bulletin de l'institut royal des sciences naturelles de belgique sciences de la terre, 74-suppl.: 165-175, 2004 bulletin van het koninklijk belgisch instituut voor natuurwetenschappen aardwetenschappen, 74-suppl.: 165-175, 2004 Primitive Equoid and Tapiroid mammals: keys for interpreting the Ypresian-Lutetian transition in Belgium by Thierry SMITH, Björn DE WILDE & Etienne STEURBAUT Smith, T., De Wilde, B. & Steurbaut, E., 2004. — Primitive Equoid and Tapiroid mammals: keys for interpreting the Ypresian-Lutetian transition in Belgium. Bulletin de l'Institut royal des Sciences natur¬ elles de Belgique, Sciences de la Terre, 74 Supplement: 165-175, 1 pl., 5 figs., Bruxelles-Brussel, December 15, 2004 - ISSN 0374- 6291. Abstract Terrestrial mammal teeth, pertaining to the primitive horses Hallensia louisi and Propachynolophus levei and the tapiroid Lophiodon remen- sis, have been discovered at the base of the marine Lede Formation in the Oosterzele and Balegem sandpits, 10 km southeast of Gent. Ac- cording to its calcareous nannofossil NP 15 dating, the basai Lede Formation in that area belongs to the Middle Lutetian. Comparison with in situ records in the Paris Basin suggests a latest Ypresian origin for the mammals (reference-level MP10) and consequently reworking. The Oosterzele and Balegem specimens most probably originate from érosion of the continental Aalterbrugge Lignitic Horizon (uppermost Ypresian), which is cropping out nearby. The additional reworked faunal and lithological components at the base of the Lede Formation suggest that also the Aalter Formation (top NP13-base NP14) and the overlying Brussel Formation (NP14), or certain parts of these, were deposited but subsequently eroded in that area. The depositional his- tory of the Lede Formation, resulting from the interplay of tectonic uplift and eustatic sea-level changes, is detailed. Key words: Mammals, Perissodactyla, reworking, Ypresian-Lutetian transition, Belgium. Résumé Des dents de mammifères, appartenant aux chevaux primitifs Hallensia louisi et Propachynolophus levei ainsi qu'au tapiroïde Lophiodon remensis, ont été découvertes à la base de la Formation de Lede dans les sablières d'Oosterzele et Balegem, situées à 10 km au sud-est de Gand. L'analyse des nannofossiles calcaires indique que la base de la Formation de Lede dans cette région appartient à la biozone NP15 et est donc d'âge Lutetien moyen. La comparaison des dents de mammi¬ fères avec des spécimens in situ du Bassin de Paris suggère que celles- ci sont remaniées de l'Yprésien terminal (niveau-repère MP10). Les spécimens mammaliens d'Oosterzele et Balegem sont donc très proba¬ blement originaires de l'érosion de l'Horizon continental lignifère d'Aalterbrugge (Yprésien terminal) qui affleure dans la région. Les autres composants fauniques et lithologiques remaniés à la base de la Formation de Lede suggèrent que la Formation de Aalter (NP13 sommital-NP14 basai) ainsi que la Formation de Bruxelles sus-jacente (NP14), ou certaines parties de celles-ci, ont été déposées et érodées par la suite dans cette région. L'histoire du dépôt de la Formation de Lede, résultant de l'interaction des mouvements tectoniques et des changements eustatiques du niveau de la mer, est discutée. Mots-clefs: Mammalia, Perissodactyla, remaniement, transition yprésienne-lutétienne, Belgique. Introduction Terrestrial mammal remains have been rarely preserved in the Early Palaeogene of Belgium because of its essen- tially marine content and érosion of continental strata during subséquent marine transgressions. The famous early Selandian Hainin mammal fauna (Godfriaux & Thaler, 1972; Steurbaut, 1998; Smith & Smith, 2003) and the earliest Eocene Dormaal mammal fauna (Teilhard de Chardin, 1927; Smith & Smith, 1996; Steurbaut et al., 1999; Smith, 2000), which figure among the internationally accepted mammal reference levels (the so-called MP-levels), are the only exceptions, being buried at the edges of the basin. Discoveries of terrestrial mammal remains in marine settings are extre- mely important in light of the very incomplete mammal record of these environments and the potential of précisé dating of the mammal finds. Some of these, even isolated teeth, have allowed exact positioning of poorly dated Continental vertebrate sites (e.g. the classic sites from the Paris Basin) into well-calibrated stratigraphie frame- works and the integrated time scale of Berggren et al. (1995) (Hooker, 1996; Smith & Smith, 2003). Isolated cheek teeth of middle-sized and large terres¬ trial mammals have recently been discovered at the base of the marine Middle Eocene Lede Formation in two sandpits south of Gent, East-Flanders (Fig. 1). The ma- jority of the material (9 teeth and 3 tooth fragments) was collected in the Scheurbroek sand quarry at Oosterzele (map-sheet 22/6; x == 111.950; y = 181.075). It was found in a badly sorted 50-cm thick basai interval, together with sandstone pebbles, lamniform and carchariniform shark teeth, Pristis rostral teeth, terebratuloid brachiopods, iso¬ lated valves of the bivalve Megacardita planicosta and very rare crocodile and turtle remains. One complete premolar was recovered from the classic Verlee quarry at Balegem (map-sheet 22/6; x = 110.800, y = 179.100). This quarry is now almost completely filled in. A new quarry has recently been excavated a few hundred meters southward.