Mandibular growth and function in Archaeolemur

Abstract

Ontogenetic changes in the morphology of the mandibular symphysis are described in Archaeolemur so as to infer the functional significance of symphyseal fusion in this subfossil Malagasy lemur. The first regions of the symphysis to show a more complex morphology were the lower and anterior borders of the joint and, to a lesser extent, the lingual borders of the superior and inferior transverse tori. During growth, these regions became increasingly rugose and encroached upon a centrally located, smooth, "oval" region, which may have been a principal pathway for neurovascular structures communicating with the unfused joint. In subadults, the symphysis was completely fused except for the lingual surface of the inferior transverse torus, where a patent suture and potential space were present between dentaries. Thus, in Archaeolemur there was an age- and size-related pattern of increased symphyseal ossification or fusion that was complete by adulthood. The morphology of the interlocking bony processes and the sequence of ossification in the symphysis suggest that increased dorsoventral shear stress during mastication was the most likely determinant of symphyseal fusion in Archaeolemur. The allometric pattern of greater symphyseal fusion may be linked to the presence of relatively greater dorsoventral shear in adults due to an increased recruitment of balancing-side jaw-muscle force. There is little indication that the symphysis of juvenile Archaeolemur was buttressed to resist forces associated with "wishboning" during mastication or vertical bending during incision. Our observations, as well as those of others, suggest that symphyseal fusion in primates occurs initially as a response to increased dorsoventral shear during mastication. Therefore, wishboning stress might only become a major determinant of symphyseal form and function in those taxa that develop a fused symphysis to counter increased dorsoventral shear.

Anthropoid Origins and the Modern Symphysis

“…Recents discoveries of stem group anthropoids indicate that Middle Eocene Eosimiidae evinced the primitive primate and mammalian condition of an unfused symphysis [10,11], while Upper Eocene Oligopithecidae and Qatraniinae possessed a partially fused symphysis [12][13][14], a more derived character state [15][16][17]. Such fossil finds not only extend the antiquity of the Anthropoidea, but also indicate the existence of a far greater range of anthropoid dietary adaptations and body sizes than has previously been demonstrated in Oligocene Parapithecinae and Propliopithecidae.…”

Pushing the limit: masticatory stress and adaptive plasticity in mammalian craniomandibular joints

“…In rabbits and other mammals, this results in elevated peak strains along the mandible and higher TMJ reaction forces (Weijs and de Jongh, 1977;Hylander, 1979a;Hylander, 1979b;Hylander, 1979c;Hylander, 1992;Hylander et al, 1998;Ravosa et al, 2000). Like marsupials, rodents, carnivorans, artiodactyls and primates, rabbits exhibit postnatal variation in the size and conformation of the articular surface and connective tissues of the symphysis, beginning as an amphiarthrosis (unfused) in neonates and developing into a synarthrosis (partially fused) by adulthood (Trevisan and Scapino, 1976a;Trevisan and Scapino, 1976b;Beecher, 1977;Beecher, 1979;Hirschfeld et al, 1977;Scapino, 1981;Weijs and Dantuma, 1981;Ravosa and Simons, 1994;Ravosa, 1996;Ravosa, 1999;Hogue and Ravosa, 2001;Hogue, 2004).…”

“…In vivo and comparative analyses indicate that the postnatal development of masticatory elements and tissues is influenced by variation in jaw-loading patterns, with weaning being a particularly important life-history stage (Ravosa and Simons, 1994;Ravosa, 1996;Ravosa, 1999;Ravosa and Hogue, 2004). Once weaned, mammals ingest 'adult' food items (e.g.…”

“…As joint ossification clearly does not compromise symphyseal function as it would with the TMJ, the disparate long-term responses of symphyseal soft versus hard tissues may explain a common (but poorly understood) intraspecific trend of older mammals developing increased fusion (cf. Beecher, 1977;Beecher, 1979;Scapino, 1981;Ravosa and Simons, 1994;Ravosa, 1996;Ravosa, 1999;Hogue and Ravosa, 2001). Thus, age-related changes in fusion, especially in old adults, may represent a compensatory osteogenic response to load-induced degradation of the FC pad and perhaps other connective tissues.…”

The Prosimian Fauna of the Fayum Eocene/Oligocene Deposits of Egypt

“…Enough mandibles of Catopithecus have been found to indicate that the symphysis was probably not fused, but the opposing dentaries interlocked in a stage which Beecher [26,27] considered intermediate between unfused and fused. This system of interlocking ridges is also an ontogenetic stage seen in juvenile Archaeolemur fossils [28] and is followed by complete fusion in adult Archaeolemur. It was Beecher's belief that symphyseal fusion appeared to correlate with a folivorous diet in both living and fossil prosimians, although certainly not all modern Malagasy folivores have fused symphyses.…”