Platychelone emarginata Dollo, 1909 is a large turtle from the Late Cretaceous (Maastrichtian) chalk sediments of Limburg, Belgium. Hitherto, only the name was given to this turtle without describing details or providing figures. A single well articulated carapace (IRScNB. Reg. 1681), lacking nuchal, peripherals, and pygal plates, is preserved. The distance from the first costal to the distal end of the eighth costal is 180 cm, indicating that the original carapace was about 210 cm long. Its gigantic size, flattened shell, reduction of distal half of costals, and loss of scute sulcus, indicate that Platychelone is a member of true marine turtles (superfamily Chelonioidea). Neurals are rectangular shape and inclination of the first thoracic vertebra is almost vertical, suggesting this turtle belongs to either Protostegidae or Dermochelyidae. Seventh and eighth costals are medially meeting due to the loss of neurals; this condition is shared with the genus Mesodermochelys from the Late Cretaceous (Santonian to Maastrichtian) of Japan. Thus, it seems most probable that Platychelone is a closest relative of Mesodermochelys among basal dermochelyids. Platychelone has presumed autoapomorphic characters such as very thickened distal ends of thoracic ribs and irregular sculptures on carapace, not seen in any other chelonioids. This genus is only known by the holotype, whereas Allopleuron hoffmanni, a very common cheloniid marine turtle from in the Maastrichtian deposits of Belgium and Netherland, is known from some hundred specimens. So far, there is no ancestral or related taxon of Platychelone from the Campanian deposits of Belgium. The occurrence of Platychelone is very rare but evokes a high taxonomic diversity of gigantic chelonioids in the Cretaceous Tethys.
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RBINS Staff Publications 2017
