Table 2. Systematic overview of skeletal differences between Nanophoca vitulinoides and extant Phocinae.
Skeletal differences between Nanophoca vitulinoides and extant Phocinae are provided, together with their myological and locomotive implications.
| Bone | Character | Development compared to extant Phocinae | Muscle | Implications for Nanophoca vitulinoides |
|---|---|---|---|---|
| Cervical vertebra | Small with small spinous process† | = | Epaxial muscles (musculus multifidus lumborum, musculus longisimus thoracics) | Limited use of fore flippers during aquatic and terrestrial locomotion |
| Lumbar vertebra | Robust with well developed spinous process*,† | = | Hypaxial muscles (musculus quadratus lumborum, musculus longissimus thoracis, musculus iliocaudalis) | Strong horizontal ambulation posterior end body during aquatic locomotion |
| Humerus | Height greater tubercle (insertion spinatus muscles)*,† | + | Musculus supraspinatus, musculus infraspinatus | Decreased speed but increased power abduction foreflipper |
| Height lesser tubercle*,† | − | Musculus subscapularis | Increased speed and decreased power medial rotation and adduction foreflipper | |
| Anterior projection deltopectoral crest*,† | − | Musculus atlantoscapularis (extension), musculus humerotrapezius (extension), musculus deltoideus (abduction) | Increased speed and decreased power extension and abduction foreflipper | |
| Width bicipital groove* | − | Musculus biceps brachii | Weakly developed flexion of foreflipper | |
| Development lateral epicondylar crest* | = | Most manual extensors and musculus pronator teres | Similarly frequent use of manual extensors and pronation of fore flipper | |
| Development medial epicondyle* | − | Most manual flexors | Less intense use of manual flexors | |
| Ulna | Development olecranon process*,† | + | Musculus. triceps brachii | More powerful extension fore flipper |
| Radius | Insertion surfaces musculus supinator, musculus pronator teres, and musculus brachioradialis* | = | Musculus supinator, musculus pronator teres, musculus brachioradialis | Supination and pronation of foreflipper about equally strong and weak, respectively, as in extant Phocinae |
| Proximal position of insertion surface musculsu brachioradialis* | + | Musculus brachioradialis | Increased power (and reduced speed of) supination of the foreflipper | |
| Sacrum | Size sacral wings*,† | =/+ | musculus erector spinae, ventral sacroiliac ligament | Flexion, extension and rotation of dorsum during horizontal aquatic pelvic undulations with firm contact of sacrum to innominate, similar to extant Phocinae |
| Anterior projection sacral wings | + | Musculus erector spinae | Increased speed of flexion, extension and rotation of dorsum during aquatic locomotion but reduced amplitude | |
| Spinous processes | + (incl. fusion) | ?Musculus multifidus | ?Increased stability lumbus during pelvic oscillations | |
| Innominate | Depth gluteal fossa*,† | =/− | Gluteus muscles | Relatively weak external rotation and extension hip joint (contra greater trochanter femur, see below) |
| Development origin surface musculus obturatorius externus* | + | Musculus obturatorius externus | Relatively powerful adduction thigh | |
| Development origin fossa musculus rectus femoris | = | Musculus rectus femoris | Relatively strong extension knee joint, as in extant Phocinae | |
| Hook-like ischiatic spine | N/A | Dorsal sacroiliac ligament | Strong contact innominate and sacrum | |
| Femur | Height greater trochanter* | + | Gluteus muscles | Relatively strong external rotation and extension hip joint (contra gluteal fossa ilium, see above) |
| Depth trochanteric fossa* | =/+ | Musculus obturatorius externus | Relatively powerful adduction thigh | |
| Sphericity femoral head (dimensions neck compared to head)* | + (−) | N/A | Increased mobility coxo-femoral joint | |
| Development pit femoral head* | + | Teres major ligament | Increased fixation femoral head during terrestrial locomotion and increased maneuverability | |
| Adductor tubercle medial epicondyle | + | Hip adductors | Powerful adduction hip | |
| Ridge above medial condyle | N/A | Musculus gastrocnemius | ?More powerful flexion knee and foot | |
| Pit on lateral side lateral condyle | + | Musculus popliteus | More powerful flexion and lateral rotation knee joint | |
| Presence suprapatellar fossa | N/A | ?Musculus articularis genus | ?Powerful extension knee joint and providing space for the extension of the knee joint | |
| Tibia | Concavity proximal portion posterior margin of tibia | + | Musculus popliteus | More powerful flexion and lateral rotation knee joint |
Notes:
For the comparison with extant Phocinae: “=” approximately equal, “+” better developed, “−” less developed. Muizon (1981) was the first to discuss the functional anatomy of the axial and appendicular skeleton of Phocidae.
In the “Character” column, an asterisk (*) indicates referral to Muizon (1981), a cross (†) indicates referral to Berta, Sumich & Kovacs (2015; and references therein), while no sign indicates an interpretation reported here for the first time. These observations may have been observed in other publications as well, but without connecting character observations with musculature implications.