Mammalodon Pritchard, 1939

Fordyce, R. Ewan & Marx, Felix G., 2016, Mysticetes baring their teeth: a new fossil whale, Mammalodon hakataramea, from the Southwest Pacific, Memoirs of Museum Victoria 74, pp. 107-116 : 108-109

Mammalodon Pritchard, 1939

Emended diagnosis of Mammalodon . Small-sized mysticetes differing from chaeomysticetes in having teeth. Differ from all toothed mysticetes except Janjucetus in having a foreshortened, dorsoventrally tall rostrum, a linguiform anterior border of the supraorbital process, a triangular wedge of the frontal separating the ascending process of maxilla from the posterolateral margin of the nasal, a roughly horizontal dorsal profile of the braincase (relative to the lateral edge of the rostrum) and posteriorly reclined mandibular cheek teeth; further differ from all other toothed mysticetes except Janjucetus and Chonecetus in having a distinctly V-shaped fronto-parietal suture in dorsal view; from Llanocetus and two previously coded, undescribed archaic mysticetes (ChM PV4745; OU GS10897) in having both relatively and absolutely smaller posterior cheek teeth with proximally fused roots, and an inner posterior prominence of the tympanic bulla that is subequal to the outer prominence in posterior view; from all aetiocetids in having a more elongate intertemporal region and an anteroposteriorly broader coronoid process of the mandible; from Aetiocetus and Fucaia in lacking a medially expanded lacrimal and a dorsoventrally constricted mandible, and in having more robust cheek teeth with distally separate roots; from Chonecetus in having a broader, less anteriorly-thrust supraoccipital bearing a well-developed external occipital crest, and in lacking a parasagittal cleft on the dorsal surface of the parietal; from Aetiocetus in having a clearly heterodont dentition and closely-spaced posterior cheek teeth with well-developed enamel ridges on both the labial and lingual sides of the crown; from Morawanocetus in having a much more robust postorbital process of the frontal; from Ashorocetus in having a less steeply inclined supraoccipital shield and a somewhat more anteromedially oriented basioccipital crest; and from the enigmatic Willungacetus in having a clearly marked orbitotemporal crest extending posteriorly on to the intertemporal constriction and a rounded (rather than triangular), less anteriorly-thrust supraoccipital bearing a well-developed external occipital crest. Finally, Mammalodon differs from the only other described mammalodontid, Janjucetus , in having a rostrum with a bluntly rounded apex and a gently convex lateral profile in dorsal view, coalesced alveoli for the upper incisors, a gracile, foreshortened and dorsoventrally flattened premaxilla, an anteriorly expanded nasal, a transversely narrow, linguiform ascending process of the maxilla extending posteriorly as far as the nasal, a more anteriorly directed orbit, a more laterally oriented postorbital process, a transversely convex dorsal profile of the parietals with no salient sagittal crest, a more laterally oriented nuchal crest, a broadly rounded apex of the supraoccipital shield in dorsal view, an anterior portion of the tympanic bulla that is squared (rather than obliquely truncated) in ventral view, an inner posterior prominence of the tympanic bulla that is subequal to the outer prominence in posterior view, a straight and comparatively gracile mandible bearing large mental foramina, and three upper and four lower molars, all of which (at least in adult specimens) are affected by heavy occlusal wear.

Remarks. Comparisons of Mammalodon with Ashorocetus eguchii , Willungacetus aldingensis and Chonecetus sookensis are currently hampered by the poor state of preservation of the available material; none of the latter, for example, includes the tympanic bulla or teeth. Ashorocetus is currently known only from the posterior portion of a braincase preserving little surface detail ( Barnes et al. 1995). Willungacetus and Chonecetus sookensis are based on somewhat more complete, but still highly fragmentary crania having lost their rostra, most or all of the ear bones and much of the (basicranial) surface detail ( Pledge, 2005; Russell, 1968). Until the discovery of better material, the comparisons made here are necessarily provisional. The use of occlusal wear as a potential diagnostic character may be queried, as this feature may primarily correlate with age or the foraging environment. Nevertheless, the extreme wear present in the two species of Mammalodon is unusual amongst fossil cetaceans as a whole, and highly so in the context of toothed mysticetes in particular.If tooth wear in Mammalodon is primarily linked to the manner of occlusion and/or food preferences, it may record a valid character that can be used for diagnostic and cladistic purposes. Without evidence to the contrary, we therefore retain it here as part of the diagnosis.