MONACHINAE

Koretsky, I. A., Rahmat, S. J. & Peters, N., 2014, Remarks On Correlations And Implications Of The Mandibular Structure And Diet In Some Seals (Mammalia, Phocidae), Vestnik Zoologii 48 (3), pp. 255-268 : 264-267

Subfamily MONACHINAE

Members of this subfamily have large teeth and a lower mandibular ramus. The condyloid process is situated at about the level of the tooth-row and these seals possess the smallest condyloid angles of the three extant subfamilies examined, demonstrating the smallest gape and diets consisting of small-sized prey. Lobodontini are unique phocids that are specialized filter-feeders, inferring a smaller condyloid angle and gape than Cystophorinae and Monachinae . The masseter and temporalis muscles in Monachinae are weak compared to those in Cystophorinae .

Previous studies have shown direct association between the power of bite force and food processing with the mandible ( Hylander, Johnson, 1994). Thus, functional implications can be found by examining the mandibular morphology of primates ( Hylander, 1979; Daegling, Grine, 1991) and some carnivorans ( Therrien, 2005). Also, in carnivores, Greaves (1985, 2000) showed that jaw length is related to gape. Despite numerous carnivoran mandibular studies and the examination of the jaw morphology of pinnipeds ( Jones et al., 2013), no one has detailed the correlation of specific mandibular characteristics to dietary adaptations in seals. As an initial study, we specifically narrowed our attention on: 1) the morphological variations of the masseteric fossa; 2) the measurement of the condyloid angle with 3) correlating known dietary studies of different true seals.

Due to the dietary preferences of seals (i. e. hard shells, fish, and krill), the ratio between the condyloid process height and the level of the alveolar row (= the condyloid angle) can be correlated to the type /size of prey consumed by each of the three extant subfamilies. Examinations of the mandibles in this study demonstrate that the condyloid angle does not change due to age. A lower condyloid process reduces the muscle stretching and therefore maximizes the bite force at a given gape angle ( Herring, Herring, 1974). Male Mirounga angustirostris (Cystophorinae) has a significantly lower condyloid process depth than in Phocinae and Monachinae , minimizing the muscle stretch while maximizing the bite force. Having a more powerful bite is of crucial importance to M. angustirostris because their diet includes larger and stronger prey such as small sharks ( Stewart, Huber, 1993). Despite being the largest phocid in terms of body size, male Mirounga possess one of the smaller condyloid angles, indicating potential dietary preferences as a suction/bottom feeder. This would correlate with species such as walruses, which are extremely large yet are suction feeders. This initial finding must be further investigated to determine the diet of male Mirounga , as previous studies show that both male and female Mirounga prefer similar diets of fish and small sized sharks ( Heptner et al., 1976; Riedman, 1990; Stewart, Huber, 1993).

For modern Phocinae and Monachinae , the position of the condyloid process in relation to the alveolar row is generally taller than in Cystophorinae as evidenced in H. grypus , P. largha , L. carcinophagus , and M. tropicalis . Thus, Phocinae and Monachinae seals feed on primarily fish and invertebrates, which include smaller-sized and weaker prey, and therefore require a less powerful bite.

Therefore, species such as Cystophora cristata and female Mirounga angustirostris (Cystophorinae) possess the largest condyloid angles, directly correlating to their diet of larger prey, despite the presence of weak teeth. In contrast, seals with smaller condyloid angles limit the stretch of masticatory muscle fibers, allowing for more forceful contractions and a smaller gape, correlating with a diet consisting of small to medium-sized prey, as in Lobodon carcinophagus . The extinct Pliophoca etrusca ( Koretsky, Ray, 2008: fig. 42) has a relatively large condyloid angle (24˚), suggesting that they ate medium to large sized prey and were deep divers. A large condyloid angle in this fossil could represent a plesiomorphic trait for all monachines.

Ecological and morphological adaptations may partly explain the differing dietary capabilities of the three extant subfamilies of seals. Female Mirounga have the greatest diving depth (up to 1270 m), correlating with the ability to obtain larger prey that live in deeper waters and having one of the largest condyloid angles. Phocinae , who eat medium sized prey, have the second deepest diving depths (up to 225 m) and second largest condyloid angles, while Monachinae have the shallowest diving depths (up to 175 m) and eat smallsized prey. The highly specialized monachine filter-feeder Lobodon is the second deepest diver (up to 520 m), despite eating the smallest prey of any phocid. Despite the general opportunistic feeding habits of seals, there appears to be a strong correlation between degree of condyloid angle, size of teeth and types of prey.

