Mesonychians are an extinct group of primitive hoofed mammals. They have been found all over Laurasia and were well diversified: more than 20 genera are presently recorded. Mesonychia are divided into two families: Hapalodectidae and Mesonychidae. Hapalodectidae are recorded from the late Paleocene to the middle Eocene in Asia (Gashatan to Irdinmanhan), and in the early Eocene in North America (from Wasatchian to early Bridgerian). Hapalodectids remained small: the species of Hapalodectes, the type genus of the family, weighed between 500 g and 1 kg. Because the hapalodectids are relatively rare mammals, the discovery of new specimens, especially in the Paleocene, is crucial for understanding the evolution of these peculiar mammals. Field work in Qianshan Basin (Anhui Province, China) led to the discovery of a new lower jaw of the mesonychian Hapalodectes in Gashatan (late Paleocene) sediments. It is worth noting that the fragmentary mandible is only the third specimen of Hapalodectidae discovered in the Paleocene, and the first in southeast China. The premolars and molars of the new fossil are morphologically similar to Hapalodectes dux, the most primitive hapalodectid, but their relative proportions recall H. paleocenus and the Eocene Hapalodectes species. As a result, the fossil described herein appears to be different from the other previously described species of Hapalodectes in being morphologically intermediate between H. dux and the other Hapalodectes species; it is thus identified as a new species. Its discovery is important because it sheds light on the initial radiation of the hapalodectids. The presence of the most primitive hapalodectids in Mongolia (e.g., H. dux) suggests that the Mongolian area is the center of origination of this carnivorous family. The differences between the new species and the Eocene hapalodectids from China, H. huanghaiensis and H. hetangensis, imply that these species do not derive from the newly described species. Therefore, the new Chinese hapalodectid allows reconstructing the existence of two dispersals from the Mongolian area to the southeast of China, before and shortly after the Paleocene–Eocene boundary. At that latter time, Hapalodectes also dispersed from Asia to North America; this event was part of the 'East of Eden' dispersals. The Paleocene/Eocene transition thus appears as a crucial event for the distribution and radiation of the hapalodectids with the establishment of two distinct groups, respectively in North America and in the southeast of China. Grant Information This abstract is a contribution to the Belgian Bilateral Cooperation Project Belspo BL/36/C54 and China International S&T Cooperation Project MOST 2009DFA32210.
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Numerous fossil localities were investigated in western central Africa during the Belgian expeditions that started in the 19th century. At least 47 localities were excavated or analyzed in the framework of Edmond Dartevelle’s paleontological expeditions of 1933 and 1937-1938, producing a large and unique collection of Mesozoic-Cenozoic vetebrates from the margin of the Congo Basin along the coastal area of Angola to Gabon. Among them, snake vertebrae from the marine Paleocene-Eocene Landana section, Cabinda enclave, Angola have been referred to the aquatic snake Palaeophis aff. typhaeus. New investigation of the old Dartevelle’s collections has led to relocation of a few undescribed snake vertebrae from Landana and the nearby locality of Sassa Zao, permitting a revision of Palaeophis aff. typhaeus. The results of this work indicate that all specimens from Landana originate from the same stratigraphic level (layers 31-32) and are of Lutetian age based on the rich associated elasmobranch fauna. The locality of Sassa Zao is also Lutetian based on elasmobranchs that are similar to those of layer 32 of Landana. All of the vertebrae, ten in total, can be attributed to a single large species of Palaeophis. The maximum width across the prezygapophyses is 35 mm and the maximum length of the centrum is 27 mm. The weak lateral compression of trunk vertebrae, low development of the pterapophyses, diapophyses not very low, and the marked lateral projection of the zygapophyses indicate that this species belongs to the ‘primitive’ grade of Palaeophis and thus differs from species of the ‘advanced’ group such as P. casei, P. ferganicus, P. littoralis, P. toliapicus, P. typhaeus, P. grandis, P. tamdy, P. nessovi, and P. udovichenkoi. Among ‘primitive’ grade species, it differs from the giant P. colossaeus by smaller size, proportionally longer vertebrae, the cotyle and condyle more oval in shape, and the zygosphene not larger than the cotyle; from P. africanus by the neural spine that does not approach the zygosphene and shorter hypapophyses that are not prolonged by a ventral carina; from P. vastaniensis, P. virginianus, and P. zhylan by less depressed vertebrae. In size and morphology it most closely resembles P. maghrebianus but differs by more developed hypapophyses and paradiapophyses that do not extend over the cotyle posteriorly. This new species was apparently poorly adapted to aquatic life and was more closely related to the North African Ypresian P. maghrebianus than to West African Lutetian species. 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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