______________________________________________________________________________________________________________________ Australian Biologist 2004 17 (1) 4 Biogeographic and biostratigraphic implications of Australian Mesozoic marine reptiles Benjamin P. Kear School of Earth & Environmental Sciences The University of Adelaide South Australia 5005 *South Australian Museum, North Terrace, Adelaide South Australia 5000. *address for correspondence kear.ben@saugov.sa.gov.au Abstract Mesozoic marine reptile fossils are known from numerous localities in Australia. The vast majority come from epicontinental marine rocks of Cretaceous age (Berriasian-Maastrichtian); however rare fragmentary specimens have also been recovered from Lower-middle Jurassic (Upper Liassic-Bajocian) freshwater and marine deposits. A tentative report of ichthyosaur material from the Lower Triassic (Spathian) of Western Australia is not yet rigorously substantiated. Because the Jurassic record is poor, meaningful biogeographic comparisons are limited largely to Early Cretaceous faunas. These currently include several cosmopolitan (ichthyosaur Platypterygius , pliosauroid Leptocleidus and Kronosaurus ) and endemic (chelonians Notochelone and Cratochelone) genera, all represented by local regional species. Interestingly, the earliest and latest representatives of several family-level clades (e.g. Polycotylidae, Rhomaleosauridae and possibly the Pachypleurosauridae) are present, perhaps indicating that the Australian region was both a centre of origin and refuge for many groups. Marine reptile faunas from Australia and elsewhere show common changes in taxonomic diversity and faunal composition during the mid-late Mesozoic. For example, ichthyosaurs dominate Early Jurassic (Hettangian-Toarcian) assemblages but are replaced by a rapid radiation of plesiosaurs and marine mesosuchian crocodilians in the Callovian (middle Jurassic). During the Early Cretaceous (Aptian), plesiosaurs formed the major component, at least in the Australian faunas, with ichthyosaurs (and elsewhere marine mesosuchians) undergoing a significant drop in diversity. By the Albian (middle Cretaceous), chelonioid turtles had emerged as a major new faunal element that persisted throughout the Late Cretaceous (Cenomanian- Maastrichtian), by which time ichthyosaurs had become extinct and mosasaurs rapidly evolved to dominate most marine reptile assemblages. Introduction Marine reptile remains have long been known from the Lower Jurassic to Upper Cretaceous epicontinental and continental margin deposits of Australia. The fossils commonly include ichthyosaurs, plesiosaurs, turtles and more rarely mosasaurs with nearly all of the well- preserved and taxonomically identifiable material recorded from units of Lower Cretaceous age. Appraisals of the Australian Mesozoic marine reptile fauna (some including maps listing current localities) have been presented by Molnar (1982a, 1991), Long (1998), Cruickshank et al . (1999) and Kear (2002a, 2003a). In this paper the known occurrences of Australian Mesozoic marine reptile fossils are reviewed and their biogeographic and biostratigraphic implications assessed. Abbreviations and terminology Repository abbreviations: AM, Australian Museum, Sydney; QM, Queensland Museum, Brisbane; SAM, South Australian Museum, Adelaide; UWA, University of Western Australia, Perth; WAM, Western Australian Museum, Perth. Lithostratigraphic nomenclature for Australian marine reptile bearing deposits follows Playford and Cornelius (1967) and