The first non-mammalian cynodonts from Australia and the unusual nature of Australian Cretaceous continental tetrapod faunas.

MATTHEW LAMANNA; S. AHYONG; AGUSTIN MARTINELLI; R. JONES; ANNE MUSSER; STEVEN SALISBURY

Congreso; 79th Annual Meeting, Society of Vertebrate Paleontology; 2019

Cynodonts therapsids appeared by the late Permian and gave rise to mammals in the Late Triassic. Most nonmammalian species were well-diverse until the Early Jurassic and become extinct by the the mid-Cretaceous. These globally distributed synapsids diversified mainly in Gondwana. However, Australia until now had no representatives of this key group. Here we present premammalian cynodonts from the mid-Cretaceous of New South Wales and Queensland, describing or reinterpreting specimens which are here included within the Haramiyoidea clade and as non-mammaliaform Probainognathia. We reassess Kollikodon ritchiei from New South Wales, first interpreted as monotreme and subsequently as a basal australosphenidan. However, distinctive characters linking Kollikodon to haramiyidans challenge these views, including postcanines with multicusped rows; orthal jaw movement (although Kollikodon lacks the palinal action of other haramiyidans); and mediolateral divergence of upper postcanines, a possible haramiyidan synapomorphy. Haramiyavia has a plesiomorphic lower jaw that may have retained substantive postdentary bones, as does Kollikodon. Other fossils include a fragmentary femur and incipiently divided molar tooth root from the Winton Formation of Queensland. The anteroposteriorly compressed femur has an unusually long lesser trochanter like that of non-mammaliaform probainognathians (such as chiniquodontids and basal prozostrodontians) from the Late Triassic of South America and South Africa. Identification of the tooth root is equivocal but likely a premammalian feature: incipient bifurcation of postcanine roots precedes the fully divided roots of mammals and it was independently acquired in separate cynodont lineages. These fossils are all younger than their closest relatives (significantly so in the case of the Winton material), adding to the unique and unprecedented faunal mix of archaic taxa (dicynodonts, labyrinthodont amphibians and tuataras), endemic species, and Gondwanan relict taxa, such as monotremes found in the Mesozoic of Australasia. Australia?s late-surviving non-mammalian cynodonts fill the last void in the global distribution of our mammalian ancestors. The highly unusual blend of Triassic, Jurassic and Cretaceous taxa co-existing in the mid-Cretaceous high latitude environs of Australasia has no parallel elsewhere in the world during the late Mesozoic.