Skull shape variation in extant and extinct Testudinata and its relation to habitat and feeding ecology

Abstract

Foth, C., Rabi M. and Joyce W.G. 2017. Skull shape variation in extant and extinct Testudinata and its relation to habitat and feeding ecology. -Acta Zoologica (Stockholm) 98: 310-325.Turtles (Testudinata) are a diverse group of reptiles that conquered a broad set of habitats and feeding ecologies over the course of their well-documented evolutionary history. We here investigate the cranial shape of 171 representatives of the turtle lineage and the relationship of shape to different habitat and diet preferences using two-dimensional geometric morphometrics. The skull shape of extant turtles correlates with both ecological proxies, but is more affected by habitat than diet. However, the application of these correlations to extinct turtles produces mostly flawed results, as least when compared to external data such as sedimentary environment, highlighting that the morphospace held by extant turtles is not necessarily the optimal location in tree space for a particular ecology. The inability of this study to correctly predict the ecology of extinct turtles is likely related to the fact that the shape of turtle skulls is dominated by the emarginations and jaw closure mechanisms, two shape features unrelated to habitat or feeding ecology. This indicates that various specializations that are apparent in the skull only contribute little to overall shape.

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…Taxa with higher/lower and wider/thinner endocasts do not possess similar skull proportions, which seem more related to the size and shape of the adductor chamber and the associated supraoccipital and squamosal crests (Figure 7). Proportional changes observed in the adductor chamber throughout the turtle lineage rather reflect the distinct volume and size of the external jaw adductor musculature in different taxa (Claude et al, 2004;Foth and Joyce, 2016;Foth et al, 2017;Ferreira and Werneburg, in press). Also, the position of the exits of the cranial nerves change only slightly, even with profound changes in the arrangement of related structures such as the eyes and muscles.…”

Still slow, but even steadier: an update on the evolution of turtle cranial disparity interpolating shapes along branches

“…As we thought a climatological control of disparity to be biologically implausible, we hypothesized following our initial analysis [ 6 ] that cranial disparity may be correlated with biogeography. In particular, as most turtle groups populate different parts of the available morphospace [ 32 ], the increasing fragmentation of Pangaea over the course of the late Mesozoic may had led to the formation of increasing amounts of endemism. This trend was only reversed during the Cenozoic when the extinction of basally branching turtle groups, perhaps caused by the global spread of cryptodires made possible by emerging continental bridges, offset gains in cranial disparity [ 7 , 8 ].…”

Comparative analysis of the shape and size of the middle ear cavity of turtles reveals no correlation with habitat ecology

“…sea turtles). As turtles with different habitat preferences show morphological differences in the skull (Foth et al 2017), shell (Claude et al 2003;Domokos & V arkonyi, 2008;Benson et al 2011;Polly et al 2016) and forelimbs (Joyce & Gauthier, 2004;Renous et al 2008), differences have been interpreted to reflect these ecologies. As the physical demands of hearing are vastly different for animals in water vs. in air, modifications can be expected in the auditory system of turtles as well.…”

“…As the full range of intermediates is apparent, it is difficult to rigorously categorize habitat preferences based on field observations (e.g. Joyce & Gauthier, 2004;Benson et al 2011;Foth et al 2017). However, Joyce & Gauthier (2004) could demonstrate that the forelimbs of turtles reflect the gradation from fully terrestrial to fully aquatic lifestyles.…”