The effect of the masticatory muscle physiological cross-sectional area on the structure of the temporomandibular joint in Carnivora

Abstract

We compared the temporomandibular joint structure between species of the order Carnivora and investigated its variation among family lineages. We also investigated the effect of the masticatory muscle physiological cross-sectional area (PCSA) on temporomandibular joint structure. The masticatory muscle is composed of multiple muscles, which contract in different directions and exert pressure on the temporomandibular joint. We investigated the effect of the ratio of each muscle’s PCSA—an indicator of muscle force—and muscle size relative to body size on temporomandibular joint structure. The temporalis PCSA relative to body size showed the highest correlation with temporomandibular joint structure. When the temporalis PCSA is large relative to body size, the preglenoid projects caudally, the postglenoid projects rostrally and the pre-postglenoid angle interval is small, indicating that the condyle is locked in the fossa to reinforce the temporomandibular joint. Most Carnivora use blade-like carnassial teeth when slicing food. However, dislocation occurs when the carnassial teeth are used by the temporalis muscle. Our results suggest that the temporomandibular joint is reinforced to prevent dislocation caused by the temporalis muscle. In Mustelidae, the temporomandibular joint with a rostrally projecting postglenoid is suitable for carnassial biting using the temporalis muscle. In Felidae, the force of the masseter onto the carnassial teeth is diverted to the canine by tightening the temporomandibular joint. In Canidae, the masticatory muscle arrangement is well-balanced, enabling combined action. Hence, reinforcement of the temporomandibular joint by bone structure is unnecessary.

Comparative three‐dimensional jaw muscle anatomy of marsupial carnivores (Dasyurus spp.) and the termite‐eating numbat (Myrmecobius fasciatus)

“…The emphasis on large temporalis muscles in generalist forms, exemplified here by species of Dasyurus and especially in carnivorans, facilitates rapid and powerful jaw closing from a wide gape (Becerra et al, 2014; Bensley, 1903; Christiansen & Adolfssen, 2005; Gorniak & Gans, 1980; Ito & Endo, 2019; Ström et al, 1988). Carnivorous species require a substantial temporalis mass to produce the force necessary to capture large prey (Christiansen & Adolfssen, 2005; Ito & Endo, 2019; Penrose et al, 2016; Turnbull, 1970; Weijs, 1994). As such, it is interesting, though perhaps not surprising, that Dasyurus species do not possess as extreme an enlargement of the temporalis as many carnivorans do.…”

“…The digastric is typically most highly developed in mammals that have a wide gape, such as carnivores, passing from the anteroventral surface of the mandible to the paraoccipital process from the ventrocaudal aspect of the skull (Evans & De Lahunta, 2012; Scapino, 1976; Ström et al, 1988). In Dasyurus , the digastric presents in a typical form, with fleshy insertion on the ventral aspect of the mandible, which provides a power advantage suited for the rapid and forceful opening of the jaw to capture live prey (Christiansen & Adolfssen, 2005; Ito & Endo, 2019; Morales‐García et al, 2021; Penrose et al, 2016).…”