(LCA) of placentals and monotremes close to the LCA of placentals and multituberculates, whereas some suggest that the LCA of placentals and multituberculates was more recent than the LCA of placentals and monotremes. ORDER MONOTREMATA Family Ornithorhynchidae: platypus Genus Ornithorhynchus Platypus, O. anatinus Family Tachyglossidae: echidnas Genus Tachyglossus Short-beaked echidna, T. aculeatus T. a. aculeatus (Common short-beaked echidna) T. a. acanthion (Northern short-beaked echidna) T. a. lawesii (New Guinea short-beaked echidna) T. a. multiaculeatus (Kangaroo Island short-beaked echidna) T. a. setosus (Tasmanian short-beaked echidna) Genus Zaglossus Sir David's long-beaked echidna, Z. attenboroughi Eastern long-beaked echidna, Z. bartoni Z. b. bartoni Z. b. clunius Z. b. diamondi Z. b. smeenki Western long-beaked echidna, Z. bruijni Fossil monotremes The first Mesozoic monotreme to be discovered was the Cenomanian (100-96.6 ma) Steropodon galmani from Lightning Ridge, New South Wales. Biochemical and anatomical evidence suggests that the monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose. The only Mesozoic monotremes are Teinolophos (Barremian, 126 ma), Sundrius and Kryoryctes (Albian, 113-108 ma), Steropodon, Stirtodon, Kollikodon, and an unanamed ornithorhynchid (all Cenomanian) from Australian deposits in the Cretaceous, indicating that monotremes were diversifiying by the early Late Cretaceous. A platypus tooth has been found in the Palaeocene of Argentina, so one hypothesis is that monotremes arose in Australia in the Late Jurassic or Early Cretaceous, and that some migrated across Antarctica to South America, both of which were still united with Australia at that time; however, several genetic studies suggest an origin in the Triassic. Fossil species Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils. Family Incertae sedis Genus Kryoryctes Species Kryoryctes cadburyi Family Steropodontidae – paraphyletic assemblage Genus Steropodon Species Steropodon galmani Genus Teinolophos Species Teinolophos trusleri – 123 million years old, oldest monotreme specimen Family Ornithorhynchidae Genus Ornithorhynchus – oldest Ornithorhynchus specimen 9 million years old Species Ornithorhynchus anatinus (platypus) – oldest specimen 10,000 years old Genus Obdurodon – includes a number of Miocene (5–24 million years ago; Riversleigh platypuses) Species Obdurodon dicksoni Species Obdurodon insignis Species Obdurodon tharalkooschild – Middle Miocene and Upper Miocene (15–5 mya) Genus Monotrematum Species Monotrematum sudamericanum – 61 million years old, originally placed in separate genus, now thought an Obdurodon Family Tachyglossidae Genus Zaglossus – Upper Pleistocene (0.1–1.8 million years ago) Species Zaglossus robustus Genus Murrayglossus Species Murrayglossus hacketti Genus Megalibgwilia Species Megalibgwilia ramsayi – Late Pleistocene