Abstract
We provide detailed morphological description, including enamel microstructure, of the earliest known embrithopod mammals (Afrotheria, Paenungulata), Stylolophus minor and S. major, n. sp., recently discovered in the early Eocene of the Ouled Abdoun phosphate basin, Morocco. Stylolophus minor and S. major, n. sp., show close morphological affinity, and the enamel microstructure supports their congeneric status. Stylolophus major, which comes from an upper level (middle Ypresian) of the Ouled Abdoun phosphate series, has more derived features than S. minor in addition to a larger size. This argues that S. minor and S. major are chronospecies. This new mammal lineage recognized in the Ouled Abdoun phosphate series is characterized by a rapid size increase, as for the proboscideans and probably in correlation with early-middle Ypresian global climatic warming events. We investigated relationships of Stylolophus in a new cladistic analysis based on an extended and revised matrix that includes new enamel microstructural features studied in Stylolophus. Resulting MPTs recover 1) basal relationships of Stylolophus within the Embrithopoda; 2) sister-group relationship of the Embrithopoda to the crown Tethytheria (Proboscidea, Sirenia); and 3) the clades Tethytheria and Paenungulata. It supports in particular that the order Embrithopoda is a basal tethytherian offshoot that rapidly evolved and specialized in parallel to the splitting and early evolution of extant tethytherian orders. Stylolophus shows that the ancestral embrithopod morphotype was already well specialized in the early Eocene and quite distinctive with respect to other paenungulates (e.g., hyperdilambdodonty, lingual hypoconulid, cristid obliqua very lingual on the trigonid, small and lingual hypoconulid lobe on M3, large paranasal sinuses, concave palate), even with respect to their earliest representatives. Stylolophus minor was larger than the contemporary proboscidean Phosphatherium escuilliei. Together with the stem tethytherian relationship of the Embrithopoda, it supports an old origin of the order, at least early in the Paleocene. This relationship is also most consistent with the evolution of the embrithopod hyperdilambdodont pattern from the dilambdodont and selenodont ancestral morphotype of the paenungulates, which is known in early hyracoids but not in proboscideans and sirenians, which have reduced the ectoloph. The most specialized embrithopods evolved hyperdilambdodonty in a pseudolophodont state in parallel to the true lophodonty of crown tethytherians. The Ouled Abdoun embrithopods further help indeed to show that the early Tertiary herbivorous niches of the African island favored convergent evolution of the folivorous diet in several paenungulate and tethytherian clades. At higher scale, convergence of African and Laurasian “ungulates” is supported by enamel microstructure of Stylolophus. The early and basalmost embrithopods S. minor and S. major, n. sp., are new evidence of the Arabo-African origin of the order.