Delphinus sp.

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 : 78

Delphinus sp. Linnaeus, 1758 or Stenella sp. Gray, 1866

Referred specimen. NMV P218265, virtually complete left periotic ( Fig. 7 ).

Description. P218265 is smaller than P218266 (cf. Tursiops sp. ) ( Fig. 6 ), closely matching the periotics of both Delphinus and Stenella in overall proportions. Morphological similarities between P218265 and Delphinus include: (1) relatively large diameter of the aperture of the fenestra cochleae; and (2) aperture of the aquaeductus vestibuli is ellipsoid to slit-like in outline and opens posteriorly. However, P218265 differs from Delphinus in: (1) the suprameatal area of the periotic, lateral to the aperture of the internal acoustic meatus, is less planar and more convex; (2) the lateral wall of the internal acoustic meatus lacks an elevated platform which obscures the vestibular foramen in endocranial view; (3) the crista transversa is lower; and (4) the aperture of the internal facial foramen is narrower.

Discussion. Whitmore (1994) noted that it is very difficult to distinguish between Delphinus and Stenella on the basis of periotic morphology. Confounding generic identification of P21265 is the fact that the periotics of Stenella show wide individual variation ( Kasuya, 1973). In both Delphinus and Stenella , the aquaeductus cochleae opens at a similar position on the posterior wall of the pars cochlearis. Most preserved features indicate closer affinities with Stenella than Delphinus . However, all of the identified differences between P218265 and Delphinus are subtle, representing differences in degree rather than kind. I do not advocate the definitive placement of P218265 within either Delphinus or Stenella . In any event, the somewhat intermediate morphology of P218265 between the periotics of Delphinus and Stenella may indicate that P218265 is from an extinct genus closely related to both of the extant genera in question. In either case, P218265 is the first record of a periotic of the Delphinus-Stenella type recorded from the Neogene of Australia.

Fossils representing Delphinus or Stenella have been recorded from: Upper Member Almejas Formation, Baja California Sur (latest Miocene to Early Pliocene, 3.5–6 Ma) ( Barnes, 1998); San Mateo Formation, California (Late Miocene to Early Pliocene, 5–9 Ma) ( Barnes et al., 1981); unnamed blue clays at Waihi Beach, Hawera, New Zealand (Early Pliocene, 3–3.6 Ma; Beu, 1995) ( McKee and Fordyce, 1987; Fordyce, 1991a: 1262); Yorktown Formation, North Carolina (Early Pliocene, 4.5 Ma) ( Whitmore, 1994); Salada Formation, Baja California Sur (Early to Late Pliocene, 3–5 Ma) ( Barnes, 1998); Tirabuzón Formation, Baja California Sur (middle Pliocene, 3–4 Ma) ( Barnes, 1998); SAO Level of the Pisco Formation, Peru (late Early to early Late Pliocene, 3.0–4.0 Ma) ( Muizon and DeVries, 1985; Muizon and Domning, 2002); and San Diego Formation, California (Late Pliocene, 1.8–3.4 Ma) (Barnes, 1973, 1977, 1998).

Figures

Figure 6. cf. Tursiops sp. (Pliocene Whalers Bluff Formation, Portland, Victoria, Australia), right periotic, NMV P218266 (AC). A, ventral view. B, cranial view. C, medial view. D, lateral view. Scale bar equals 10 mm.

Figure 7. Delphinus sp. or Stenella sp. (Pliocene Whalers Bluff Formation, Portland, Victoria, Australia), left periotic, NMV P218265 (AC). A, ventral view. B, cranial view. C, medial view. D, lateral view. Scale bar equals 10 mm.