Pleurosternon bullockii ( Owen, 1842 )

Allain, Ronan, Vullo, Romain, Rozada, Lee, Anquetin, Jérémy, Bourgeais, Renaud, Goedert, Jean, Lasseron, Maxime, Martin, Jeremy E., Pérez-García, Adán, Fabrègues, Claire Peyre De, Royo-Torres, Rafael, Augier, Dominique & Bailly, Gilles, 2022, Vertebrate paleobiodiversity of the Early Cretaceous (Berriasian) Angeac-Charente Lagerstätte (southwestern France): implications for continental faunal turnover at the J / K boundary, Geodiversitas 44 (25), pp. 683-752 : 700

Pleurosternon bullockii ( Owen, 1842)

( Fig. 13 )

DESCRIPTION

Pleurosternids are the most abundant turtles in Angeac-Charente representing nearly 85% of the turtle material( Table 1 ). Most of the material consists of isolated shell plates characterized by an external surface covered by regular pits with fine linear striations perpendicular to the plate margins. This character combination is only shared with the Portuguese Late Jurassic (Kimmeridgian) Selenemys lusitanica and the British and French Late Jurassic-Early Cretaceous (Tithonian to Berriasian) Pleurosternon bullockii (Pérez-García&Ortega 2011) .An almost complete shell collected in 2014 confirmed the attribution to the species Pleurosternon bullockii ( Fig. 13 ; Gônet et al. 2019). The carapace is oval in outline and depressed with no nuchal emargination ( Fig.13A ). The nuchal is trapezoidal, wider posteriorly than anteriorly.The cervical scute is absent. The vertebral scutes are wide and cover about half of the costals. Unlike British specimens, suprapygal 2 is a wide hexagonal element that presents several morphotypes in Angeac-Charente.The plastron has a pair of mesoplastra and a wide anal notch posteriorly ( Fig.13B ). The large entoplastron is triangular anteriorly and rounded posteriorly. The posterior plastral lobe is longer than the anterior one. The near-complete shell and the remaining pleurosternid material in general will be described in detail elsewhere. The pleurosternid from Angeac-Charente differs from Selenemys lusitanica in characters including: nuchal width less than twice its maximum length; anterior nuchal edge constituting the anterior carapace rim; posterior margin of the first peripheral two times shorter than the anterior one; overlap of the first vertebral on the first pair of peripherals; first vertebral wider than the nuchal; first pair of marginals wider than long; and development of an anal notch.

Pleurosternon bullockii is known by numerous specimens from the Berriasian Purbeck Group of Dorset ( United Kingdom), including complete shells, carapaces, plastra, elements of the appendicular skeleton,and one skull ( Evans&Kemp 1975; Milner 2004; Sterli et al. 2010; Evers et al. 2020), but it is also identified in the Tithonian record of Dorset and the French locality of Wimille (Boulogne-sur-Mer) (Guerrero & Pérez-García 2020).

Pleurosternids are freshwater turtles, of which most of the remains have been collected from fluvial and lacustrine sedimentary deposits ( Joyce & Anquetin 2019). This hypothesis is confirmed by the depressed shell shape, appendicular bone morphology, and feeding specializations. Pleurosternon bullockii presents an elongate skull with low labial margins which suggests a gape and suction feeding ( Pritchard 1984; Foth et al. 2017; Joyce& Anquetin 2019).The abundance of pleurosternid material and the presence of different growth stages suggest that the Angeac-Charente paleoecosystem probably corresponds to the preferred habitat of this turtle.

Figures

FIG. 13. — Pleurosternid turtle from the Berriasian of Angeac-Charente: A, carapace of Pleurosternon bullockii (Owen, 1842) (ANG14 R-454) in dorsal view; B, plastron and right peripheral plates 1 and 2 of Pleurosternon bullockii (ANG14 R-474 & ANG15 R-1082) in ventral view. Scale bar: 10 cm.