Basal phylogenetic relationships among artiodactyls remain a current topic of investigation. However, recent studies showed that stem taxa to extant artiodactyl groups are to be found in the paraphyletic assemblage of dichobunoid artiodactyls to which Diacodexis belongs (e.g. Geisler et al. 2007; Geisler & Theodor, 2009; Spaulding et al. 2009). While being the earliest representatives of Artiodactyla, Diacodexis species appear either as the first branch at the base of the Artiodactyla (e.g. Geisler et al. 2007; Geisler & Theodor, 2009), or more highly nested in the artiodactyl tree and closely related to cetaceans (Spaulding et al. 2009; Orliac & Ducrocq, 2011).
Morphology of the cochlear canal
The cochlear canal has at least one complete 360 ° turn in living therian mammals (Gray, 1907, 1908; Rowe, 1988), and a low value of cochlear coiling is hypothesized to be primitive for Eutheria, reaching 735 ° in the hypothetical common ancestor of placental mammals (Ekdale, 2009). The cochlear canal of Diacodexis completes two full whorls (720 °), a value smaller than that observed in extant Sus and Moschiola (Table 2). Based on the morphology of the bony labyrinth of S. scrofa (living suid), Bathygenys reevesi (extinct merycoidodontid, Eocene), Tursiops truncatus (living cetacean) and a fossil Balaenopteridae (extinct cetacean, age not specified), Ekdale (2009) reconstructed the ancestral morphology of the cochlea for the common ancestor of Artiodactyla. Ekdale (2009) estimated an ancestral cochlear coiling for artiodactyls of 845.8 ° (2.35 turns), a value less than that of living terrestrial members of the group. The low value of cochlear coiling of Eocene fossil artiodactyls described so far, B. reevesi (665 °; Ekdale, 2009) and D. ilicis (720 °), is congruent with the hypothesis that the earliest artiodactyls had relatively limited cochlear coiling.
In Diacodexis, the basal turn of the cochlea forms an acute angle with the plane of the lateral canal (12 °), and the vestibular and cochlear fenestrae are almost parallel to the LSC plane. This condition differs greatly from that of extant artiodactyls, which display a more opened angle (Table 2); a morphology similar to that of extant artiodactyls is also observed in the Oligocene merycoidodontid Bathygenys (26 °; Ekdale, 2009). Like living artiodactyls, most modern therian mammals, especially Ferungulata, display a widely open angle (e.g. Felis: 45.8 °; Canis: 20.8 °; Equus: 37.9 °; Ekdale, 2009). However, a small angle was observed in the Cretaceous eutherian mammals Kulbeckia, Ukhaatherium and Zalambdalestes (respectively, 12.1 °, 6.63 ° and 13.5 °; Ekdale, 2009, fig. 4.5, 2011), and the polarity of this character remains poorly understood in the absence of more specimens of Paleogene artiodactylans.
The basal turn of the cochlear canal of Diacodexis shows a clear lamina secundaria. Among Artiodactyla, a lamina secundaria is present in extant and extinct cetaceans (Ketten et al. 1992; Spoor et al. 2002; Ekdale, 2009) and described here in the tragulid ruminant M. meminna, but is absent in the suid S. scrofa (Ekdale, 2009). Its presence could not be assessed on the CT reconstruction of Bathygenys (Ekdale, 2009). Outside Artiodactyla, this structure is observed in most therian mammals (Meng & Fox, 1995; Ekdale, 2009).
Macrini et al. (2010) observed that the inner ear of the South American native ungulate Notostylops (Notoungulata, Eocene) differed from other eutherians in the extension of the cochlear aqueduct posterior to the plane of the PSC. The location and orientation of the cochlear aqueduct in artiodactyls is similar to that of Notostylops: it ascends posteriorly and forms a right angle with the plane of the PSC in dorsal view. A similar orientation exists in the perissodactyl Equus caballus (Ekdale, 2009, fig. 5.32). It would be important to assess the polarity of this character within ‘ungulates’. Diacodexis, however, differs from other ‘ungulates’ studied to date in the relatively short length of its cochlear canal.
The fenestra cochleae of Diacodexis are directed laterally relative to the PSC plane. This orientation is also observed in S. scrofa and M. meminna, as well as in E. caballus (Ekdale, 2009; fig. 5.32). However, the fenestra cochleae extend posterior to the plane of the PSC in Diacodexis, a relationship that does not occur in Sus and Moschiola. A similar extension of the fenestra cochleae posterior to the PCS plane occurs in the Eocene archeocete cetaceans Ichtyolestes (pakicetid) and Indocetus (protocetid), as illustrated by Spoor et al. (2002). In the living dolphin Tursiops, the fenestra cochleae are located completely posterior to the PSC plane, while in balaenopterid cetaceans the fenestra cochleae have a posteriorly directed opening (Ekdale, 2009; fig. 5.28). A posterior location of the fenestra cochleae also occurs in some Cretaceous zhelestids and stem placentals: Kulbeckia, Ukhaatherium, Zalambdalestes (Ekdale, 2009; fig. 5.6; Ekdale & Rowe, 2011).
As described above, the secondary common crus corresponds to a fusion of the lumen of the bony channels of the PSC and LSC. This structure has been widely observed in a range of Cretaceous therians (Ekdale et al. 2004; Ekdale, 2009; Ekdale & Rowe, 2011) and in Cenozoic metatherians (Sánchez-Villagra et al. 2007; Schmelzle et al. 2007; Ekdale, 2009). It is present in extinct primates (Lebrun et al. 2010), extinct afrotherians (Benoit et al. in press) and extinct ‘ungulates’ (Russell, 1964), as well as in some extant ferungulate mammals such as Canidae and Orycteropodidae (Ekdale, 2009).
In his reconstruction of the bony labyrinth of the hypothetical common ancestor of Artiodactyla, Ekdale (2009) predicted that the common crus was absent and that the LSC opened into the vestibule directly at a superior position compared with the PSC. However, our work shows that a secondary common crus is present in the earliest artiodactyl D. ilicis and it is also present in the Eocene archaeocetes Ichtyolestes pinfordi and Indocetus ramani, as figured by Spoor et al. (2002, fig. 1). In Ic. pinfordi, the common crus is slender and shows the same morphology as in D. ilicis. In Ic. pinfordi and D. ilicis, the lateral canal enters the vestibule in a low position when compared with Bathygenys (Ekdale, 2009; fig. 5.24), Moschiola and S. scrofa, (i.e. at the level of the posterior ampulla). A secondary common crus occurs in all dichobunoid taxa investigated so far (e.g. Homacodon, Dichobune, Cebochoerus, Gobiohyus, Helohyus; Orliac & O'Leary, 2011), and we predict that the secondary common crus will optimize as present in revised phylogenetic studies reconstructing the hypothetical common ancestor of Artiodactyla once data on inner ear morphology are directly incorporated into a phylogenetic analysis. We also predict that this condition is later lost in certain artiodactyls, as has been observed independently in several other therian clades (e.g. Meng & Fox, 1995; Ruf et al. 2009; Ekdale & Rowe, 2011).