Abstract
Studies on living turtles have demonstrated that shells are involved in the resistance to hypoxia during apnea via bone acidosis buffering; a process which is complemented with cutaneous respiration, transpharyngeal and cloacal gas exchanges in the soft-shell turtles. Bone acidosis buffering during apnea has also been identified in crocodylian osteoderms, which are also known to employ heat transfer when basking. Although diverse, many of these functions rely on one common trait: the vascularization of the dermal shield. Here, we test whether the above ecophysiological functions played an adaptive role in the evolutionary transitions between land and aquatic environments in both Pseudosuchia and Testudinata. To do so, we measured the bone porosity as a proxy for vascular density in a set of dermal plates before performing phylogenetic comparative analyses. For both lineages, the dermal plate porosity obviously varies depending on the animal lifestyle, but these variations prove to be highly driven by phylogenetic relationships. We argue that the complexity of multi-functional roles of the post-cranial dermal skeleton in both Pseudosuchia and Testudinata probably is the reason for a lack of obvious physiological signal, and we discuss the role of the dermal shield vascularization in the evolution of these groups.This article is part of the theme issue ‘Vertebrate palaeophysiology’.
Highlights
Studies on living turtles have demonstrated that shells are involved in the resistance to hypoxia during apnea via bone acidosis buffering; a process which is complemented with cutaneous respiration, transpharyngeal and cloacal gas exchanges in the soft-shell turtles
Bone acidosis buffering during apnea has been identified in crocodylian osteoderms, which are known to employ heat transfer when basking
We argue that the complexity of multi-functional roles of the post-cranial dermal skeleton in both Pseudosuchia and Testudinata probably is the reason for a lack of obvious physiological signal, and we discuss the role of the dermal shield vascularization in the evolution of these groups
Summary
The vertebrate post-cranial dermal skeleton is composed of bony scutes which ossify within the dermis [1,2,3,4]. We exported the pictures in TIFF format (electronic supplementary material, S2) and analysed them with Bone profiler [49] in order to measure the area occupied by the empty spaces proportionally to the entire area covered by bone and vascular spaces (as detailed in electronic supplementary material, S3) For both Pseudosuchia and Testudinata, time-scaled phylogenetic relationships of the sampled specimens were reconstructed in Mesquite [50] by relying on published references [5,41,42,51,52,53,54,55,56,57,58,59,60,61,62,63] (figure 1). This is a statistical test that reveals a correlation between quantitative and qualitative data while retracting the influence of the phylogeny, which is quantified either by K or λ—we decided to consider both options [68] (table 2)
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