Un nouveau gisement karstique polyphasé miocène moyen de Four (Isère) : étude des Cricetidae (Mammalia, Rodentia) et description de Democricetodon fourensis nov. sp.

Abstract

Karstic fill at a new locality called “Four” has been brought to light in Bajocian limestone in the La Grive-St-Alban area. Thirty eight species were found. The faunal composition is very similar to the other Miocene localities of the region (Guérin et Mein, 1971). Some turtles (one Emydidae and one Testudinidae) and birds (Galliformes) have been identified but the majority of the species are mammals. Amongst Insectivora, Galerix socialis is the most abundant and a Desmanella is present though it has not been identified at the other localities of the La Grive-St-Alban area. The Lagomorpha group shows a great diversity with three species. The rodents are both abundant and show significant diversity. Amongst the rodents, the cricetid family is particularly abundant with six genus: Democricetodon Fahlbusch 1964, Megacricetodon Fahlbush 1964, Fahlbuschia Mein et Freudenthal 1971, Cricetodon Lartet 1851, Lartetomys Mein et Freudenthal 1971 et Eumyarion Thaler 1966. One of the Democricetodon species is a new (Democricetodon fourensis nov. sp.) and could have a common ancestor with Democricetodon gaillardi from Sansan. Thus Democricetodon fourensis nov. sp. may result from differential evolution of Democricetodon gaillardi in the La Grive-St-Alban area. Only one tooth of a new Lartetomys, with a specific morphology, has been collected showing the largest size of cricetids ever recorded in the Miocene. Fahlbuschia cf. larteti is a little different compared to other Fahlbuschia from the La Grive-St-Alban localities. Except for Megacricetodon gersii-lemartineli, the genus Megacricetodon is similar to others found on this area. The large mammals are rare with only two Carnivora and four Artiodactyla. One of the Carnivora is a new Ailuridae. The Artiodactyla show no differences compared to others from this area. The faunal list is given in the table 1. The particularity of this karstic fill is that it is layered; four red clay beds with fossils are separated by three yellow siliceous clay beds devoid of fossils. The red clay beds are named n1 to n4 and the yellow siliceous clay beds i1 to i3. Initial sampling was done without separatig the different beds, but after the sampling they were separated according to the different coloured clay fills. The genus Fahlbuschia, Democricetodon brevis and D. aff. freisingensis are only present on the lower red clay bed (n1). The new species (Democricetodon fourensis nov. sp.) can be found in all the red clay beds. Megacricetodon is present with one species in the n1 bed (Megacricetodon gersii-lemartineli), with three other species in the other red clay beds (Megacricetodon minor, Megacricetodon gregarius and Megacricetodon sp.). Democricetodon fourensis nov. sp. which is the only cricetid species found in all red beds shows an evolution in the size of its teeth. The size of this species increases from the n2 fill to the n3-n4. The size increase of this new species is indicated by a Mann-Whitney test on the molar measurements (p<0.2). These variations in size and faunal composition suggest a different age for each red clay beds, indicating that this karstic locality is a true polyphased filling. Indeed, the yellow siliceous clay levels can be interpreted as periods of decantation between two periods of active filling demonstrating that there is no weatharing. F. cf. larteti found in the n1 red clay bed shows more primitive morphology than F. larteti (La Grive-St-Alban) known in MN7-8 zone. Thus the n1 bed can be attributed to the upper MN6 or lower MN7-8 zone whereas the other upper levels (n2 to n4), with M. gregarius and M. minor, belong to the MN7-8 zone. Three phases of filling can be individualised in the locality : a lower filling (n1) with the exclusive presence of F. cf. larteti, D. aff. freisingensis, D. brevis and M. gersii-lemartineli; and a medium (n2) and a upper filling (n3-n4) separated by the size evolution of Democricetodon fourensis nov. sp. Differences in the faunal composition suggest temporal but also environmental variations. The lack of Fahlbuschia in the medium and upper levels could be attributed to an increase in humidity (Freudenthal et Mein, 1989). This episode between Fahlbuschia and Democricetodon (fig. 1) can be correlated with the grow of the turtle population on the n2 to n3-n4 beds, supporting the idea of an environmental change through increasing humidity. The presence of Desmanella in the karstic fill can also be attributed to a more humid environment than in the other localities of the region. In fact, a stratified locality like “Four” is a way of observing the effects of general climatic changes on a same biotope over a short period time.

Key words: Middle Miocene, Layered karstic locality, Environmental variations Cricetidae, New species, Dating