The stapes of stem and extinct Marsupialia: implications for the ancestral condition

This paper describes, for the first time, the stapedes of several non-marsupial species of the metatherian clade Sparassodonta (<i>Sipalocyon gracilis</i>, <i>Arctodictis sinclairi</i>, and <i>Borhyaena tuberata</i>) which were fortuitously encountered during routine microtomography. To augment our comparative set we also scanned and reconstructed single examples of the stapedes of the fossil taxa <i>Sparassocynus bahiai</i> and <i>Thylophorops</i> cf<i>. T. chapalmalensis</i> (Didelphimorphia), <i>Argyrolagus scagliai</i> (?Paucituberculata), as well as single examples drawn from extant members of Caenolestidae (<i>Caenolestes</i> sp.) and Microbiotheriidae (<i>Dromiciops gliroides</i>). The sparassodont, didelphid, and microbiotherian samples exhibit a common bauplan (stapes triangular, with intracrural foramen), whereas the paucituberculatan samples differ in possessing columelliform, imperforate stapes as also previously reported for various australidelphians. The stapedial footplate is rounded in sparassodonts (stapedial ratio, ∼1.6), in both of the fossil didelphimorphians (∼1.7), and in the fossil ?paucituberculatan (∼1.5). According to our optimization of our results of mammalian phylogeny, and in contrast to some other reconstructions, a triangular stapes with intracrural foramen and rounded footplate is likely the ancestral condition for Marsupialia. No particular function can be correlated with possession of the intracrural foramen as opposed to an imperforate stapes, apart from accommodation of the proximal stapedial artery (as seen also in many eutherians). The frequent presence of the intracrural foramen in members of both infraclasses suggests that the ontogenies of the second arch’s blood supply (stapedial artery) and its main skeletal element (stapes) have remained strongly integrated throughout therian evolution, even in cases in which the proximal part of the vessel involutes.