Paroxyclaenidae is an enigmatic archaic group of middle size placental mammals. They are known only from Europe, and are recorded from the early Eocene (Ypresian) to the middle Eocene (Bartonian). Paroxyclaenids are divided into two distinct subfamilies: Paroxyclaeninae and Merialinae. They have been variously placed by different authors in Carnivora, Creodonta, ‘Condylarthra’, and Insectivora, but are considered since 1970’s as members of Pantolesta. The dentition of paroxyclaenids is complete (4 premolars, 3 molars); it is highly specialized, with relatively enlarged posterior premolars, spaced out cheek teeth, but primitive, for instance, in the absence of hypocone on upper molars. The molars decrease in size from M1/m1 to M3/m3; the M3 and m3 are sometimes well reduced. A particularity of the dentition of some paroxyclaenids is the tendency to enlargement and molarisation of the third and fourth upper and lower premolars, generally exceeding the succeeding molars in size. We recently studied unpublished fossils from the first half of the Ypresian: these fossils originate from the French localities of Le Lien (Hérault), Pourcy, Mutigny, Avenay (Marne), and Condé-en-Brie (Aisne). They allow to describe new specimens of Merialus martinae (the oldest paroxyclaenid) and three new species – the oldest paroxyclaenine and two merialines. Their study is the opportunity to review the evolution of this family – the last extensive and comprehensive review of the paroxyclaenids has been published in 1988. The two paroxyclaenid subfamilies – Paroxyclaeninae and Merialinae – are rarely recorded together: this case only occurs in the Paris Basin during the early Eocene (Mutigny, Avenay, Condé-en-Brie). Half of the merialines are present in the Southern European Province, while the paroxyclaenines are only recorded in Northern European Province. The two subfamilies reach their maximum size (+/- 3-4 kg) (e.g., Spaniella, Kopidodon) around the early/middle Eocene boundary (47.8 Myr). However, some smaller paroxyclaenids (body mass around 1 kg) have co-existed together with the larger ones. The small middle Eocene paroxyclaenids, which are as small as the taxa found in the early Eocene, have been the last representatives of the group (Bartonian). The maximum of diversity of the Paroxyclaenids occurred during the Lutetian (middle Eocene). Finally, because the new fossils provide information on the morphology of the earliest paroxyclaenids, their study is the opportunity to question the origin of this group and its relationships among Placentalia. Grant Information: This abstract is a contribution to the project BR/121/A3/PALEURAFRICA funded by the Belgian Science Policy Office.
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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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