Small is beautiful: the Erquelinnes mammal fauna from the earliest Eocene of the Southern Mons Basin, Belgium

In 1880, the early Eocene fluvial deposits of the Erquelinnes sand quarry in the southern part of the Mons basin in Belgium yielded their first mammal fossil, a well preserved jaw of a primitive perissodactyl. By 1927, about 40 mammal specimens had been recovered from Erquelinnes and were attributed to Adapisorex, ‘Protomomys’, Paramys, Plesiadapis, Arctocyonides, Hyracotherium, Coryphodon and ‘Oxyaena or Miacidae’. By that time however, the Erquelinnes fauna had already been eclipsed by the contemporaneous Dormaal fauna from northeastern Belgium, which yielded thousands of specimens rather than only a few dozen. Since then, attention for the Erquelinnes fauna has therefore been limited to the passing mentions of referred specimens in the formal descriptions of the new plesiadapiform Platychoerops georgei and of the miacid Gracilocyon solei. Here we present an updated faunal list of the complete Erquelinnes mammal fauna. We show that also hyaenodontids, mesonychids, hyopsodontids, and dichobunid artiodactyls are present in the Erquelinnes fauna, and some of the earlier identifications are corrected or detailed further. This update of the Erquelinnes mammal fauna almost doubles its diversity, and strengthens the correlation with the earliest Eocene (MP7) Dormaal reference fauna. Results from δ13Corg analysis of the mammal-bearing level at Erquelinnes and the strata immediately below it, seem to independently support the faunal correlation. Faunal differences between Erquelinnes and Dormaal are most likely the result of subtle differences in depositional environments and thus in taphonomic bias. Details of the stratigraphic origin of the Erquelinnes perissodactyl specimens however show that these are derived from two distinct stratigraphic levels, which potentially have significantly different age correlations