Palaeobatrachids are an extinct group of aquatic frogs. They occurred from the Late Cretaceous to the Pleistocene, only in Europe with the exception of one questionable species recorded in the late Maastrichtian Lance Formation of Wyoming and a second possible occurrence in the early Paleocene of Montana.Here, we describe about ninety isolated palaeobatrachid bones well-preserved in three-dimensions (maxillae, surangulars, vertebrae, urostyles, ilia and humeri) from the early Paleocene locality of Hainin (Belgium), which is the reference-level MP1-5 of the mammalian biochronological scale for the European Paleogene. These remains are clearly attributable to a single species of palaeobatrachid that presents the following typical characters: a surangular with a coronoid process bearing muscle scars on dorsal surface; a bicondylar sacro-urostylar articulation; an urostyle with a low neural crest and lacking transversal processes; a humerus with the humeral condyle in the alignment of the bone, epicondyles similar in size; an ilium presenting a large acetabular area, a short and posteriorly oriented pars ascendens, an elongate tuber superius, an horizontal depression on the inner surface of the iliac shaft and lacking the dorsal crest and the pars descendens; and procoelous vertebrae with typical crescent-like cotyle and condyle. The four known palaeobatrachid genera have all been recently synonymized with Palaeobatrachus for which at least seven species are now recognized. The taxon from Hainin differs from most Palaeobatrachus species in the absence of cubital fossa on the humerus, the presence of diapophyses on the first vertebra, and the maxilla that presents a higher number of tooth positions. It is thus referred to a new Palaeobatrachus species or a new genus depending of the definition of the genus Palaeobatrachus. Prior to this study, fragmentary remains of palaeobatrachids had been identified in the Campanian of France and Spain and in the Late Paleocene of France. The early Paleocene species from Hainin is therefore the earliest formally described species from Europe. This abstract is a contribution to the project BR/121/A3/PALEURAFRICA funded by the Belgian Science Policy Office.
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The Paleocene in general and in Europe in particular, is generally considered as an epoch with high endemism and few intercontinental dispersals of mammals, although some faunal interchange is known between Asia and North America, mainly from the Tiffanian-Clarkforkian boundary. Up to now, however, faunal interchange between Europe and North America during the Paleocene has almost never been demonstrated. The study of the early Paleocene fauna of Hainin (Belgium) reveals high endemism in European mammals at the end of the Danian (early Paleocene). The age of the fauna of Walbeck (Germany) is reevaluated and is likely to be Selandian (middle Paleocene), significantly older than previously suggested. Therefore, Walbeck is closer in age to Hainin than to the typical late Paleocene fauna of Cernay (France). However, the faunas from Walbeck and Cernay share many common genera that are not present in Hainin, showing a faunal turnover around the Danian-Selandian boundary in Europe, marked by the first occurrence of Plesiadapis, Arctocyon and Adunator in Walbeck, and of neoplagiaulacid multituberculates and Dissacus in Cernay. The three genera present in Walbeck are abundant and diversified in North America from the beginning of the Tiffanian, i.e., older than the expected age of Walbeck. Therefore, it is inferred that these genera dispersed from North America to Europe around the Danian–Selandian boundary, corresponding roughly to the Torrejonian–Tiffanian boundary. The absence of multituberculates in the fauna of Walbeck does not allow the drawing of definite conclusions about the moment of their dispersal, but it is likely that it happened at the same time as Plesiadapis and Arctocyon, because neoplagiaulacids are abundant and diversified during the whole Paleocene in North America. Similarly, Dissacus is very rare in Cernay, and could also have dispersed at the same time as others, but remained unnoticed because of its rarity. The Clarkforkian in North America is marked by massive arrival of taxa from Asia, among which are rodents, carnivorans, and tillodonts. The recent discovery of the latest Paleocene fauna of Rivecourt (France), where typical Paleocene taxa cohabit with rodents and a carnivoran, indicates that the large-scale dispersal event marking the Paleocene–Eocene boundary began in Europe about at the same time as in North America, with the arrival of rodents and carnivorans. The morphology of the new carnivoran species suggests that this group dispersed separately from Asia to Europe and from Asia to North America.
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