The Recent seals from the three extant subfamilies examined here show two different basic morphologies of the mandible: 1) high ramus with a condyloid process superior to the alveolar plane (as in herbivores) and 2) low ramus with condyloid process just at or inferior to the alveolar plane (as seen in other carnivores). The varying heights of the mandibular ramus and condyloid process, both between and within each subfamily, suggest strong association of the function, morphology and diet of true seals. Seals with a high mandibular ramus and greater condyloid angle feed on larger-sized prey, while seals with a low ramus and lesser condyloid angle feed on small to medium-sized prey regardless of the overall size of the animal.

Thus, our results demonstrate differences in morphological features of the mandible and their correlation with taxonomy between the three extant subfamilies and with further extrapolation to fossil representatives. The importance of these findings to future phocid studies is extremely evident when considering the shortage of published material on this vital region for modern and fossil seals.

The limited sampling of representatives from each extant subfamily of true seals used in this preliminary study demonstrates significant variation in the condyloid angle. Despite this shortcoming, this study can be used as the basis for future extrapolative examinations of fossil seal mandibles in order to determine potential dietary implications. For example, the extinct phocine Leptophoca lenis (late-early Miocene ~18 Ma, Calvert County, Maryland, USA; Koretsky, 2001: fig. 42), representing the Phocinae subfamily, has a condyloid angle of 26°, which is similar to the fossil Pliophoca etrusca (Pliocene ~5.2–3.4 Ma, Western Europe and Eastern United States), representing the Monachinae subfamily, with a condyloid angle of 24° ( Koretsky, Ray, 2008: fig. 42). It should be noted that the overall morphology

В

Рис. 7. Подсемейство Phocinae : А — правая челюсть ископаемого Leptophoca lenis, CMM –V 202, вид с губной стороны, из формации Калверт, поздний–ранний миоцен (~18 млн. л.), Калверт Каунти, штат Мэриленд, США; подсемейство Devinophocinae : развернута правая челюсть ископаемого sp. n.? (NMNH 553687), В — вид с губной стороны, из формации Бадениан, ранний–средний миоцен (~16 млн. л.), Центральный Паратетис, Венский бассейн Западной Словакии, Европа (жевательные ямки показаны пунктирными контурами).

of Leptophoca lenis ( fig. 7 ) has mixed characters of Phocinae and Monachinae as well. Thus, fossil L. lenis was probably a shallow to deep diver with a diet consisting of small to mediumsized prey due to the medium condyloid angle and morphology of dentition and muscle attachment, similar to Recent Phocinae and Monachinae . The condyloid angle of a new fossil Devinophocinae (sp. n.?) is 20° (table 1), which is smaller than most cystophorines and larger than most monachines, suggesting that this extinct seal was a medium-depth diver and ate medium sized prey, despite the small size of this seal as compared to other Phocidae . The fossil skulls and mandibles of Devinophocinae share characters with all three extant subfamilies. The first fossil record representing the subfamily Cystophorinae recently described by Koretsky and Rahmat (2013) did not include mandibular descriptions due to the lack of preserved material. The mandible of the fossil Miophoca vetusta ( Zapfe, 1937) , a possible fossil Cystophorinae , has been described, but there remains questionable classification of this seal and therefore, presently is situated in an uncertain taxonomic position. The condyloid angle of the extinct M. vetusta (9°, table 1) is the smallest of any seal examined, suggesting that they fed on small sized prey, perhaps even as a suction/bottom feeder.

While the morphology of the mandibular ramus of some Recent seals has been discussed, future studies are needed to accurately describe mandibular biomechanics. These examinations would need to include: distinguishing the comparative sizes of different types of teeth (cheek, incisors, and canines); specifying lines of actions of the muscles of mastication; and estimating vector inclination and joint locations. Although not included in this paper, we realize the importance of such studies and the potential for extrapolating the findings to determine the functional, morphological, and dietary correlations for modern and possibly fossil seals.

The authors are grateful to Dr. James Mead and Charles Potter for access to modern comparative material, to Dave Bohaska for access to fossil pinniped collections (all National Museum of Natural History, Smithsonian Institution), and to Drs. E. Gilland and Daryl Domning ( Laboratory of Evolutionary Biology , Howard University, Washington, D.C.) for their helpful comments on several versions of this manuscript. We also thank medical students Tina Hong, Robert E. Singleton II, and Brandon J. Anderson (Howard University, Washington, D.C.) for their assistance .

Figures

Fig. 7. Subfamily Phocinae: right mandible of fossil Leptophoca lenis, CMM–V 2021 in labial view from Calvert Formation, late-early Miocene (~18 Ma), Calvert County, Maryland, USA (A); subfamily Devinophocinae: reversed right mandible of fossil sp. n.? (NMNH 553687) in labial view from Badenian Formation, early– middle Miocene (~16 Ma), Central Paratethys, Vienna Basin of Western Slovakia, Europe. Masseteric fossae are shown by dashed outlines (B).