A digital cranial endocast of Pucadelphys andinus, a Paleocene metatherian from Bolivia, is described. This is the first cranial endocast described for the taxon and the only one (to date) from a stem marsupial (i.e., non-marsupial metatherian). The Pucadelphys endocast is compared with others generated from skulls from six taxa of extant marsupials (Monodelphis domestica, Didelphis virginiana, Dasyurus hallucatus, Vombatus ursinus, Phascolarctos cinereus, and Dromiciops australis). The Pucadelphys endocast is generally similar to the didelphid marsupial Monodelphis. However, the olfactory bulb casts and the cerebellar space (approximated using vermis cast length) of Pucadelphys are relatively large in comparison to those of these extant marsupials. The cerebral hemisphere casts of Pucadelphys are dorsoventrally thin in comparison to those of the extant marsupials examined here. With other dimensions of the cerebral hemisphere casts being comparable between Pucadelphys and the extant marsupials, this suggests that the overall volume of the cerebral hemispheres was smaller in Pucadelphys. Thus, two possible synapomorphies for crown Marsupialia are a decrease in size of the cerebellar space and an increase in cerebral hemisphere volume.
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