Balaenopteridae Gray, 1864

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 : 72

Family Balaenopteridae Gray, 1864

Genus and species indeterminate

Referred specimen. NMV P218268, incomplete right periotic; lacking medial three-quarters of pars cochlearis and posterior process ( Fig. 3B ).

Description. P218268 is polished, rolled and may be secondarily phosphatised. The anterior process is elongated and somewhat attenuated anteriorly. The dorsal surface of the anterior process is smooth, with only slight rugosity, as seen in the periotics of extant Balaenopteridae . An oblique groove on the dorsolateral surface of the anterior process near its preserved apex is interpreted as a trace of a vascular sulcus. The latter feature has previously been considered a sulcus for the capsuloparietal emissary vein ( Geisler and Luo, 1998; Geisler and Sanders, 2003) or as a sulcus marking the path of an artery, specifically part of the middle meningeal artery (Fordyce, 1994). Fordyce (1994) and Watson and Fordyce (1994) described this feature as the anteroexternal sulcus whereas Geisler and Sanders (2003) treated the anteroexternal sulcus and sulcus for the capsuloparietal emissary vein as separate features. Further work is required to better establish the venous/arterial correlate of this osteological feature which in this study is referred to as the sulcus for the capsuloparietal emissary vein. In extant balaenopterids, this sulcus usually courses posteriorly to a point level with the position of the mallear fossa. However, in P218268 any more posterior continuation of the sulcus for the capsuloparietal emissary vein, if formerly present, no longer occurs due to abrasion.

The ventral presentation of the periotic exhibits several features. As occurs in extant Balaenoptera and Megaptera , the lateralmost eminence of the ventrolateral ridge of the superior process (sensu Geisler and Luo, 1996) is situated at the same level as the anterior margin of the pars cochlearis. The mallear fossa is poorly differentiated from the rest of the epitympanic recess.

The preserved posterolateral margin of the periotic is formed by the hiatus epitympanicus. The course of the facial nerve on the ventral surface of the periotic is marked by the facial sulcus which is bounded anteriorly by the aperture of the ventral facial foramen. In ventral view, a distinct bridge of bone at the anterolateral corner of the preserved pars cochlearis represents the ventral roof of the ventral facial foramen. The endocranial aspect of the pars cochlearis preserves the aperture of the internal facial foramen. Anterior to this aperture is a deep excavation in the medial surface of the periotic at the base of the anterior process. This region (composed of cancellous bone in extant balaenopterids) marks the site of ankylosis between the anterior process and the body of the periotic. As in other Balaenopteridae , the pars cochlearis appears to have been elongated towards the cranial cavity.

Discussion. That P218268 is a balaenopterid periotic is evident by the possession of: (1) elongated, triangular and anteriorly attenuated anterior process; (2) a triangular lateral eminence of the ventrolateral ridge; and (3) a relatively large pars cochlearis elongated towards the cranial cavity. Because P218268 is represented only by an incomplete periotic, it is not possible for it to be identified below family level. The size of P218268 is comparable to that of periotics of subadult Balaenoptera edeni Anderson, 1879 or B. brydei Olsen, 1913 (P171503) and juvenile Megaptera novaeangliae Borowski, 1781 (C28892). However, the periotics of extant Megaptera novaeangliae and an undescribed species of Megaptera from the Early Pliocene of Victoria (P179005) possess the following features which differentiate them from P218268: (1) the anterior process is relatively shorter; (2) anterior process is more dorsoventrally compressed; (3) in endocranial view, there is an anteroposteriorly thickened region of cancellous bone in the pars cochlearis anterior to the internal facial foramen; and (4) no deep excavation in the endocranial surface of the periotic anterior to the pars cochlearis. Many of these features may be related to ontogenetic variation. At least, the lack of these four features in P218268 may indicate that this periotic is not referrable to Megaptera and may belong to an indeterminate species in the genus Balaenoptera .

Figures

Figure 3. Mysticeti and Physeteridae from the Pliocene Whalers Bluff Formation, Portland. A, Balaenidae gen. et sp. indet., incomplete right periotic, NMV P218269, in ventrolateral view (AC); B, Balaenopteridae gen. et sp. indet., incomplete right periotic, NMV P218268, in ventral view (AC); C, cf. Physeter sp., apical crown of tooth, NMV P218298, in side view (AC). Scale bars equal 10 mm.