GENERAL CONCLUSION

Biostratigraphic Implications

Before the discovery of the fossiliferous locality of Le Quesnoy, our knowledge of the European Oxyaenidae was poor and restricted to few complete or fragmentary teeth. The Le Quesnoy material increases significantly our knowledge of the first European oxyaenids, and their systematics and evolution.

The oxyaenids from Le Quesnoy are Oxyaena woutersi and Palaeonictis gigantea. They recall taxa from the Paleocene/Eocene transition of North America. They belong to species previously known in Dormaal, although slight size differences do exist between the two samples.

The presence of almost the same oxyaenid fauna at a stage of evolution similar to that of Dormaal supports the placement of Le Quesnoy very close to MP7. The larger size of the specimens from Le Quesnoy might be explained by a slightly younger age with respect to Dormaal.

O. woutersi departs from Dipsalidictis by its more advanced carnassial shearing adaptation and from Oxyaena gulo by its less secant M₁. It seems intermediate between two Oxyaeninae from the early Wasatchian of North America, D. transiens (Wa0-Wa2) and O. gulo (Wa2-Wa3). Palaeonictis gigantea is less derived than the Wasatchian P. occidentalis (Wa1-Wa4) and appears closer to the latest Clarkforkian P. peloria (Cf3) and P. wingi (Wa0). However, P. gigantea and P. wingi are of same size and are probably equivalent in age.

The correlation of MP7 and Wa0 proposed by Smith and Smith (2001) is supported by the oxyaenids from Le Quesnoy fauna.

Le Quesnoy has yielded numerous specimens of the oldest European Oxyaeninae (Oxyaena woutersi). The species is more primitive than those from the "Conglomérat de Meudon", Sinceny, Pourcy and Abbey Wood.

Oxyaenidae are known in Dormaal (reference-level locality of MP7), but are unknown in Avenay (reference-level locality of MP8+9). Meudon, Sinceny and Abbey Wood are considered presently as MP8+9 localities. However, it seems that the carnivorous mammal faunas changed between Dormaal (MP7) and Mutigny (MP8+9), because no oxyaenid is known in Europe after Mutigny. This turnover is notably characterized by the disappearance of the Oxyaenidae. As Meudon, Abbey Wood and Sinceny are considered as older than Mutigny (Hooker 1998), they may probably be considered as MP7 localities. Moreover, Oxyaenidae could be considered as a characteristic group of MP7 level.

Paleoecologic Implications

As indicated above, the MP7-MP8+9 transition is marked by the disappearance from Europe of the Oxyaenidae, but also by that of the largest hyaenodontid genera (Arfia Van Valen, 1965; Galecyon Gingerich and Deutsch, 1989; and Prototomus Cope, 1874; North American and partly European 'proviverrine' genera sensu Morlo and Gunnell [2003]); the two groups are well diversified in North America during Early and Middle Eocene. This decrease of diversity of Early Eocene carnivorous mammals from Europe is coincident with the disappearance of Coryphodon from Europe.

Only the smallest hyaenodontids, which belong to the 'True' Proviverrinae sensu Morlo and Gunnell (2003), and miacids span the MP7-MP8+9 transition in Europe. The 'True' Proviverrinae probably filled the larger body-size carnivorous niches in Europe after the disappearance of Oxyaenidae, because they represented the main group of carnivorous mammals in Europe during the Middle Eocene (Morlo 1999). The decrease of diversity is unknown for carnivorous mammals in North America and probably had significant implications for the ecology of European mammal faunas.

Chew (2009) has recently exemplified the existence of biotic turnover events in the Early Eocene mammalian fauna of the Willwood Formation (Wyoming, United States) during the earliest part of Early Eocene (Wa3-Wa5) (55-54 Ma). Chew (2009) identified increasing disappearance rates followed by a diversity crash and a subsequent faunal reorganization. This extinction event coincided with a climate cooling (decrease in MAT of 5-8 °C) between PETM and EECO evidenced by Wing et al. (1999). However, this cooling has not been presently recorded presently in Europe.

The age of the fossiliferous locality of Mutigny (MP8+9) is considered to be 54-52 m.y. (Neal 1996; Duprat 1997). This age is younger than the faunal turnover exemplified by Chew (2009). The faunal turnover, which affected the Early Eocene carnivorous mammals from Europe, and which is marked by the disappearance of Prototomus, Galecyon, Arfia and Oxyaenidae, could represent in Europe the faunal response to the global cooling which succeeded the PETM. Moreover, the differences observed between North American and European carnivorous faunas could be explained by the fact that Europe mostly consisted of several isolated islands during the Early Eocene. Thus, the small size of the ecosystems was probably defavorable to the mammalian faunas when the climate has changed during the beginning of the the Early Eocene. However, these are preliminary results, which must be confronted with those obtained for the other groups of mammals.

Paleobiogeographic Implications

The North American origin of the Le Quesnoy oxyaenids is well supported. The European taxa are rooted in Clarkforkian oxyaenines and ambloctonines. The dispersal via Artic Canada and Greenland (e.g., McKenna 1975) is supported by the discovery of Palaeonictis sp. from the Eureka Sound Group (Early Eocene, Ellesmere Island, Nunavut) (Eberle and McKenna 2002).

A revision of the European Oxyaenidae presently known supports a single dispersal followed by endemic and convergent evolution. However, European Ambloctoninae and Oxyaeninae evolution and diversification is much more limited than in North America.

The Oxyaenidae are present in Northern Europe, but absent in Southern Europe, supporting a North-South provincialism (Marandat 1997).

As in North America, the oxyaenids represent the largest specialized carnivorous mammals in the Early Eocene of Europe. They are distinctly larger than the hyaenodontids and "miacids". However, as for the other carnivorous mammals, European Oxyaenidae remain smaller than the North American ones.