Abstract
The origin of monotremes—egg-laying mammals from the Australian region—and their relationship to therians (marsupials and placentals) have been intriguing zoological problems. Recently Archer et al.1 described a fragment of a lower jaw of an early Cretaceous monotreme from Australia (Steropodon galmani) and interpreted its teeth as tribosphenic and thus supposedly derived from a relatively advanced stage in therian evolution. This discovery is of major importance, and radically alters currently held opinion that monotremes and therians diverged at the earliest stage of mammalian evolution. We agree that Steropodon is a therian but would argue thai its teeth are not tribosphenic. Steropodon appears to have been derived from therians before the development of tribosphenic teeth, possibly during the Jurassic period.
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Kielan-Jaworowska, Z., Crompton, A. & Jenkins, F. The origin of egg-laying mammals. Nature 326, 871–873 (1987). https://doi.org/10.1038/326871a0
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DOI: https://doi.org/10.1038/326871a0
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