However, there are also differences : there is no entoconid, and an absence of wear seems to suggest that the upper molars ( as yet unknown ) did not have a protocone, and so implying that there was a double origin of tribosphenic teeth, one group represented by the living placentals and marsupials, and the other group being the monotremes bearing'pseudo-tribosphenic'teeth.
42.
However, there are also differences : there is no entoconid, and an absence of wear seems to suggest that the upper molars ( as yet unknown ) did not have a protocone, and so implying that there was a double origin of tribosphenic teeth, one group represented by the living placentals and marsupials, and the other group being the monotremes bearing'pseudo-tribosphenic'teeth.
43.
In 2001, Zhe-Xi Luo and colleagues alternatively proposed that a tribosphenic molar pattern had arisen twice ( compare Figure 3, top ) once giving rise to the marsupials and placentals ( Boreosphenida ), and once producing " Ambondro ", the Cretaceous Australian " Ausktribosphenos ", and the living monotremes, which first appeared in the Cretaceous ( united as Australosphenida ).