The hooked element in the pes of turtles (Testudines): a global approach to exploring primary and secondary homology

Abstract

The hooked element in the pes of turtles was historically identified by most palaeontologists and embryologists as a modified fifth metatarsal, and often used as evidence to unite turtles with other reptiles with a hooked element. Some recent embryological studies, however, revealed that this element might represent an enlarged fifth distal tarsal. We herein provide extensive new myological and developmental observations on the hooked element of turtles, and re-evaluate its primary and secondary homology using all available lines of evidence. Digital count and timing of development are uninformative. However, extensive myological, embryological and topological data are consistent with the hypothesis that the hooked element of turtles represents a fusion of the fifth distal tarsal with the fifth metatarsal, but that the fifth distal tarsal dominates the hooked element in pleurodiran turtles, whereas the fifth metatarsal dominates the hooked element of cryptodiran turtles. The term 'ansulate bone' is proposed to refer to hooked elements that result from the fusion of these two bones. The available phylogenetic and fossil data are currently insufficient to clarify the secondary homology of hooked elements within Reptilia.

A Review of the Fossil Record of Turtles of the CladeBaenidae

“…These are consistent in general morphology with those of other freshwater aquatic turtles, but lack specializations as seen in marine turtles or soft-shelled turtles. The ansula (sensu Joyce et al 2013) is well developed and digit V lacks a claw.…”

Bony skull development in the Argus monitor (Squamata, Varanidae, Varanus panoptes) with comments on developmental timing and adult anatomy

“…Arguments differ regarding character correlation, especially skeletal characters associated with limbless burrowing and feeding (Müller et al, 2011;Gauthier et al, 2012). In such cases of conflicting phylogeny, developmental data can contribute to the solution of the problem by addressing primary homology hypotheses (Freeman and Herron, 2007;Joyce et al, 2013;Werneburg et al, 2013a) or by generating 'characters' of relative timing that can be analysed in cladistic studies (Velhagen, 1997;Maxwell and Harrison, 2009;Werneburg and Sánchez-Villagra, 2015).…”

“…Bones that serve as attachment sites for large and strong muscles, particularly those with tendinous attachments, appear to ossify earlier to enable the development of necessary mechanical properties (Joyce et al, 2013). Also, the movements of an embryo need to be taken into account when evaluating ossification patterns (Nowlan et al, 2010).…”

“…As such, the limitation of the threshold that we use in surface rendering corresponds to that of the double staining procedure. The most accurate method would be to clear and stain the specimens with Alizarin red only to avoid acid effects (e.g., Joyce et al, 2013), because Alizarin red has just a small, mostly alcalic effect. The disadvantage is that the pure Alizarin approach prevents the observation of chondrification.…”

The origin of the turtle body plan: evidence from fossils and embryos

“…However, the former feature is also present in various archosauromorph reptiles (Pritchard & Sues ) and the ontogenetic development of the latter bone (which involves the fifth distal tarsal) differs in detail not only between squamates and turtles but even among turtles (Joyce et al . ). In a detailed assessment of the features cited in support of a close relationship between (extant) turtles and archosaurs, Rieppel () only accepted the connection of the nictitating membrane of the eye to a pyramidalis muscle (derived from the retractor bulbi muscle) and the shift of the origin of the subclavian arteries from the aortic to the carotid arch as potential synapomorphies.…”