Early loss and multiple return of the lower temporal arcade in diapsid reptiles

Abstract

The temporal arches of diapsid reptiles have received attention for several decades. In particular, it has been observed that the lower temporal bar at the ventral margin of the cheek is frequently reduced due to the absence of a contact between jugal and quadratojugal. The loss of the arcade was formerly considered to be of high systematic value, but is now often interpreted as being autapomorphic for the respective taxon, and the presence of both arcades is generally regarded as a plesiomorphic feature. Here I show, based on a cladistic analysis as well as on further anatomical evidence, that the lower temporal arcade was lost only once in diapsid evolution, and that the presence of the arch in "higher" diapsids is secondary, which is indicated by the different ratio between jugal and quadratojugal as well as by ontogeny. This result also sheds new light on the understanding of the cheek configuration of enigmatic taxa such as ichthyosaurs and turtles.

Biomechanical assessment of evolutionary changes in the lepidosaurian skull

“…Indeed, the same seems to have been true for the last common ancestor of lepidosaurs and archosaurs ( Fig. 1) (9). A complete lower temporal bar was developed de novo one or more times within Rhynchocephalia (7,8).…”

“…In the tuatara, Sphenodon, the quadrate is rigidly fixed to the skull by its bony attachments to the pterygoid, squamosal, quadratojugal/jugal, the last of these being a secondary development (7)(8)(9). In most squamates, by contrast, the streptostylic quadrate can swing anteriorly and posteriorly during jaw opening and closing, although this movement is restricted, to various degrees, by the stiffness of the joints between the quadrate and neighboring bones (squamosal, pterygoid, supratemporal, opisthotic) and by associated quadratopterygoid and temporal (quadratojugal/jugomandibular) ligaments.…”

“…Loss of that bar supposedly ''freed'' the quadrate, resulting in streptostyly. Elements of this hypothesis persist in the literature, even though it has been demonstrated unequivocally (1,(7)(8)(9) that the ancestral lepidosauromorph, the ancestral lepidosaur, and the ancestral rhynchocephalian (7,8) all had a fixed quadrate (strong quadratepterygoid and squamosal-quadrate joints) but no lower temporal bar (Fig. 1).…”

“…Thus, the question at issue in relation to the lepidosaurian lower temporal bar is not the functional advantage of its loss (13), but rather of its gain in some rhynchocephalians and, very rarely, in lizards (12,14). This, in turn, raises questions as to the selective advantages of the different lepidosaurian skull morphologies.Morphological changes in the lepidosaurian skull have been the subject of theoretical and experimental studies (6,8,9,(15)(16)(17)(18)(19)) that aimed to understand the underlying selective criteria. It is widely accepted that such factors as genetic history and lifestyle play a pivotal role in the evolution of cranial structure (20), but there has been less discussion on the degree to which biomechanical necessities are among the selective factors that direct evolutionary pathways.…”

“…Morphological changes in the lepidosaurian skull have been the subject of theoretical and experimental studies (6,8,9,(15)(16)(17)(18)(19) that aimed to understand the underlying selective criteria. It is widely accepted that such factors as genetic history and lifestyle play a pivotal role in the evolution of cranial structure (20), but there has been less discussion on the degree to which biomechanical necessities are among the selective factors that direct evolutionary pathways.…”