MONACHINAE Gray, 1869

Rule, James P. & Fitzgerald, Erich M. G., 2022, Pinniped (Mammalia: Carnivora) fossils from Black Rock, a new late Neogene vertebrate locality in Victoria, Australia, Palaeontologia Electronica (a 32) 25 (3), pp. 1-12 : 5-6

Subfamily MONACHINAE Gray, 1869

Referred specimen. NMV P254995, a fragmentary right mandible collected by B.S.J. Francischelli in 2019 ( Figure 2 ).

Diagnosis. A fragmentary right mandible based on the position of the masseteric fossa. Referred to Pinnipedia based on a transversely flattened mandibular body, and a coronoid crest that does not ascend vertically from the toothrow. Referred to Phocidae based on a double rooted m1 with clearly separated anterior and posterior alveoli. Referred to Monachinae based on a shallow coronoid crest that ascends below a 45° angle.

Description. NMV P254995 is missing the proximal portion of the mandible, including the articular surface and a portion of the coronoid crest, and the distal portion from the p4 onwards. The body of the mandible is thick (12 mm) compared to the low height (23 mm), making the medial-lateral thickness roughly half of the dorsal-ventral length. The medial surface of the body is flat compared to the lateral surface, which is slightly dorso-ventrally curved ( Figure 2C ). While the preserved body of the mandible is antero-posteriorly straight, the coronoid crest curves medially. The masseteric fossa is shallow ( Figure 2E ), and barely visible on the lateral surface of the coronoid. The coronoid crest ascends shallowly from the toothrow (<45°), with a short space separating the crest from the posterior margin of the toothrow. Both alveoli of the m1 and the preserved posterior alveolus of the p4 are round, with the p4 alveolus slightly larger than the m1 ( Figure 2F ). A short diastema separates the m1 and p4. The posterior portion of the anterior alveolus of the p4 is preserved ( Figure 2F ).

Comparison. The shallow coronoid crest of NMV P254995 is similar to that condition seen in Neomonachus , Mirounga , Acrophoca , and Lobodontini, and is therefore mostly consistent with Monachinae (Phocinae have a steep coronoid crest). The only exceptions are Monachus , Homiphoca , Piscophoca , and Hadrokirus . The toothrow is separated from the coronoid crest by a short space (less than twice the length of the m1), similar to the monachines Lobodon , Hydrurga , Hadrokirus , and tribe Monachini ; and similar to phocines such as Erignathus , Halichoerus , Phoca , and Pusa .

The body of the mandible NMV P254995 below the toothrow is thick, similar to Monachini and Hadrokirus . All other phocids, including other monachines, have either thin mandibles below the toothrow (width less than half the height), or mandibles that grade from thin posteriorly to thick anteriorly. While most phocids have a mandible that is fairly straight antero-posteriorly, NMV P254995 has a slightly curved mandible similar to Ommatophoca , Neomonachus , and Erignathus . Overall, the preserved morphology of NMV P254995 has the most similarities with the genus Neomonachus .

Two other fragmentary fossil pinniped mandibles are known from Australia: NMV P251642 (Beaumaris: Rule et al., 2020b) and NMV P218465 (Portland: Fitzgerald, 2005). The shallow coronoid crest and round profile of the posterior m1 alveolus of NMV P254995 is shared with NMV P251642 and NMV P218465. Further, several additional characters are shared between NMV P254995 and NMV P218465: a posterior p4 alveolus larger than both m1 alveoli (also in common with other phocids), and an m1 alveoli at a slightly imbricated angle compared to the p4 alveoli. There are no discernible differences between NMV P254995, NMV P251642, and NMV P218465. The similarities between NMV P254995 and NMV P218465 are enough to confidently revise the identification of NMV P218465 to Phocidae cf. Monachinae , confirming the tentative diagnosis of Phocidae of Fitzgerald (2005).

Figures

FIGURE 2. The Black Rock phocid specimens. NMV P254995, right mandible in A) lateral, B) medial, C) dorsal, D) ventral, E) annotated lateral, and F) annotated dorsal views. NMV P254178, phalanx in, G) side, H) ventral, I) opposing side, and J) dorsal views. Scale bar equals 20 mm.