Chewing function and jaw articulation in fossil and extant shrews
Final Report Abstract
The masticatory musculature of different species of Soricidae was reconstructed. Tooth wear was observed, and jaw movements were inferred. In general, the masticatory muscles of Soricidae are characterized by a strong subdivision. During mastication, the main force is applied by the huge m. temporalis, resulting in a powerful dorsally directed movement of the mandible. The m. masseter and the m. pterygoideus internus, inserting on the lateral and the medial side of the long angular process respectively, induce primarily a medially oriented yaw rotation and a roll rotation. Regarding the latter, the contraction of the m. masseter leads to an inversion, while the contraction of the m. pterygoideus internus produces an eversion. Both muscles are subdivided into many subunits, each possessing a different line of action and torque. This enables the degree of rotational movements to be adjusted according to the properties of the ingested food, which has been demonstrated by Oron and Crompton (1985) in Tenrec ecaudatus. Roll rotational motions of the mandible might enhance the fragmentation of food during the shearing phase, as bolus between the shearing surfaces is affected by an internal twisting. During the crushing and grinding phase, inversion and eversion lead to an increased variety of the relative movement directions of the protocone through the talonid basin, enhancing the fragmentation. It also allows for an efficient rotational grinding stroke, observed by Bhullar et al. (2019) in Monodelphis domestica. Furthermore, the guided, precise rotational mandibular motions facilitate the maintenance of precise occlusion, despite advanced tooth wear. In the Crocidurinae (with exception of Suncus etruscus), the m. masseter is dominated by a large m. masseter (2), which is proportionally larger than in the Soricinae. Furthermore, m. masseter (2) and (3) originate from a large area in the Crocidurinae, extending further anteriorly and laterally, compared to the Soricinae. This suggests a more unilateral inversion of the Crocidurinae. Besides that, no significant differences between the variations in the orientations of scratches on the molars could be observed, suggesting differentiated jaw motions in all investigated species of Soricidae. These are made possible by the separation of the articular facet on the condylar process. Overall, the unique jaw morphology in conjunction with the complex muscle arrangement permits a more efficient energy gain and the maintenance of a high metabolic rate which is crucial for small-bodied mammals such as shrews.
Publications
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Jaw mechanics in shrews and the role of the double articulation. 93rd Annual Meeting of the Paläontologische Gesellschaft, Stuttgart, Germany
Pommerening S. & Martin T.
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Jaw mechanics in shrews and the role of the double articulation. Bull Int Assoc Paleodont. 2022;16(2): 115
Pommerening S. & Martin T.
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Jaw mechanics in shrews and the role of the double articulation. 83rd Annual Meeting of the Society of Vertebrate Paleontology
Pommerening S.
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Jaw mechanics in shrews and the role of the double articulation. 94th Annual Meeting of the Paläontologische Gesellschaft, Jena, Germany
Pommerening S. & Martin T.
