The stapes of stem and extinct Marsupialia: Implications on the ancestral condition.

GAILLARD, C.; MACPHEE, R.D.E.; FORASIEPI, A.M.

Ciudad Autónoma de Buenos Aires (virtual)

Jornada; Reunión Virtual de Comunicaciones de la Asociación Paleontológica Argentina 2020; 2020

Asociación Paleontológica Argentina

For the first time, the stapes of several species of the non-marsupial metatherian clade Sparassodonta (Sipalocyon gracilis, Arctodictis sinclairi, and Borhyaena tuberata) are available to study. The stapes were fortuitously encountered during routine microtomography, inside the osseous labyrinths into which they had fallen post mortem after detaching from the fenestra vestibuli. To augment our comparative set we also scanned and reconstructed single examples of the stapes of the fossil taxa Argyrolagus scagliai (?Paucituberculata), and the Didelphimorphia Sparassocynus bahiai, and Thylophorops cf. T. chapalmalensis, as well as single examples drawn from extant members of Caenolestes sp. (Paucituberculata) and Dromiciops gliroides (Microbiotheridae). The sparassodont, didelphid and microbiotherian samples exhibit a common bauplan (i.e., stapes triangular, with intracrural foramen), whereas the caenolestid samples differ in possessing columelliform, imperforate stapes as previously reported for various australidelphians. The stapedial footplate is rounded in sparassodonts (stapedial ratio, ~1.6), in both fossil didelphimorphians (S. bahiai and T. chapalmalensis, ~1.7) and in the fossil ?paucituberculatan (A. scagliai, ~1.5). According to our optimization in the framework of mammalian phylogeny, a triangular stapes with intracrural foramen and rounded footplate is likely the ancestral condition for Marsupialia. in contrast to some earlier predictions, the columelliform stapes is a derived condition within the clade.