A new crocodylian from the Early Eocene of south-eastern Queensland and a preliminary investigation of the phylogenetic relationships of crocodyloids

Abstract

Kambara implexidens sp. nov. is the second crocodylomorph from the Early Eocene (Ypresian) Tingamarra Local Fauna at Boat Mountain, near the township of Murgon, southeastern Queensland. Kambara is now the best represented genus of early Tertiary crocodylomorphs yet collected from Australia. The new species differs from Kambara murgonensis in several features, the most significant of which is possession of an interlocking dentition. Both species occur in a single stratigraphic horizon, possibly indicating two sympatric populations. The presence of adults and hatclings, coupled with the rarity of intermediately sized animals in the Murgon sample suggests the area may have been used as a nesting ground by one or both species. The new material permits a detailed reassessment of the phylogenetic relationships of Australia's Tertiary crocodylians, and provides impetus for a preliminary investigation into the relationships of many putative crocodylid stem taxa. We define Crocodyloidea and Crocodylidae as the descent community and crown group respectively of extant crocodylids (species of Crocodylus, Osteolaemus tetraspis and Tomistoma schlegelii) so as to place these taxa in a taxonomy reflecting phylogenetic relationships. Our results tentatively suggest that Mekosuchinae, as defined previously, is polyphyletic; Harpacochampsa camfieldensis is more closely related to Crocodylidae, whereas all other mekosuchines appear to represent a relic Gondwanan radiation of plesiomorphic crocodyloids, outwardly similar to several early Tertiary Northern Hemisphere forms of comparable grade such as Asiatosuchus germanicus and ‘Crocodylus’ affinis. Within Mekosuchinae (excluding Harpacochampsa camfieldensis), species of Kambara form a clade with Australosuchus clarkae. This clade forms an unresolved trichotomy with species of Pallimnarchus and Mekosuchini (species of Baru, Mekosuchus, Quinkana and Trilophosuchus rackhami). A previous trend in the study of crocodylomorph phylogeny has been to exclude from consideration characters thought to be associated with adaptive complexes, as this was seen to introduce unnecessary homoplasy. We challenge this assumption. Detailed investigation of the distribution of such characters among crocodylians can provide significant evolutionary information at varying taxonomic levels.