Abstract
The interpretation of the wall of the brain case between orbit and ear has important implications for mammalian phytogeny. The fossil mammalian groups morganucodonts, triconodonts and multituberculates possess a reduced alisphenoid and a large anterior process of the petrosal. In this they resemble monotremes. By contrast ‘therians’ (marsupials, placentals and their putative ancestors) have a large alisphenoid and no anterior process. Palaeontologists reject forms with a large anterior process from a position close to the ancestry of therians, though uncertain of the timing of the divergence1,2. The development of the brain case of monotremes is crucial here. The view that their anterior process is a laminar outgrowth of the petrosal3 has been widely accepted. Recent work has shown that this is not true in echidnas4,5 and here I confirm the suggestion6 that it is not true in platypus. The ossification pattern of living mammals indicates the possibility of close affinity between ‘therian’ and ‘non-therian’ mammals.
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Presley, R. Alisphenoid equivalents in placentals, marsupials, monotremes and fossils. Nature 294, 668–670 (1981). https://doi.org/10.1038/294668a0
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DOI: https://doi.org/10.1038/294668a0
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