Temporal Bone Arrangements in Turtles: An Overview

Abstract

The temporal region of turtles is characterized by significant anatomical diversity. Turtles show a pure anapsid morphotype that exhibits various different marginal reductions known as emarginations. As a result of this diversity, turtles can be taken as a model by which to understand the processes that may have resulted in the highly debated anatomy of the amniote temporal region in general. In this review on almost forgotten literature, I summarize ten potential factors that may act on the skull to shape the temporal region of turtles. These are: (1) phylogenetic constraints, (2) skull weights, (3) type of food, (4) skull dimensions, (5) muscle bulging, (6) ear anatomy and jaw muscle bending mechanisms, (7) extent and nature of muscle attachment sites, (8) internal forces within the jaw adductor chamber, (9) environmental pressure, and (10) neck bending mechanisms. Particular focus is laid on the interrelationship of the jaw musculature and the dermatocranial armour, which were assumed to influence each other to a certain degree. In the literature, cranial dimensions were assumed to influence temporal bone formation within major tetrapod groups. Among these, turtles seem to represent a kind of intermixture, a phenomenon that may be reflected in their specific anatomy. The references presented should be understood as product of the scientific environment in which they developed and the older literature does not always insist current empirical demands. However, the intuitive and creative ideas and the comprehensive anatomical considerations of these authors may inspire future studies in several fields related to this topic.

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

“…The arrangement of the temporal bones in Testudinata (Figs. 1D, 2G-J), however, is barely comparable to the "typical" anapsid skull (Müller, 2003) and raises difficulties in reconstructing the position of turtles among amniotes in phylogenetic analyses (Werneburg, 2012) and resulted in a variety of hypotheses for turtle origin .…”

“…Temporal openings are herein defined as reductions of the plesiomorphically complete dermatocranial temporal armour, which was present in early tetrapods. These openings involve temporal fenestrae and/or emarginations, the origins of which are not well understood and remain the subject of recent debates (reviews by Rieppel, 1993;Werneburg, 2012).…”

“…Emarginations are marginal excavations of the temporal dermatocranial armor, which appear in several tetrapod groups. In turtles, ventrolateral and posterodor-sal emarginations can appear to different extents (Kilias, 1957;Rieppel, 1993; reviewed by Werneburg, 2012). If in turtles both emarginations meet in the middle (e.g., Terrapene, Fig.…”

Jaw musculature during the dawn of turtle evolution

“…The discussion of the present study on the phylogenetic signal of the jaw musculature (see above) should not be applied as a general feature to other vertebrate taxa and muscles, as, when compared to other vertebrate groups, turtles show a high degree of cranial diversity, which evolved relatively recently (e.g. Zdansky 1923Zdansky -1925Gaffney 1979;Joyce 2007;Werneburg 2012). Among other vertebrate taxa, muscle anatomy contains much phylogenetic signal (e.g.…”

“…This extension and related changes in bite stresses were often assumed to be in the cause of the loss of various dermatocranial elements resulting in a caudal or/and a lateral emargination (e.g. Zdansky 1923Zdansky -1925Kilias 1957;reviewed by Werneburg 2012) and were correlated to a potential secondary loss of skull fenestration. The Constrictor primus lateralis consists of mm.…”