Portlandemys gracilis n. sp., a New Coastal Marine Turtle from the Late Jurassic of Porrentruy (Switzerland) and a Reconsideration of Plesiochelyid Cranial Anatomy

Abstract

BackgroundSeveral groups of stem cryptodires became adapted to coastal marine environments as early as the Late Jurassic, 40 million years before the Pan-Chelonioidea. The Plesiochelyidae are a major component of this first radiation of crown-group turtles into marine habitats. They are abundant in many European localities, but their systematics is still greatly confused. Only three species are represented by cranial material: Plesiochelys etalloni, Plesiochelys planiceps, and Portlandemys mcdowelli.Methodology/Principal FindingsIn the present study, we describe a cranium and a mandible from the Kimmeridgian of Porrentruy (Switzerland), which we refer to a new species, Portlandemys gracilis n. sp. This new taxon differs from Portlandemys mcdowelli in several aspects of the cranium and mandible, notably in being generally more gracile, but the two species share a narrow skull, a more acute angle between the labial ridges on the mandible, and a unique configuration of the anterodorsal part of the basicranium. The cranial anatomy of plesiochelyid turtles is discussed in details based primarily on these new specimens and new cranial material of Plesiochelys etalloni from Solothurn, Switzerland.Conclusions/SignificanceSeveral characters (e.g., the contribution of the parietal to the foramen nervi trigemini, the configuration of the dorsum sellae and sella turcica, the presence of an infolding ridge on the posterior surface of the quadrate) appear as potential candidates to help elucidate plesiochelyid relationships. Some of these characters are included in a previously published phylogenetic dataset and help to stabilize the relationships of plesiochelyid turtles and closely related taxa. For the first time, our results suggest that plesiochelyids, 'Thalassemys' moseri, and Solnhofia parsonsi (representing the Eurysternidae) form a clade at the base of Eucryptodira.

The comparative osteology ofPlesiochelys biglerin. sp., a new coastal marine turtle from the Late Jurassic of Porrentruy (Switzerland)

“…As preserved, there is no contact between the exoccipital and pterygoid, which may be a difference with Plesiochelys etalloni , but this contact is often difficult to observe in this taxon (Anquetin, Püntener & Billon-Bruyat, 2015). An exoccipital–pterygoid contact is otherwise present in Portlandemys ssp.…”

Skull shape variation in extant and extinct Testudinata and its relation to habitat and feeding ecology

“…In a first step, the habitat preference of extinct turtles is typically assessed by reference to the sediments in which they were found. Similarly, although the rich Late Jurassic turtle faunas of Europe are consistently found in marine sediments (Milner 2004;Anquetin et al 2014Anquetin et al , 2015Joyce 2015), geochemical evidence, among others, strongly hints at the presence of some freshwater turtles (Billon-Bruyat et al 2005;Joyce 2015). For example, the Late Cretaceous Hell Creek Formation exposed in Montana and the Dakotas has yielded an exceptionally rich record of fossil turtles Holroyd et al 2014).…”

“…The same sediments, however, also regularly yield the nanhsiungchelyid Basilemys sinuosa, which shows clear morphological adaptations to terrestriality (Holroyd and Hutchison 2002), in particular short limbs that are covered by osteoderms, thereby highlighting the limits to this approach. Similarly, although the rich Late Jurassic turtle faunas of Europe are consistently found in marine sediments (Milner 2004;Anquetin et al 2014Anquetin et al , 2015Joyce 2015), geochemical evidence, among others, strongly hints at the presence of some freshwater turtles (Billon-Bruyat et al 2005;Joyce 2015).…”

A new pancryptodiran turtle from the Late Jurassic of Poland and palaeobiology of early marine turtles

“…In S. parsonsi , the labial and lingual ridges are lacking (Gaffney, ; Joyce, ; Parsons & Williams, ). On the other hand, all the plesiochelyids with preserved mandibles ( Plesiochelys etalloni , P. planiceps , and P. mcdowelli ), with the exception of Portlandemys gracilis (Anquetin, Püntener, & Billon‐Bruyat, ), from the Kimmeridgian of Switzerland, have a much more pronounced lingual ridge of the mandible, a relatively wide triturating surface and the symphyseal hook (Anquetin et al, ; Gaffney, ; Parsons & Williams, ). The expansion of the mandibular triturating surface in all three cases appears to be attained differently.…”

“…from Cerin, France (Broin, 1994), but no large differences are expected. The other Late Jurassic turtles with preserved mandible, Portlandemys gracilis and Eurysternum wagleri (Meyer, 1839), have shorter symphyses and, in the case of E. wagleri, a wider angle between the rami (figure 1 in Anquetin & Joyce, 2014;Anquetin et al, 2015). The lower jaws of Jurassichelon oleronensis (Pérez-García, 2015a), and Palaeomedusa testa (Meyer, 1860), are unknown, but the skulls of these taxa possess wide, rounded, and short snouts unable to accommodate an elongated and narrow mandibular symphysis (Joyce, 2003;Meyer, 1860;Rieppel, 1980).…”