An order comprising the duck-billed platypus (Ornithorhynchus anatinus) and the echidnas or spiny ant-eaters, Tachyglossus and Zaglossus. There were some extinct forms of which very few are known in detail, though of those some attained large sizes. The echidnas have no fossil record older than the Pleistocene, but a fossil platypus, Obdurodon, is known from the Miocene and in the early 1990s teeth of an undoubted platypus-like form, Monotrematum sudamericanum, were discovered in Palaeocene deposits in Patagonia. Two Cretaceous genera, Steropodon and Kollikodon, are also known. In view of their reptilian affinities they are thought to represent a separate and direct line of descent from the earliest Mesozoic animals, independent of the line leading to other mammals. They retain many primitive features and are quite unlike marsupials or eutherians (placental mammals).