MACRINI, T. E.*; ROWE, T. B.; The University of Texas at Austin; The University of Texas at Austin: Preliminary comparison of cranial endocasts of marsupials

Descriptions of the brains of extant marsupials are not uncommon in the literature but little is known of how these correspond to their endocasts. A thorough understanding of the relationship between the central nervous system (CNS) and the endocranial space of extant taxa is required to make inferences about the brains of extinct taxa. Digital cranial endocasts were extracted from high-resolution X-ray CT scans of twelve individuals representing the following taxa: Monodelphis domestica, Dasyurus hallucatus, and Dromiciops australis. Ten of these endocasts are from a growth series of M. domestica that includes individuals from at least six different ages. Brains of M. domestica specimens of comparable ages were extracted to compare with the endocasts. Additional comparisons were made with published descriptions and illustrations of marsupial brains. The marsupial endocasts were also compared with those taken from a variety of other fossil and extant mammals and non-mammalian cynodonts in a phylogenetic context. The brains of marsupial taxa examined here have lissencephalic cortices, enlarged olfactory bulbs, and well-developed parafloccular processes. These features are well represented on the endocasts and all represent plesiomorphies for Mammalia. The mesencephalon is not visible on the exterior of the marsupial endocasts or brains. A similar condition is seen in monotremes, many extinct groups of mammals, and many other therians. The Dromiciops endocast shows that the corresponding brain has a prominent median sulcus, and cortices that are transversely wide in contrast to the other two taxa. The endocasts from the growth series of M. domestica show considerable variation in shapes and relative sizes of CNS structures such as the olfactory bulbs.