Cretaceous Crocodyliforms from Lightning Ridge, NSW

Abstract

<p>Modern crocodyliforms belong to a group known as the Eusuchia, which first arose during the Cretaceous period, roughly 100 million years ago. This group is significant as it is one of the only two lines of archosaurs, along with birds, which still exist today. </p> <p>Extant eusuchians show only a fraction of the diversity that this group displayed during the Cretaceous. Some members of this group grew to very large sizes (such as the enigmatic Aegyptosuchidae), and some appear to have evolved the capacity to process food other than meat (the Hylaeochampsidae). Fossils of eusuchians have been found on almost every continent, with some evidence indicating an Australian origin for this group. The systematics, diversity and distribution of crocodyliforms in Gondwana are explored in a literature review, which forms the first chapter of this thesis.</p> <p>Mesozoic crocodyliforms as a group are, in general, poorly known from Australia. Prior to this study, only one confidently described taxon was known from this time period, <i>Isisfordia duncani</i> , from the Albian-Turonion Winton Formation, at Isisford in Queensland. Upon its discovery, <i>I. duncani</i> was interpreted to have features that placed it as the basal-most member of Eusuchia. Other Cretaceous crocodyliform remains have been uncovered at Dinosaur Cove (Eumeralla Formation, Aptian) in Victoria, and from the Cenomanian Griman Creek Formation near Lightning Ridge, New South Wales. Of these, only the latter has produced a significant number of fossils, on which this study is based.</p> <p>There have been two taxa that have been previously described from the Griman Creek Formation. The first, ‘<i>Crocodylus (Bottosaurus) selaslophensis</i>’ (Etheridge, 1917) is represented by a unique jawbone fragment bearing six teeth. This taxon is now considered a nomen dubium. The second taxon, an indeterminate mesoeucrocodylian, was described in 2001 based on fragments from the maxilla and mandible. Apart from these published accounts, there are many other cranial and postcranial remains that have not yet been closely analysed.</p> <p>The second chapter of this thesis presents a reanalysis of the ‘<i>C. selaslophensis</i> ’ holotype and assigns the aforementioned jawbone, along with a previously undescribed braincase, to a new taxon, <i>Isisfordia molnari</i> . The final chapter addresses a considerable number of other cranial and postcranial remains, also from the Griman Creek Formation, many previously undescribed. This includes the first partial skeleton of an Australian Mesozoic crocodyliform outside of the Winton Formation, also ascribable to I. molnari</p> <p>The work presented here clarifies the taxonomic affinities of the crocodyliform fossils from the Griman Creek Formation. The studies contained herein present strong evidence that suggests that there is a single crocodyliform taxon present within this formation, or at the very least a form that is closely related to <i>Isisfordia</i> . This adds to the broader knowledge of early eusuchians and fills some of the gaps in understanding life in Australia during the Cretaceous. </p>