The adaptive significance of mandibular symphyseal fusion in mammals

Abstract

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.

A Reevaluation of the Morphology, Paleoecology, and Phylogenetic Relationships of the Enigmatic Walrus Pelagiarctos

“…Fusion and ankylosis of the mandibular symphysis, among pinnipeds, is only typically found in Pelagiarctos thomasi , Dusignathus seftoni , and the odobenines Odobenus and Valenictus ; in odobenines it is hypothesized to reduce torsion during suction feeding [42]. Among terrestrial carnivores, mandibular symphyseal fusion occurs in a variety of taxa, including big cats ( Panthera ), aardwolves ( Proteles ), bears (Ursidae), kinkajou ( Potos ) and a variety of mustelid taxa [61]. Within terrestrial taxa, this feature is thought to reduce strain at the symphyseal joint and focus force along the horizontal midline of the mandibles [62].…”

What shapes the oral jaws? Accommodation of complex dentition correlates with premaxillary but not mandibular shape

“…Ossification of the mandibular symphysis restricts the individual movement of each half of the mandible; this helps create a forceful bite by transferring strength from the jaw muscles (Lieberman and Crompton, 2000;Scott et al, 2012). The degree of ossification of the symphysis can be used to predict diet in mammals (Scott et al, 2012) and reptiles (Holliday et al, 2010).…”

“…The degree of ossification of the symphysis can be used to predict diet in mammals (Scott et al, 2012) and reptiles (Holliday et al, 2010). The mandibular symphysis of the Mexican tetra and zebrafish has received little attention in the literature (Cubbage and Mabee, 1996;Gregory and Conrad, 1936;Milligan et al, 2012).…”

“…We hypothesized that there would be morphological shape differences associated with oral jaws that have teeth versus those without teeth in teleosts. Furthermore, since the mandibular symphysis is known to influence the overall strength of the vertebrate lower jaw (Scott et al, 2012), we also investigated the histological differences of this structure in these two species.…”

Kedilerde Symphisis Mandibula Ayrilmalarinin ve Parasymphiseal Kiriklarin Sağaltiminda İnterdental ve İnterfragmenter Stabilizasyon (İVİS): Yeni Bir Teknik

“…While in some mammals fusion in this joint is never completed  İletişim (Correspondence)  +90 212 4737070/17296  ozer_kursat@yahoo.com (amphiarthrosis), there is a union in some (synarthrosis), and complete ossification in others (synostosis). In small adult cats, the mandibular symphysis is in the form of a synarthrosis [3,4] . Road traffic accidents occupy first place in the etiology of mandibular symphysis separations, followed by falling from a height, fighting with other animals, firearm injuries and, rarely seen in cats, periodontal diseases and neoplasias [5,6] .…”