MONACHINAE GEN. & SP. INDET.

Rule, James P, Adams, Justin W & Fitzgerald, Erich M G, 2021, Colonization of the ancient southern oceans by small-sized Phocidae: new evidence from Australia, Zoological Journal of the Linnean Society 191 (4), pp. 1160-1160 : 1160-

MONACHINAE GEN. & SP. INDET.

Referred specimen: NMV P232849, isolated right humerus, collected by R. Wilkie ( Fig. 11 ).

Description: NMV P232849 ( Fig. 11 ) is a right humerus. The most anterior extent of the deltopectoral crest is missing, as is the lateral epicondyle. NMV P232849 represents a monachine phocid on the basis of a deltopectoral crest that curves gradually down the shaft, and the lack of an entepicondylar foramen. The lesser tubercle is slightly damaged anteriorly and posteriorly. The lesser tubercle, greater tubercle and the head are the same level. The bicipital groove is narrow and shallow, lacking a deep insertion for the m. supraspinatus. The head is rounded, but compressed proximodistally in both posterior and lateral views.

The deltopectoral crest is anteroposteriorly deep, tapering towards the distal end of the shaft. The lateral surface posterior to the crest (the fossa for m. brachialis) is shallow. A deltopectoral overhang usually forms the anterior border of the m. brachialis fossa. The absence of an overhang suggests a more weakly developed m. brachialis. The deltoid tuberosity of NMV P232849 is laterally bulbous and large relative to the small size of the deltopectoral crest. The medial surface of the shaft is concave, smoothly transitioning into the medial epicondyle. Despite the lack of a preserved lateral epicondylar crest, it appears that the olecranon fossa is shallow. What is preserved of the trochlea on the medial side is small, but there may be a substantial portion missing due to the lack of the lateral epicondyle. The coronoid fossa is deep relative to the size of the medial epicondyle, and a visible outline is present between the trochlea and the epicondyles.

The fossa for the m. triceps brachii caput mediale in NMV P232849 is deep in comparison to the majority of phocids. Other taxa with a deep m. triceps brachii caput mediale are the phocine Pagophilus groenlandicus (NHMUK 1938.12.10.1), the extinct monachine Auroraphoca atlantica Dewaele et al., 2018b (USNM 181419) and the extinct lobodontin Piscophoca pacifica de Muizon, 1981 (MNHN.F.SAS564), but the condition is not as well developed as in NMV P232849. The proximal level of the lesser tubercle is level with the head (when resting on the medial epicondyle), similar to stem-Lobodontini ( Hendey, 1972; de Muizon, 1981; Amson & de Muizon, 2014) and Neomonachus Slater & Helgen, 2014 (USNM 395999 and USNM 22543). This differs from extant and extinct Phocinae , extant Lobodontini , Monachus monachus , Pliophoca etrusca Tavani, 1941 and Mirounga Gray, 1827 , in which the lesser tubercle greatly extends proximal to the level of the head (see: Berta et al., 2015). Like most of Phocidae, NMV P 232849 has a greater tubercle level with the head proximally.

The compressed condition of the head of NMV P232849 is similar to that seen in Monachus monachus (NHMUK 1894.7.27.1), Ommatophoca rossii (NHMUK 1965.8.2.1) and Virginiaphoca magurai Dewaele et al., 2018b (USNM 639750). There is little division between the head and lesser tubercle, such as in V. magurai and Australophoca changorum . The narrow, shallow bicipital groove is a feature not found in extant Monachinae , but shared with A. changorum ( Valenzuela-Toro et al., 2016) .

The anteroposteriorly wide deltopectoral crest is similar to the state in Australophoca changorum ( Valenzuela-Toro et al., 2016) . The fossa for the m. brachialis is much deeper in multiple fossil phocid taxa ( Dewaele et al., 2017b, 2018b), as it is for extant phocines. NMV P232849 lacks the deltopectoral overhang that is present in extant phocines, the extinct phocine Leptophoca proxima (van Beneden, 1876) ( Dewaele et al., 2017b) and the extinct monachines Homiphoca capensis and Virginiaphoca magurai . A variation of the deltopectoral overhang is present in Auroraphoca atlantica ( Dewaele et al., 2018b) , with a distinct separation of the feature on the lateral surface present on the proximal and distal end.

The medial side of the diaphysis is concave in posterior view, a feature present in multiple extant and fossil monachines, such as Pliophoca etrusca ( Berta et al., 2015) and Homiphoca capensis ( de Muizon & Hendey, 1980) , as well as multiple North Atlantic species ( Dewaele et al., 2018b). The medial diaphysis smoothly transitions into the medial epicondyle, unlike the more abrupt transition observed in other Phocidae . The medial epicondyle of NMV P232849 is relatively small compared to other Monachinae ( de Muizon, 1981; Berta et al., 2015; Valenzuela-Toro et al., 2016; Dewaele et al., 2018b). The shallow olecranon fossa is a common feature of fossil and extant Phocidae , but is not necessarily exclusive to them ( Dewaele et al., 2017b, 2018b). The coronoid fossa is deep relative to other monachines ( de Muizon, 1981; Dewaele et al., 2018b).

Figures

Figure 11. Monachinae, gen. & sp. indet., NMV P232849, isolated right humerus. A, posterior view. B, lateral view. C, anterior view. D, anteromedial view. E, posteromedial view. F, proximal view.