Nouvelles données sur Phosphatherium escuilliei (Mammalia, Proboscidea) de l'Eocène inférieur du Maroc, apports à la phylogénie des Proboscidea et des ongulés lophodontes.

We report here significant new material belonging to the oldest and most primitive known Proboscidean, Phosphatherium escuilliei Gheerbrant, Sudre & Cappetta, 1996, from the early Eocene of the Ouled Abdoun phosphatic basin, Morocco. This material permits the first reconstruction of the skull and most of upper and lower dentition of Phosphatherium escuilliei. The species, which is one of the oldest and most primitive known representatives of modern orders of ungulates, becomes one of the best known among them. Its dentition shows a noticeable dental variability, which is interpreted, at least provisionally, to be intraspecific. In addition to the individual variation, some features of Phosphatherium escuilliei such as the maxillary (infraorbital area) show a noticeable variability which is related to sexual dimorphism, as was also reported in Numidotherium. The skull of Phosphatherium escuilliei is very primitive in many respects. It is long with an elongated facial part and a narrow rostrum. The toothrow does not extend posteriorly beyond the middle of the skull. The nasals are long and located anteriorly (i.e. nasal fossa not retracted). There is no contact between the premaxilla and frontal. There is a strong postorbital constriction and a distinct postorbital process on the frontal. The zygomatic arches are noticeably expanded laterally. The sagittal and nuchal crests are strong. The alisphenoid canal is present. The external auditory meatus is open ventrally. The periotic has a primitive therian construction with the presence of a round window and a cochlear canaliculus. The hypoglossal foramen is present. The ethmoid foramen is in anterior position. The occipital is flat and lacks a median crest. The braincase is strongly compressed laterally. Some primitive features of the dentition are also noticeable: C1/c1 and P1 are retained, diastemata are smaH (upper) or absent (lower), lower central incisor is only moderately enlarged, lower premolars are simplified, and a dilambdodont ectoloph linked to the mesostyle is present. In the lower jaw, the symphysis is unfused, the horizontal ramus is narrow and the condyle is low. However, Phosphatherium escuilliei displays several strikingly advanced features, especially proboscidean and tethytherian features. P. escuilliei is actually one of those exceptionally known fossils that illustrates the initial radiation of modern placentals, at an early stage close to the generalized eutherian condylarth pattern that typifies the early Paleogene. P. escuilliei is one of the few (well) known fossil taxa that documents the emergence of a modern order of mammals at this stage. A cladistic study of 129 features of Phosphatherium escuilliei within an extended systematic framework, including most lophodont ungulate lineages and their presumed condylarth relatives (e.g., Phenacodontidae, Ocepeia), provides new insight into the intra-ordinal and supra-ordinal phylogenetic relationships of the Proboscidea. It supports the monophyly of Proboscidea and the inclusion of Phosphatherium within the order. The most significant Proboscidean synapomorphies found in Phosphatherium are: 1) the well developed zygomatic process of the maxillary which contributes significantly to the ventral border of the orbit and to the zygomatic arch; 2) the relatively large size of the pars mastoidea of the periotic; and 3) the hypoconulid in a labial position (a state unique to Proboscidea).