MAMMALODONTIDAE MITCHELL, 1989

Fitzgerald, Erich M. G., 2010, The morphology and systematics of Mammalodon colliveri (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia, Zoological Journal of the Linnean Society 158 (2), pp. 367-476 : 373-374

MAMMALODONTIDAE MITCHELL, 1989

Janjucetidae Fitzgerald (2006: 2955)

Type genus: Mammalodon Pritchard, 1939 .

Included genera: Mammalodon Pritchard, 1939 and Janjucetus Fitzgerald, 2006 .

Definition of family: The clade Mammalodontidae includes Mammalodon and all other genera more closely related to Mammalodon than to any other mysticete.

Emended diagnosis of family: Relatively small mysticetes (condylobasal length <500 mm). More archaic than described species of Chaeomysticeti : mammalodontids are differentiated from all these mysticetes in possessing mineralized rostral and mandibular teeth as adults. Mammalodontids are placed in the Mysticeti because they possess: a maxilla with a dorsoventrally thin lateral edge, a laterally projecting antorbital process with a steep face on its anterior edge clearly distinguishing it from the rostral part of the maxilla, and a posteriorly elongated infraorbital process developed ventral to the supraorbital process of the frontal; upper cheek teeth that lack an entocingulum; and transversely thickened basioccipital crests.

Primitive archaeocete-like features of Mammalodontidae include: a relatively small skull and inferred body size, functional heterodont teeth, a short rostral portion of the maxilla (as a proportion of condylobasal length), a rostrum that, although shelved, is not dorsoventrally flattened, a posteriormost upper tooth that lies posterior to the level of the antorbital notch, bony external nares that open at a level well anterior to the antorbital notch, elongate and dorsoventrally thin nasals, an anteroposteriorly elongated optic infundibulum, parietals extensively exposed on the dorsal surface of the cranium within an elongated intertemporal constriction, a braincase that is not inflated, a supraoccipital shield with a semicircular outline, a supraoccipital that is (secondarily) not thrust forward anteriorly (i.e. no occipital component of cranial telescoping), a short anterior process of the periotic, a tympanic bulla with a distinct median furrow on its ventral surface and with a bilobed posterior edge, a posterior half of the alveolar margin of the mandible forming an angle with the ventral margin of the mandible, a large mandibular foramen and associated pan bone forming its lateral wall, and a high and elongate coronoid process of the mandible (see Fig. 2 for skull characters).

Mammalodontidae are differentiated from the toothed mysticete family Llanocetidae in having the following derived features: relatively and absolutely small lower cheek teeth; posterior lower cheek teeth with two roots joined below the crown base by a transversely narrow isthmus; lower cheek teeth are closely spaced along the alveolar margin without elongated intervening diastemata; mandible has a salient lateral edge to the alveolar margin such that lower cheek teeth are implanted within an alveolar groove.

Mammalodontids differ in morphology from described aetiocetid mysticetes in possessing the following derived characters: an extremely short rostral part of the maxilla (length of rostral part of maxilla is <43% of condylobasal length); presence of a triangular wedge of frontal separating the posteromedial margin of the ascending process of maxilla from the posterolateral margin of the nasal; a linguiform preorbital process of the frontal; a low-profile braincase; a V-shaped frontoparietal suture on the dorsal surface of the cranium; a cranium with secondarily reduced occipital telescoping; a secondarily semicircular supraoccipital shield; lower teeth implanted within an alveolar groove that has a salient, ridge-like, lateral edge; cheek teeth have salient longitudinal ridges developed on both the buccal and lingual surfaces of the crown enamel; and posterior upper and lower cheek teeth with two distinct roots joined by an isthmus for part or all of their length (see Fig. 2 for skull characters).

In summary, Mammalodontidae is diagnosed by the following synapomorphies: a short rostral portion of the maxilla, which is <43% of condylobasal length minus the premaxillae; the preorbital process of the frontal has a linguiform outline with a rounded-off anterior edge; a triangular wedge of frontal separates the posteromedial edge of the ascending process of maxilla from the posterolateral margin of the nasal; viewed laterally, the dorsal edge of the braincase is low to flat, its dorsal profile at an angle of <10° to the lateral edge of the rostrum; a V-shaped frontoparietal suture on the dorsal surface of the skull; the skull lacks cranial telescoping, with the anterior edge of the supraoccipital at a level posterior to the anterior edge of the squamosal fossa (homoplasious); the anterior edge of the supraoccipital has a semicircular outline (homoplasious); and the posterior upper cheek teeth are double rooted, with the roots being joined by an isthmus for part or all of their length.

Remarks: Mitchell (1989: 2231), in a nondifferential diagnosis, characterized a monotypic family Mammalodontidae on the basis of the following characters: (1) small size; (2) long and low skull displaying little ‘cranial telescoping’; (3) broad and flat palate; (4) externally convex upper tooth row; (5) no vascular grooves in palate; (6) loosely sutured rostral bones; (7) lack of a bony mandibular symphysis; (8) a heterodont but not polydont dentition; (9) 11 mandibular teeth; (10) large, closely appressed lower cheek teeth; (11) double-rooted lower cheek teeth with roots fused for most of length; (12) lower cheek tooth crowns possess multiple accessory denticles; (13) cheek tooth crown enamel with finely fluted or wrinkled ornamentation; (14) lower cheek teeth reclined posteriorly in a rake of about 20°; (15) a mandible with a welldeveloped coronoid process; (16) an enlarged mandibular foramen and canal; and (17) a mandibular condyle projecting posteriorly from the longitudinal axis of the mandible.

Subsequent to Mitchell’s (1989) establishment of Mammalodontidae it has been noted that this family was not diagnosed by synapomorphies ( Fordyce & Barnes, 1994; Barnes et al., 1995). Of Mitchell’s (1989) 17 diagnostic characters listed above: characters 2, 5, 12, 13, 15, 16, and 17 likely represent mysticete symplesiomorphies; characters 7, 8, 9, and 11 are based on spurious interpretations of anatomy; and characters 3 and possibly 6 are synapomorphies of a more inclusive clade (i.e. Mysticeti: Fitzgerald, 2006). Character 4 is an autapomorphy of Ma. colliveri . Character 1, ‘small size’, is difficult to assess. It is unclear whether Mitchell (1989) was referring to relatively small skull size or inferred small body size, compared to that of extant Mysticeti. It remains possible that ‘small size’ is a synapomorphy of Mammalodontidae . Character 10 is inadequately defined, lacking a quantitative description of relative tooth size and spacing between lower cheek teeth. Character 14 is a potential synapomorphy of Mammalodontidae , as both Ma. colliveri and Janjucetus hunderi possess posteriorly reclined lower cheek teeth.

Figures

Figure 2. Skulls of archaeocetes and toothed mysticetes in dorsal (upper) and right lateral (lower) views. Skulls are scaled to the same length. Dashed lines represent reconstructed regions of cranial anatomy. Basilosaurus isis modified from Stromer (1908); Zygorhiza kochii modified from Kellogg (1936); Janjucetus hunderi modified from Fitzgerald (2006); Mammalodon colliveri by E. M. G. F.; Aetiocetus cotylalveus modified from Emlong (1966) and Barnes et al. (1995); Aetiocetus weltoni dorsal view modified from Deméré & Berta (2008) and lateral view by E. M. G. F.; Aetiocetus polydentatus modified from Barnes et al. (1995); Chonecetus goedertorum modified from Barnes et al. (1995).