Comparative study of notoungulate (Placentalia, Mammalia) bony labyrinths and new phylogenetically informative inner ear characters

Abstract

The phylogenetic relationships of notoungulates, an extinct group of predominantly South American herbivores, remain poorly resolved with respect to both other placental mammals and among one another. Most previous phylogenetic analyses of notoungulates have not included characters of the internal cranium, not least because few such features, including the bony labyrinth, have been described for members of the group. Here we describe the inner ears of the notoungulates Altitypotherium chucalensis (Mesotheriidae), Pachyrukhos moyani (Hegetotheriidae) and Cochilius sp. (Interatheriidae) based on reconstructions of bony labyrinths obtained from computed tomography imagery. Comparisons of the bony labyrinths of these taxa with the basally diverging notoungulate Notostylops murinus (Notostylopidae), an isolated petrosal from Itabora ı, Brazil, referred to Notoungulata, and six therian outgroups, yielded an inner ear character matrix of 25 potentially phylogenetically informative characters, 14 of them novel to this study. Two equivocally optimized character states potentially support a pairing of Mesotheriidae and Hegetotheriidae, whereas four others may be diagnostic of Notoungulata. Three additional characters are potentially informative for diagnosing more inclusive clades: one for crown Placentalia; another for a clade containing Kulbeckia, Zalambdalestes, and Placentalia; and a third for Eutheria (crown Placentalia plus stem taxa). Several other characters are apomorphic for at least one notoungulate in our study and are of potential interest for broader taxonomic sampling within Notoungulata to clarify currently enigmatic interrelationships. Measures of the semicircular canals were used to infer agility (e.g. capable of quick movements vs. lethargic movements) of these taxa. Agility scores calculated from these data generally corroborate interpretations based on postcranial remains of these or closely related species. We provide estimates of the lowfrequency hearing limits in notoungulates based on the ratio of radii of the apical and basal turns of the cochlea. These limits range from 15 Hz in Notostylops to 149 Hz in Pachyrukhos, values comparable to the Asian elephant (Elephas maximus) and the California sea lion (Zalophus californianus) when hearing in air, respectively.

Supplemental Material for 'Exceptional skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) from the late Miocene of Argentina : anatomy, systematics, and paleobiological implications. (Bulletin of the American Museum of Natural History, no. 404)'

“…All litopterns measured to date exhibit similar values, including Early Eocene taxa (2.25 to 2.4 turns), "Proterotherium" 2 (2.1), Diadiaphorus (2.25), and Macrauchenia (2.3) as measured by Billet et al (2015). The notoungulates Notostylops (2.25; Macrini et al, 2010) and Hegetotherium (2.5; Simpson, 1936;Macrini et al, 2010) are also similar, whereas in some other notoungulates (Altitypotherium, Pachyrukhos, and Cochilius) the degree of coiling is less, ~2 turns (Macrini et al, 2013).…”

“…It projects beyond the PSC as seen in dorsal view ( fig. 21E), as in other described litopterns and Notostylops (Macrini et al, 2010;Billet et al, 2015), in contrast to Pachyrukhos in which it does not extend as far (Macrini et al, 2013). However, it should be noted that the difference may be size related, in the sense that Pachyrukhos, a very small notoungulate, does not exhibit greatly thickened capsular walls.…”

“…21A, C). The external aperture of the fossula fenestrae cochleae is a section of a distorted cone (rather than oval, as usually stated) and larger than that of the fenestra vestibuli, as is also the case in other described litopterns and the notoungulate Notostylops (Macrini et al, 2013;Billet et al, 2015).…”

“…Billet et al, 2015: fig. 17), and the notoungulates Pachyrukhos and Cochilius (Macrini et al, 2013). In dorsal view ( fig.…”

“…The semicircular canals are arranged approximately at mutual right angles (table 3). The shape, relative size, and radius of curvature of the semicircular canals vary among litopterns and notoungulates (see Macrini et al, 2010Macrini et al, , 2013Billet et al, 2015).…”