A systematic compendium of turtle mandibular anatomy using digital dissections of soft tissue and osteology

Turtles are a charismatic reptile group with a peculiar body plan, which most notably includes the shell. Anatomists have often focused descriptive efforts on the shell and other strongly derived body parts, such as the akinetic skull, or the cervical vertebrae. Other parts of turtle osteology, like the girdles, limbs, and mandibles, are documented with less rigor and detail. The mandible is the primary skeletal element involved in food acquisition and initial food processing of turtles, and its features are thus likely linked to feeding ecology. In addition, the mandible of turtles is composed of up to seven bones (sometimes fused to as little as three) and has thus anatomical complexity that may be insightful for systematic purposes and phylogenetic research. Despite apparent complexity and diversity to the mandible of turtles, this anatomical system has not been systematically studied, not even in search of characters that might improve phylogenetic resolution. Here, we describe the mandibular osteology for all major subclades of extant turtles with the help of digitally dissected 3D models derived from high-resolution computed tomography (μCT) scans of 70 extant species. We provide 31 fully segmented mandibles, as well as 3D models of the mandibular musculature, innervation, and arterial circulation of the cryptodire Dermatemys mawii. We synthesize observed variation into 51 morphological characters, which we optimize onto a molecular phylogeny. This analysis shows some mandibular characters to have high systematic value, whereas others are highly homoplastic and may underlie ecological influences or other factors invoking variation.

A new rhynchocephalian (Reptilia: Lepidosauria) from the Late Jurassic of Solnhofen (Germany) and the origin of the marine Pleurosauridae

A new rhynchocephalian is described based on a recently discovered and well-preserved specimen from the Late Jurassic (Kimmeridgian) marine limestones of Solnhofen, Bavaria. Phylogenetic analysis recovers the new taxon as the sister group to Pleurosauridae, a small radiation of rhynchocephalians representing the oldest marine invasion of crown-clade Lepidosauria. The relatively strong evidence for this taxonomically exclusive lineage, within a generally volatile rhynchocephalian tree, places the new taxon in a position to inform the early history of the pleurosaur transition to the sea. The early steps in this transition are distributed throughout the skeleton and appear to increase hydrodynamic efficiency for both swimming and aquatic feeding. This early history may also have included a global truncation of plesiomorphic ontogenetic trajectories that left a number of skeletal features with reduced levels of ossification/fusion. The exact degree to which Vadasaurus had adopted an aquatic ecology remains unclear, but the insight it provides into the origin of the enigmatic pleurosaurs exemplifies the potential of Rhynchocephalia for generating and informing broad-based questions regarding the interplay of development, morphology, ecology and macroevolutionary patterns.

A phylogenetic approach to ontogeny and heterochrony in the fossil record: cranial evolution and development in anguimorphan lizards (Reptilia: Squamata)

The incorporation of ontogeny into the interpretation of the vertebrate fossil record promises major advances in palaeontology, systematics, and macroevolution. Here, a key additional component, the incorporation of phylogenetic bracketing into ontogenetic considerations, is demonstrated using cranial anatomy in anguimorphan lizards, a diverse modern clade with an extensive fossil record. The obstacles of fragmentary disarticulated fossil material and low representation in museum collections are overcome by using detailed analysis of individual elements and binning into broad ontogenetic stages, respectively. Results indicate the prevalence of classical macroevolutionary phenomena, notably heterochrony and homoplasy (convergence), throughout anguimorphan evolution. Furthermore, two problematic fossil anguimorph taxa are examined, both of which are unusually small for their clades, suggesting either immaturity or dwarfism. Using extant phylogenetic brackets of ontogenetic trajectories to distinguish between these hypotheses, it is shown that the holotype of one of these taxa is indeed a juvenile (also calling into question its taxonomy) and that the other is a dwarf. It is expected that a phylogenetic approach to ontogeny will yield similar insights across a broad range of fossil and extant organisms.