Bed in Victoria is an Albian (Lower Cretaceous) freshwater lake deposit containing Waldmanichthys koonwarri, first described by Waldman about 50 years ago. Following the tradition of the time, both species were first ascribed to the genus Leptolepis. In 2015, these two species were placed into a newly erected family, Luisiellidae, with the Oxfordian-Tithonian fish Luisiella feruglioi from Argentinian Patagonia. This new family was interpreted as a stem teleost group, closer to Leptolepis coryphaenoides than to the crown-group Teleostei. That hypothesis was contrary to previous interpretations, which placed Cavenderichthys as a teleost incertae sedis in the crown Teleostei. This thesis re-examines the morphological characters of the Australian taxa, including 43 specimens of W. koonwarri, from Museum Victoria. Some new characters include the special configuration of the jaws and the position of the quadrate-mandibular articulation, the pattern of vertebrae at the level of the abdominal/caudal regions, and a stegural-like uroneural in the caudal skeleton. Finding new characters in Waldmanichthys called for reappraisals of the morphology of Cavenderichthys and Luisiella. In the case of Cavenderichthys, 126 specimens were re-examined, but for specimens of Luisiella, a conservative approach was followed, based on its last morphological description as well as photographs taken recently. The systematic position of the three Gondwanan taxa was re-evaluated using a pre-existing data matrix including 240 characters and 56 taxa. The new results give a very different scenario, with the three taxa included in the crown-group Teleostei. The family Luisiellidae is restricted to its type species Luisiella feruglioi. The two Australian fish taxa cluster together with Late Jurassic European taxa Leptolepides and Orthogonikleithrus, and are now ascribed to the family Orthogonikleithridae. The new results suggest that the three Gondwanan genera are stem taxa within the Osteoglossocephala (osteoglosomorphs plus more advanced teleosts), and their combination of morphological characters has a major effect on the interpretation of basal euteleosts, questioning some previous interpretations, as for instance, the homology of the stegural as an euteleostean character. The family Archaeomaenidae has been accepted as holding the Australian species Archaeomaene tenuis, Madariscus robustus, and Wadeichthys oxyops, the Antarctic Oreochima ellioti, the Chinese Zaxilepis ginglongensis and the Mongolian Gurvanichthys mongoliensis. The anatomical redescription of Archaeomaene has provided evidence that Madariscus robustus is a synonym of Archaeomaene tenuis, as specimens formerly ascribed to Madariscus robustus are the larger specimens of an ontogenetic series of Archaeomaene tenuis. Similar anatomical redescriptions of Wadeichthys oxyops and Oreochima ellioti have provided further evidence confirming their positions in Archaeomaenidae. Two lesser known fish, Zaxilepis ginglongensis from China and Gurvanichthys mongoliensis from Mongolia have also been included in the phylogenetic analysis and confirmed as members of Archaeomaenidae, which are stem teleosts or teleosteomorphs phylogenetically close to the older Eurasian family Pholidophoridae. The new knowledge presented in this thesis illuminates the importance of Australian taxa in the Tree of Life of Fishes. Show more