The loss of the lower temporal arcade in diapsid reptiles

The fossil record of diapsid reptiles does not provide conclusive evidence that the loss of the lower temporal arcade is correlated with the development of streptostyly. Analysis of the structure of the external jaw adductor in extant lizards results in the formulation of an alternative hypothesis to explain the loss of the lower temporal arcade in lepidosaurs. Extant lizards show the development of a posteroventral portion of the superficialis layer (1b) of the external adductor, which invades the lateral surface of the lower jaw, thus escaping the original confines of the external adductor. The development and expansion of a posteroventral 1b muscle would have to be correlated with the loss of at least the posterior portion of the lower temporal arcade. Fossil lepidosaurs such as Clevosaurus, Prolacerta and Macrocnemus show the reduction of the lower temporal arcade from back to front, which is consistent with the hypothesis of the development of a posteroventral 1b muscle.