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

ABSTRACT

This paper describes, for the first time, the stapedes of several non-marsupial species of the metatherian clade Sparassodonta (Sipalocyon gracilis, Arctodictis sinclairi, and Borhyaena tuberata) 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 Sparassocynus bahiai and Thylophorops cf. T. chapalmalensis (Didelphimorphia), Argyrolagus scagliai (?Paucituberculata), as well as single examples drawn from extant members of Caenolestidae (Caenolestes sp.) and Microbiotheriidae (Dromiciops gliroides). 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.

ACKNOWLEDGMENTS

We would like to thank curators that permitted the study and scanning the specimens from the collection under their care, A. Kramarz and S. Álvarez (former), L. Chornogubsky (present) (MACN-Pv), P. Teta (MACN-Ma), M. Taglioretti and F. Scaglia (MMP), M. Reguero (MLP), D. Brinkman (YPM), R. O’Leary and J. Galkin (AMNH), as well as A. Boscaini and S. Ladevèze for their help with the tomographies, and J. Blanco for the drawings of the stapes in Figure 5. We also would like to thank R. M. D. Beck for his helpful revision and advice, as well as a second anonymous reviewer. This study is a contribution to the project PICT 2015-966, funded by the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) through the Fondo para la Investigación Cientifica y Tecnológica (FONCyT).

Notes

*Percentage of the most likely state. Corresponding state is in parentheses.