Nowadays, the ruminants are the most ecologically diverse hoofed mammals of the world. All the extent of families (including related species from the Burdigalian) and feeding habits can be deduced from the mandible shape. The Tragulidae possess a small coronoid process, a shortened diastema, and a weak incisura vasorum. The Eupecora have an elongated diastema (extremely extended within the Giraffidae), the coronoid process is elongated. The Cervidae are generally more slender than the Bovidae. A trend from the massive mandible of the grazers, to the mixed feeders, the folivore, and the slender selective browsers can be observed within the Cervidae and the Bovidae. For older taxa, some doubt still exists as to their phylogeny and palaeoecology. The transitional position of the latest Oligocene-Aquitanian ruminants’ mandible shape, between the primitive Gelocus and the extent of families, suggests a basal radiation of the Eupecora without extant representatives. The mandibles of Early Oligocene and Eocene ruminants have a similar shape, retaining a primitive form close to the common ancestor.
The earliest confirmed record of ruminants in Western Europe are the German and French Gelocidae sensu stricto (Phaneromeryx gelyensis, “Gelocus” minor, and Rutitherium nouleti being unique, lost, and cannot been placed in a geological timeframe and the location of their discovery is unknown). The reassessment of the European ruminants leads to new phylogenetic attributions and an unexpected diversity during the Oligocene. Iberomeryx minor was often considered as a Lophiomerycidae. However, based on new material and new descriptions, this tiny ruminant is a Tragulidae (the shape of the p4 without mesolingual conid, huge Dorcatherium fold on lower molars, general shape of the mandible). The description of new specimens of a small Lophiomerycidae confirms the existence of “Lophiomeryx” gaudryi. Similar observations can be made for the latest Oligocene Eupecora “Amphitragulus” feningrei. A new combination is proposed for the Chattian Mosaicomeryx quercyi (Jehenne, 1987). It is the sister taxon of Prodremotherium elongatum, which are not considered as Gelocidae anymore. A new genus and species (Babameryx engesseri Mennecart, 2012) has been published. This primitive species could be at the base of the Eupecora radiation.
An exhaustive study of the European ruminants in a clear temporal framework allows a new interpretation of their biostratigraphy. The earliest undeniable European ruminants occurred only after the “Grande-Coupure” event (MP20/21), at the same age as the Oi1 glaciation event (ca. 33.5 My; part of Migrations1), whereas the first true ruminants appeared earlier in the Middle and Late Eocene in North America and Asia. Following this extinction/origination event, the earliest European ruminant families Lophiomerycidae and Gelocidae diversified rapidly. Later, at MP23, the new families Bachitheriidae and Tragulidae migrated from Asia (ca. 30.5 My; part of Migrations1). Swamps and forests dominated the European landscape, but at the beginning of MP24, a global change coinciding with the Oi2 glaciation event, occurred. In Switzerland, it was marked by the regression of the UMM (Lower Marine Molasse) and by the Renish Sea, and drastic changes of the sedimentological context that generated a drier climate and a faunal renewal. Within the European ruminant community, we note the disappearance of the Gelocidae and the Tragulidae, and only the larger species of Bachitheriidae and Lophiomerycidae survived (Extinction1).
At MP24/25 (ca. 29.2 My), the Bachitheriidae diversified and the Mosaicomerycidae appeared in Europe (Speciation1).
The European mammal evolution was relatively quiet and steady until MP27. The environment was generally wooded and the sedimentation in the Swiss Molasse was typically dominated by fluvial deposits.
Around MP28, large changes occurred in the faunal communities. The diversity of the evolution pattern of the ruminants can be precisely defined for this period, thanks to the record of numerous rich and temporally well-defined localities. First, a specific renewal resulted in ruminants more specialised for open habitats (Extinction2 and Migration2). In Pech Desse, only primitive taxa (Prodremotherium, Bachitherium, and Lophiomeryx) associated with Amphitragulus have been found. Then a rapid extinction occurred, during the emergence of new migrants, the High Ruminantia: Eupecora (Extinction3 and Migrations3). In Gaimersheim 1, only Bachitherium still survived, when Dremotherium and Babameryx arrived. In Pech du Fraysse, only Eupecora are present. In Switzerland, this short time interval coincided with sedimentological changes related to a drier environment.
The transition between the Oligocene and the Miocene (ca. 22.9 My) was marked by the Mi1 glaciation event, which coincided with a general fragmentation of the environment. In spite of this, the ruminant community was not strongly affected. However, a general trend highlights size changes; the small species derived into smaller species and the medium size species into larger ones. At the end of MN2, a huge diversity of hornless saber-toothed ruminants belonging to uncertain families is recorded (Speciation2).
Some Swiss Aquitanian localities (Engehalde, Tavannes Sous le Mont, and Wallenried) have been studied in detail.
The new excavation of Engehalde provides a partial skeleton of Dremotherium feignouxi. Moreover, a comparison with the old collections led to the conclusion that different fauna were probably linked to a small diachronism or a mosaic environment. The first late Aquitanian locality from the Central Jura Molasse provided a very rich and diversified fauna. This area proved that the Aquitanian hiatus within the Jura should be reconsidered. A new discovery in Wallenried allowed the complete diagnosis of Friburgomeryx wallenriedensis and a reassessment of the species of Andegameryx.
To conclude, comparison with previous studies on the diversity of the European ruminants highlights the inaccurate interpretation of this regional evolution. As opposed to the regional evolution which suggests few migrations, this study highlights an evolutionary pattern marked by several huge Asiatic migrations, strongly related with global and environmental changes (clearly defined in Switzerland) and punctuated by some speciation phases. Europe should be considered more as a Dead End (migration/extinction) than an area with a normal evolutionary diversification (speciation).