Phocidae Gray, 1821

Fitzgerald, Erich M. G., 2005, Pliocene marine mammals from the Whalers Bluff Formation of Portland, Victoria, Australia, Memoirs of Museum Victoria 62 (1), pp. 67-89 : 81-84

Family? Phocidae Gray, 1821

Genus and species indeterminate

Referred specimens. NMV P218465, incomplete left horizontal ramus of mandible ( Fig. 10 ). NMV P218273, isolated upper incisor or canine tooth (not figured).

Description. P218465 is an incomplete left horizontal ramus with a preserved length of 69 mm, depth at level of dorsal concavity of 21 mm, and mediolateral thickness at level of posterior alveolus for m1 of 10 mm. The surface detail of P218465 is well preserved relative to most of the other marine vertebrate fossils recovered from the Whalers Bluff Formation. P218465 lacks all of the horizontal ramus anterior to the anterior alveolus for p4 and all of the ascending and horizontal rami posterior to the anteriormost corner of the fossa for m. masseter pars profundus. No teeth are preserved in situ in the mandible. In overall proportions, the ramus is lightly built. In posterior and dorsal view, slight medial inflection of the ramus is visible. In lateral aspect, there is a small mental foramen located ventral to the position of the interalveolar septum between the anterior and posterior alveoli of p4 ( Fig. 10B ). This position is almost identical to that of the posteriormost mental foramen on a mandible referred to Piscophoca sp. by Walsh and Naish (2002). Such small mental foramina are also present on the mandibles of Zalophus californianus Lesson, 1828 ( Howell, 1929; pers. obs.). Immediately posterior to the posterior alveolus of m1 is a prominent dorsal concavity 25 mm long (measured from the m1 alveolus to the anterior margin of the fossa for m. masseter pars profundus). This dorsal concavity resembles that of Piscophoca . The preserved anterior region of the fossa for m. masseter pars profundus differs from that of Piscophoca in being dorsoventrally broader with a more rounded outline.

P218273 represents an isolated incisor or canine tooth, 28 mm long; its mesiodistal width at a level just below the base of the crown is 8 mm while its buccolingual width at the same level is 6 mm. The crown has smooth enamel and is recurved towards the apex. The root has a consistent thickness and is buccolingually compressed.

Discussion. The pinniped fossils recovered from the Whalers Bluff Formation do not include elements preserving unequivocal synapomorphies of suprageneric pinniped taxa. Berta (1991) and Berta and Wyss (1994) listed the possession of a bony flange below the angular process of the mandible as an unequivocal synapomorphy of Superfamily Phocoidea (sensu Wyss and Flynn, 1993) but the posterior region of the mandible is not preserved in P218465. However, P218465 is tentatively referred to the Phocidae on the basis of its morphology and proportions being most similar to the mandibles of phocid seals, as opposed to otariids and odobenids. It remains possible that P218465 represents an otariid mandible. If this were the case, then a major rethinking of otariid evolutionary biogeography would be necessary, as current estimates place the otariid dispersal into the Southern Hemisphere at around the Pliocene/Pleistocene boundary ( Deméré et al., 2003; contra Repenning and Tedford, 1977, who indicated a latest Miocene dispersal event at the earliest), and the age of P218465 (and P218273) probably predates that horizon.

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Given that P218465 is not similar in morphology to Otariidae but shares certain features with some phocids (see description and discussion below), there is no firm evidence to suggest that the Portland mandible represents an otariid. The fact that phocid seal fossils have previously been reported from Pliocene-aged sediments in Victoria ( Fordyce and Flannery, 1983) whereas otariids have not lends further support to the assignment of P218465 to the Phocidae . However, P218465 and P218273 are not referred unquestionably to Phocidae because the late Neogene fossil record of marine mammals in the SW Pacific remains too poorly documented to provide any absolute idea of the composition of the marine mammal fauna during the Pliocene.

Among extant and fossil phocid mandibles, P218465 is most similar to those of the Pliocene taxa Acrophoca longirostris Muizon, 1981 , Homiphoca capensis Hendey and Repenning, 1972 , and Piscophoca pacifica Muizon, 1981 . P218465 may be clearly distinguished from Acrophoca , as it lacks the wide diastema between cheek teeth characteristic of Acrophoca ( Muizon, 1981) . The Portland mandible can be further distinguished from Homiphoca ( Hendey and Repenning, 1972; Muizon and Hendey, 1980) by the possession of a well-developed dorsal concavity posterior to m1. P218465 is generally very similar to the mandible of Piscophoca ( Muizon, 1981) in its overall proportions, relative length of the dorsal concavity posterior to m1, subequal diameters of the alveoli and relatively closely spaced alveoli along the tooth row. However, it is not possible at this stage to determine whether the Portland mandible belongs to a species of Piscophoca or not.

The Australian fossil record of pinnipeds is currently poor. Fordyce (1991b) summarised the state of knowledge at the beginning of the 1990s, and virtually nothing has been added since that time. The oldest fossil pinnipeds from the SW Pacific are latest Miocene (c. 6 Ma) at the earliest and are from Australia ( Fitzgerald, 2004b). Fordyce and Flannery (1983) provided a preliminary assessment of these fragmentary fossils suggesting that they represented monachine phocids. The fossils represent one?incisor tooth (P16198), two right temporals (P160399 and P160441), two fused sacral vertebrae (P41759) and an articulated series of eight thoracic vertebrae with five ribs (P160433). None of these specimens has yet been described and only one of the temporals (P160399) has been figured ( Fordyce and Flannery, 1983: 99). Recently, two other pre-Pleistocene?phocid fossils have been discovered: P42523, isolated right metatarsal V; and P215759, isolated left metatarsal V. Both P42523 and P215759 were derived from beds immediately overlying a phosphatic nodule horizon at the base of the Black Rock Sandstone (Beaumaris, Victoria), and are thus early Early Pliocene in age. The exact relationships of the phocids represented by temporals to extant Monachinae and their fossil sister-taxa ( Acrophoca , Homiphoca and Piscophoca ) have yet to be determined. The report herein of two probable phocid pinniped fossils from Portland brings the number of known Australian pre-Pleistocene pinniped specimens to nine.

Figures

Figure 10.?Phocidae gen. et sp. indet. (Pliocene Whalers Bluff Formation, Portland, Victoria, Australia), incomplete left mandible, NMV P218465 (AC). A, dorsal view. B, lateral view. C, medial view. Black arrow in B points to mental foramen. Scale bar equals 10 mm