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Yinotheria is a proposed subclass that contains a few fossil mammals of the Mesozoic and the extant monotremes. Today, there are only five surviving species, which live in Australia and New Guinea, but fossils have been found in England, China, Madagascar and Argentina. The surviving species consist of the platypus and four species of echidna. ==Evolutionary history== According to genetic studies Yinotheria first diverge from other mammals around 220 to 210 million years ago, at some point in the Triassic or Early Jurassic.〔〔http://timetree.org/pdf/Madsen2009Chap68.pdf〕 The oldest known fossils are a bit younger, dating around 168 to 163 million years in the Middle Jurassic. These fossils are the genera ''Pseudotribos'' of China〔 and ''Shuotherium'' of both China and England. Both belong to the family Shuotheriidae, and they are the only known northern hemisphere group of yinotherians. Around the same time as Shuotheriidae, the infraclass Australosphenida first appeared. The family Henosferidae occupied in the southern hemisphere in Argentina and Madagascar with genera ''Henosferus'', ''Ambondro'', and ''Asfaltomylos''. This suggests that this family could have been more widespread and diverse in Gondwana during that time, although due to their fragile state some species might have been destroyed due to geological events. In the Early Cretaceous in the region that is now known as Australasia the family Ausktribosphenidae and the first monotremes appeared. Despite being found in the same region of the world and same time period, recent work has found that the older Henosferidae is the sister taxon to Monotremata, with Ausktribosphenidae being the next sister taxa in Australosphenida.〔Averianov ''et. al'', 2014〕 Ausktribosphenidae includes the genera ''Bishops'' and ''Ausktribosphenos''. A monotreme fossil of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales. These fragments, from the species ''Steropodon galmani'', are the oldest known fossils of monotremes. Fossils from the genera ''Kollikodon'', ''Teinolophos'', and ''Obdurodon'' have also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern Argentina (since named ''Monotrematum'', though it is now considered to be an ''Obdurodon'' species). (See fossil monotremes below.) Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago, but another putting it at 17–89 million years ago.〔http://timetree.org/pdf/Springer2009Chap69.pdf〕 All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yinotheria」の詳細全文を読む スポンサード リンク
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