The mandibular symphyseal joint is remarkably variable across major mammalian clades, ranging in adults from unfused (amphiarthrosis) to partially fused (synarthrosis) to completely ossified (synostosis). Experimental work conducted on primates suggests that greater ossification of the symphysis is a response to increased recruitment of the balancing‐side (i.e. nonchewing side) jaw‐adductor muscles during forceful unilateral biting and chewing, with increased fusion strengthening the symphysis against correspondingly elevated joint stresses. It is thus expected that species with diets composed primarily of foods that require high‐magnitude bite forces and/or repetitive loading to process will be characterized by greater degrees of symphyseal ossification than species with relatively easy‐to‐process diets (i.e. food items typified by low toughness and/or low stiffness). However, comparative support for this idea is limited. We tested this hypothesis in four dietarily diverse mammalian clades characterized by variation in symphyseal fusion – the Strepsirrhini, Marsupialia, Feliformia, and Caniformia. We scored fusion in adult specimens of 292 species, assigned each to a dietary category based on literature accounts, and tested for an association between these two variables using Pagel’s test for the correlated evolution of binary characters. Results indicate that greater fusion is associated with diets composed of resistant items in strepsirrhines, marsupials, and feliforms, providing some support for the hypothesis. However, no such relationship was detected in caniforms, suggesting that factors other than dietary mechanical properties influence symphyseal ossification. Future work should focus on such factors, as well as those that favour an unfused mandibular symphysis.
Based on extensive experimental work on primates, two masticatory loading regimes have emerged as the likely determinants of mandibular symphyseal fusion-dorsoventral shear and lateral transverse bending (wishboning) (Ravosa and Hylander, 1994; Hylander et al., 1998, 2000). Recently, however, it has been argued that, rather than functioning to strengthen the symphysis during mastication, fusion serves to stiffen the symphyseal joint so as to facilitate increased transverse jaw movements during occlusion (Lieberman and Crompton, 2000). As part of this transverse stiffness model, it has been suggested that taxa with fused symphyses should also exhibit more horizontally oriented occlusal wear facets. Using a series of univariate and bivariate analyses, we test predictions of these three models in a sample of 44 species of selenodont artiodactyls. Consistent with the wishboning and transverse stiffness models, taxa with fused symphyses (camelids) have more horizontally oriented M(2) and M(2) occlusal wear facets, anteroposteriorly (AP) elongate symphyses, and relatively wider corpora. Contrary to the dorsoventral shear model, camelids do not have relatively deeper corpora (due to greater parasagittal bending). While taxa with ossified symphyses have relatively larger symphysis cross-sectional areas, this appears to be the byproduct of an increase in AP symphysis length due to greater lateral transverse bending of the mandible. Theoretical consideration of the biomechanics of mastication further suggests that strength, not stiffness, is the critical factor in determining symphyseal ossification. Thus, like anthropoid primates, fusion in selenodont artiodactyls appears to function in resisting increased wishboning stresses arising from an emphasis on transverse occlusal/mandibular movements and loads.
The discovery of ecomorphological relationships in mammals is important not only in its own right, but also for its potential in shedding light on the ecology of extinct and poorly known species. Two such potential relationships are the proposed connection between molar shear crests and body mass to diet. Research on primates suggests that, owing to the digestive challenges of structural carbohydrates (in foliage and insects), folivores and insectivores differ from other species in needing relatively long molar shear crests. These studies also suggest that metabolic constraints force folivores to be consistently larger than insectivores, with 500-700 g being the proposed body mass boundary between the two. This study tests these proposed ecomorphological links in a diverse sample of 64 marsupial species. As predicted, folivores and insectivores have significantly relatively longer molar shear crests than other species. Similarly, folivores are significantly larger than insectivores, as predicted, though the proposed body mass boundary was not supported. These results suggest that shear crest length and body mass are indeed linked to diet in the proposed manner. It may now be possible to use these findings to gain a greater understanding of the feeding behaviours of a variety of extinct marsupials and other therians.
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