(Barremian 126 ma) and the most primitive monotreme differed substantially from non-monotreme australosphenidans in having five molars as opposed to three, Aptian and Cenomanian monotremes of the family Kollikodontidae (113-96.6 ma) have four molars. The Cenomanian age (100.2-96.6 ma) Steropodon has three pseudotribosphenic molars. This suggests that the monotremes are likely to be unrelated to the australosphenidan tribosphenids. The traditional "theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria (marsupial) and Eutheria (placental mammal) lineages happened prior to the divergence between marsupials and placental mammals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such as egg-laying. Most morphological evidence supports the theria hypothesis, but one possible exception is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing "Marsupionta" hypothesis in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placental mammals. Van Rheede (2005) concluded that the genetic evidence favors the theria hypothesis, and this hypothesis continues to be the more widely accepted one. The time when the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago. The oldest known monotreme is Teinolophos (126 ma). A fossil jaw fragment attributed to a platypus from Cenomanian deposits (100-96.6 ma) at Lighting Ridge, New South Wales, is the oldest platypus-like fossil. The durophagous Kollikodon, the pseudotribosphenic Steropodon, and the possibly small-pig sized Stirtodon occur in the same Cenomanian deposits. Oligo-Miocene fossils of the toothed platypus Obdurodon have also been recovered from Australia, and fossils of a 63 million-year old platypus occur in southern Argentina (Monotrematum), see fossil monotremes below. The platypus genus Ornithorynchus in known from Pliocene deposits, and the oldest fossil tachyglossids are Pleistocene (1.7 ma) in age. 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. It has been suggested that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. The precise relationships among extinct groups of mammals and modern groups such as monotremes are uncertain, but cladistic analyses usually put the last common ancestor