The Tendinous Framework in the Temporal Skull Region of Turtles and Considerations About Its Morphological Implications in Amniotes: A Review

Abstract

In 1926, Tage Lakjer hypothesized a replacement of the infratemporal bar in diapsids by a ligament spanning between quadrate and the upper jaw. As a similar ligament is also present in turtles, he argued for a diapsid origin of this group. Based on recent advances in the homologization of the tendinous framework in the reptile jaw adductor chamber - reviewed in this paper - one could argue for independent origins of the cheek ligaments in sauropsids. The quadratomaxillar ligament of turtles could, with reservation, be homologized with the quadrate aponeurosis of other sauropsids, as well as to the superficial tendon of m. masseter in mammals. These structures have a strong morphogenetic influence to cranial anatomy. Given such an identity, the hypothesis of a structural replacement of the lower temporal arcade in lizards would be refuted. Moreover, such a homology could be correlated to the evolution of the middle ear and to the origin of the chewing mechanism in mammals, which contributed to the evolutionary success of that group. The homologization presented herein is critically discussed and is open for revision. Nevertheless, the value of tendinous structures for fundamental homologisations in the vertebrate head is highlighted.