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

Abstract

Background. The mostly Berriasian (Early Cretaceous) Purbeck Group of southern England has produced a rich turtle fauna dominated by the freshwater paracryptodires Pleurosternon bullockii and Dorsetochelys typocardium. Each of these species is known by numerous relatively complete shells and by a single cranium. The two other turtles found in the Purbeck Group (Hylaeochelys belli, a species of uncertain affinities, and the terrestrial helochelydrid "Helochelydra" anglica) are known only from shell remains.Methods. In the present contribution, we describe a new turtle cranium from the Purbeck Group of Swanage, Dorset (southern England). We also explore the phylogenetic relationships of this new cranium and of Hylaeochelys belli in the context of a recently published global turtle matrix.Results. Before complete preparation, the new Purbeck cranium was provisionally referred to Dorsetochelys typocardium, but our analysis clearly contradicts a referral to this species in particular and to paracryptodires in general. In contrast, the new cranium shares a number of features with the Late Jurassic, coastal marine Thalassochelydia, including a posterolaterally open foramen palatinum posterius, a strong ridge on the posterior surface of the processus articularis of the quadrate, a strong posterior orientation of the processus articularis in ventral view, and a processus trochlearis oticum limited to the medial part of the otic chamber and bordered by a deep recess laterally. Our phylogenetic analysis confirms a placement of the new Purbeck cranium within the clade Thalassochelydia.Discussion. In terms of morphology, the new Purbeck cranium does not correspond to any known taxon. However, we refrain from naming a new species based on it because there is a good chance that this cranium actually belongs to the shell-based species Hylaeochelys belli (also recovered as a thalassochelydian in our phylogenetic analysis). Unfortunately, we lack any objective evidence to support this conclusion for the moment. In any case, the new Purbeck cranium confirms what others have previously suggested based on Hylaeochelys belli: thalassochelydian turtles survived the Jurassic-Cretaceous transition.

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

“…Additionally, 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 thus be re-evaluated on a broader scale. Ambiguity in character support of Paracryptodira is also implicit from many global phylogenetic studies that fail to retrieve the constituent taxa as monophyletic altogether (Sterli, 2010;Anquetin, 2012;Sterli, Pol & Laurin, 2013;Cadena & Parham, 2015;Rabi et al, 2014;Zhou & Rabi, 2015;Joyce et al, 2016;.…”

“…A palatine artery canal has also been reported for Dorsetochelys typocardium by Anquetin & André (2020), but these authors do not describe the canal system in detail and it thus remains unclear if the proposed palatine artery canal foramen reported for Dorsetochelys typocardium is indeed that or pertains to the canal system of the vidian nerve. Indeed, the posterior entry of the palatine artery described by Anquetin & André (2020) resembles that of the vidian canal described herein for Pleurosternon bullockii by piercing the pterygoid. As the situation in Dorsetochelys typocardium demands clarification, ideally by means of CT scanning, we will not further comment on this taxon herein.…”

“…The main osteological feature that has historically been used to unite paracryptodires is a far anteriorly positioned foramen for the entry of the internal carotid artery (foramen posterius canalis carotici interni; Rabi et al, 2013) formed midway along the parabasisphenoid-pterygoid suture (Gaffney, 1975a). This feature persists to be cited as diagnostic for paracryptodires (see review of Joyce & Anquetin, 2019), although the foramen has repeatedly been interpreted to be absent altogether in at least some paracryptodires (Sterli et al, 2010;Rabi et al, 2013;Anquetin & André, 2020). Additionally, 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).…”

A new species of the large-headed coastal marine turtleSolnhofia(Testudinata, Thalassochelydia) from the Late Jurassic of NW Switzerland

“…Solnhofia parsonsi is usually considered to be a representative of Thalassochelydia, a clade of coastal marine turtles almost exclusively known from the Late Jurassic of western and central Europe (but at least one species reached Argentina, while at least another one survived into the Early Cretaceous) and that represents the first radiation of chelonians into marine environments (Anquetin, Püntener & Joyce, 2017;Anquetin & André, 2020;González Ruiz, De la Fuente & Fernández, 2020). Thalassochelydians are traditionally considered to be basal Pan-Cryptodira, but recent phylogenetic works have somewhat challenged this conclusion by recovering them as stem Testudines, sister group to Pleurodira, or stem Chelonioidea Evers, Barrett & Benson, 2019;Gentry, Ebersole & Kiernan, 2019).…”

A new species of the large-headed coastal marine turtle Solnhofia (Testudinata, Thalassochelydia) from the Late Jurassic of NW Switzerland

“…Solnhofia parsonsi is usually considered to be a representative of Thalassochelydia, a clade of coastal marine turtles almost exclusively known from the Late Jurassic of western and central Europe (but at least one species reached Argentina, while at least another one survived into the Early Cretaceous) and that represents the first radiation of chelonians into marine environments (Anquetin et al, 2017;Anquetin and André, 2020;González Ruiz et al, 2020). Thalassochelydians are traditionally considered to be basal Pan-Cryptodira, but recent phylogenetic works have somewhat challenged this conclusion by recovering them as stem Testudines, sister group to Pleurodira, or stem Chelonioidea Gentry et al, 2019).…”