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

Craniodental Morphology and Phylogeny of Marsupials

“…A large obturator foramen is present (state 0) in the stapedes of most Recent didelphids, Dromiciops, and most diprotodontians. Schmelzle et al (2005: Table 1) coded Didelphis as possessing a collumeliform stapes, but most specimens examined by us exhibit a large obturator foramen (see also Voss and Jansa, 2009: character 81;Gaillard et al, 2021: fig. 5).…”

“…Stapes triangular, perforated by a large obturator foramen (0); or stapes columelliform and microperforate or imperforate (1). The morphological diversity of the marsupial stapes was reviewed by Gaudin et al (1996; see also Gaillard et al, 2021), whose character-state definitions we followed for scoring foraminal size: the maximum width of a "large" obturator (stapedial or intercrural) foramen-characteristic of triangular or "bicrurate" stapedes-is equal to or greater than the width of a surrounding crus (Novacek and Wyss, 1986: figs. 5D-G), whereas the maximum width of the opening in a microperforate stapes is less than the width of a surrounding crus (Novacek and Wyss, 1986: figs. 5C, H-J).…”

“…However, unlike Gaudin et al (1996), we do not distinguish between the microperforate and imperforate conditions, because these are often very difficult to tell apart (see e.g., comments by Segall [1970: 177] and Gaudin et al [1996: 41] regarding Notoryctes and Thylacinus); both morphologies are subsumed by state 1 here. We found other aspects of stapedial morphology incorporated by Gaudin et al (1996: 38) into their scoring scheme, as well as other features that have been scored in previous analyses-for example, the relative sizes of the openings of the obturator foramen on either side of the stapes (Schmelzle et al, 2005: character 12), and the stapedial ratio (e.g., Springer et al, 1997: character 79;Rougier et al, 1998, character 127;Horovitz and Sánchez-Villagra, 2003: character 211;Schmelzle et al, 2005: character 11;Gaillard et al, 2021: character 3)-to be too subjective or too difficult to score consistently due to the presence of intermediate morphologies.…”

“…However, the overall morphology of the stapes of Dactylonax most closely resembles state 0. By contrast, the stapes is columelliform and either imperforate or microperforate on both sides of the same bone in a few didelphids (Caluromysiops, Lestodelphys and some specimens of Tlacuatzin; Voss and Jansa, 2009), all Recent caenolestids (e.g., Caenolestes; Gaillard et al, 2021), Notoryctes, all peramelemorphian and dasyuromorphian taxa for which we were able to examine the ossicles (see also Gaillard et al, 2021), and a few diprotodontians (Vombatus, Lasiorhinus, Phascolarctos, Burramys, and Dactylopsila).…”

“…However, Caenolestes, Lestoros and Rhyncholestes form a clade to the exclusion of our fossil terminals in our dated total-evidence analysis, which is congruent with current published evidence. This clade is not supported by any unambiguous synapomorphies, but three traits optimize as synapomorphies under Delayed Transformation: stapes columelliform and microperforate or imperforate (Character 65: 0→1; ci = 0.143; see Gaillard et al, 2021); mastoid fenestra present dorsal and medial to the mastoid exposure of petrosal (Character 91: 0→1; ci = 0.200); and fourth lower molar (m4) trigonid structurally simplified, with only the metaconid distinct (Character 164: 0→1; ci = 0.667).…”