Jérémy Anquetin - A new mature specimen of the Late Jurassic thalassochelydian turtle Jurassichelon oleronensis

SCIMED - Geosciences - Turtle Evolution Symposium 2024

12:13, 1 July, English, 7 views

The holotype and hitherto only known specimen of the Late Jurassic turtle Jurassichelon oleronensis figures prominently in most global phylogenetic analyses of fossil turtles scoring several thalassochelydians as terminal taxa. This is mostly due to the exquisite preservation of the material consisting of a sub-complete cranium, the anterior half of a shell, and a series of four cervical vertebrae. Here, we describe a new specimen from the type locality (headland of Chassiron, Oléron Island, France) that is very similar to the holotype in terms of preservation. It consists of a nicely preserved cranium missing only the skull roof, the associated complete right mandibular ramus, a near-complete cervical series, a partial shell, and elements of the girdles and limbs. Not only does this new specimen allow us to complete the description of Jurassichelon oleronensis, but it also documents a more advanced ontogenetic stage revealing that this species retains juvenile characters late during ontogeny. Adults of Jurassichelon oleronensis are notably characterized by the closure of costo-peripheral and lateral plastral fontanelles, a sutural contact between the carapace and plastron and along the midline of the plastron, a quadrangular nasal, the presence of a vomer-pterygoid contact, a posteroventral covering of the basioccipital by the parabasisphenoid, and a more complete ossification of the condylus mandibularis of the quadrate and labial ridge of the maxilla. In addition, the mandible is remarkable in having a low labial ridge, a strong lingual ridge, and a wide, pennate-shaped triturating surface in between.

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