Abstract
Pleurosternon bullockii is a turtle from the Early Cretaceous of Europe known from numerous postcranial remains. Only one skull has so far been referred to the species. Pleurosternon bullockii belongs to a group of turtles called pleurosternids, which is thought to include several poorly known taxa from the Late Jurassic and Early Cretaceous of Europe and North America. Pleurosternids and baenids, a group of North American turtles that lived from the Late Cretaceous to the Eocene, define a clade called Paracryptodira. Additionally, Paracryptodira likely includes compsemydids, and, potentially, helochelydrids. Character support for Paracryptodira is relatively weak, and many global phylogenetic studies fail to support paracryptodiran monophyly altogether. Proposed paracryptodiran synapomorphies are largely cranial, despite the poor characterization of pleurosternid cranial material. In addition to their questionable monophyly, the global position of paracryptodires is debated. Early studies suggest crown-turtle affinities, but most phylogenies find them as stem-turtles, irrespective of their monophyly. Here, we document the cranial osteology of Pleurosternon bullockii with the use of three-dimensional models derived from segmenting high-resolution X-ray micro-computed tomography (CT) scans. Pleurosternon bullockii has a primitive basipterygoid region of the skull, but a cryptodire-like acustico-jugular region. A surprising number of similarities with pleurodires exist, particularly in the laterally expanded external process of the pterygoid and in the posterior orbital wall. Our observations constitute an important step toward a phylogenetic re-evaluation of Paracryptodira.
Highlights
Paracryptodires are often referred to as ‘one of the three primary clades of turtles’ (e.g., Joyce, 2017)
The fossil turtle clade Paracryptodira was initially proposed based on the anterior position of the foramen posterius canalis carotici interni halfway along the contact of the basisphenoid with the pterygoid, in contrast to pleurodires, which have an entry that involved the prootic and/or quadrate, and crown cryptodires, which have an entry towards the posterior margin of the pterygoid (Gaffney, 1975a)
Whereas an anteriorly positioned foramen posterius canalis carotici interni seems to be present in Compsemys victa (UCM 53971) and Eubaena cephalica (Rollot, Lyson & Joyce, 2018), this foramen is not present in Pleurosternon bullockii (UMZC T1041) and Uluops uluops (UCM 53971), as the internal carotid artery is not embedded in bone in these taxa
Summary
Paracryptodires are often referred to as ‘one of the three primary clades of (crown) turtles’ (e.g., Joyce, 2017). Many of the other originally proposed characters in support of the group are symplesiomorphically present in Testudinata (e.g., presence of nasal, reduced prefrontal exposure on skull roof, presence of mesoplastra, presence of paired gular scutes; Gaffney, 1975a) Some proposed synapomorphies, such as the reduction of the palatine artery (Gaffney, 1975a) or a reduced fenestra perilymphatica (Joyce, 2007), are, similar to the aforementioned position of the foramen posterius canalis carotici interni midway along the parabasisphenoid-pterygoid suture (Gaffney, 1975a; Joyce & Anquetin, 2019), not universally present in paracryptodires (e.g., Anquetin & André, 2020; this study) and should be re-evaluated on a broader scale. Our description and comparisons provide anatomical evidence for unique morphologies of pleurosternids, which likely include at
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