A re-description ofSandownia harrisi(Testudinata: Sandownidae) from the Aptian of the Isle of Wight based on computed tomography scans

Abstract

Sandownidae is an enigmatic group of Cretaceous–Paleogene turtles with highly derived cranial anatomy. Although sandownid monophyly is not debated, relationships with other turtles remain unclear. Sandownids have been recovered in significantly different parts of the turtle tree: as stem-turtles, stem-cryptodires and stem-chelonioid sea turtles. Latest phylogenetic studies find sandownids as the sister-group of the Late Jurassic thalassochelydians and as stem-turtles. Here, we provide a detailed study of the cranial and mandibular anatomy of Sandownia harrisi from the Aptian of the Isle of Wight, based on high resolution computed tomography scanning of the holotype. Our results confirm a high number of anatomical similarities with thalassochelydians and particularly Solnhofia parsonsi , which is interpreted as an early member of the sandownid lineage. Sandownids + Solnhofia show many cranial modifications related to the secondary palate and a durophagous diet. Sandownia is additionally highly derived in features related to its arterial circulation and neuroanatomy, including the endosseous labyrinth. Our results imply rapid morphological evolution during the early history of sandownids. Sandownids likely evolved in central Europe from thalassochelydian ancestors during the Late Jurassic. The durophagous diet of sandownids possibly facilitated their survival of the Cretaceous/Paleogene mass extinction.

Cranial osteology of the Early Cretaceous turtlePleurosternon bullockii(Paracryptodira: Pleurosternidae)

“…Although Uluops uluops has so far not been considered a pleurosternid (e.g., Carpenter & Bakker, 1990;Lyson & Joyce, 2011;Pérez-García, Royo-Torres & Cobos, 2015;Joyce & Rollot, 2020), our comparisons indicate that it might be attributable to this clade. Among the features that are shared between Uluops uluops and Pleurosternon bullockii are a squamosal-parietal contact (possibly plesiomorphically present in baenids; Gaffney, 1972); the exclusion of the prootic from the trigeminal foramen by a posteroventral ramus of the parietal (also present in thalassochelydians and sandownids; Anquetin, Püntener & Joyce, 2017;Evers & Joyce, 2020); the presence of an extended sulcus palatino-pterygoideus formed in part by an enlarged septum orbitotemporale (similar to pleurodires, but also present in some cryptodires; Gaffney, Tong & Meylan, 2006); presence of a deep fossa in the roof of the sulcus palatino-pterygoideus (also in pleurodires; Gaffney, Tong & Meylan, 2006); dorsal position of the jugal above the level of the labial margin of the maxilla (also present in baenids; Gaffney, 1972); the size and shape of the quadratojugal; the presence of a small quadrate-supraoccipital contact (unclear in early baenids); the complete surrounding of the fenestra ovalis by the prootic and opisthotic (absent in Arundelemys dardeni [USNM 41614]; unclear in early baenids); the presence of a hyomandibular nerve sulcus in the posterior part of the canalis cavernosus (unclear in early baenids); the presence of anterior tubercula basioccipitale (shared also with helochelydrids, see below); the presence of a basipterygoid process that is laterally invaded by the cerebral artery (absent in baenids and Compsemys victa, but symplesiomorphic for turtles); and the general shape of the parabasisphenoid (see description for full details). Although some of these characters may prove to be more widespread among paracryptodires in particular or stem-turtles more generally, the number of similar features between Pleurosternon bullockii and Uluops uluops lend support to the hypothesis that the latter is a pleurosternid.…”

“…Full-size DOI: 10.7717/peerj.9454/ fig-4 margin has been proposed as a synapomorphy of Thalassochelydia (Anquetin, Püntener & Joyce, 2017), based on the presence of this process in Solnhofia parsonsi and plesiochelyids (Anquetin, Püntener & Billon-Bruyat, 2015). The same process has also been observed in sandownids (Evers & Joyce, 2020). The presence of this process in paracryptodires as well as the aforementioned groups, all of which have recently been interpreted as stem-turtles (e.g., , indicates that this could be a feature that is more widespread along the crownward stem-lineage of turtles.…”

“…5B-5C), which has been hypothesized to be related to the perilymphatic system of reentrant fluid-flow systems in turtle ears (e.g., Foth et al, 2019), and which is also present in Uluops uluops (UCM 53971). Because the fossa continues dorsolateral to the fenestra ovalis onto the opisthotic, we use the term perilymphatic fossa instead of posterior prootic recess as in some previous works (e.g., Evers & Joyce, 2020). Laterally to the perilymphatic fossa, the pterygoid and prootic form the posterior foramen for the canalis cavernosus.…”

The last surviving Thalassochelydia—A new turtle cranium from the Early Cretaceous of the Purbeck Group (Dorset, UK)

“…Therefore, the position of Thalassochelydia must be considered uncertain and prone to change for the time being. Finally, it should also be noted that a recent study suggested that sandownids more likely derived from thalassochelydian ancestors (Evers and Joyce, 2020 information). As preserved, the cranial roof contacts the palate, and the entire internal anatomy is unfortunately lost to us.…”

Figure 1: Geographical map of the Ajoie region, Canton of Jura, Switzerland.

“…The mandible is only known in TM 4023 and JM SCHA 70. For TM 4023, we used the 3D model of Evers & Benson (2018) as primary source of data, completed by the available literature (Parsons & Williams, 1961;Gaffney, 1975;Evers & Joyce, 2020). We had NMS 8741 at hand during this study.…”