干旱区生态环境知识资源中心

All squamate reptiles are ectothermic animals whose body temperature is connected to external one. They lack internal mechanism able to produce heat and their vital functions more or less depend on the environment temperature too. Morever, they are fundamentally oviparous animals. The success of embryonic development depends to climatic conditions (mainly temperature). Their dependence on climatic factors for the preservation of vital functions and for reproductive success gives to this group significance as climatic marker. In this way, the study of squamate faunas present in Western Europe from the Middle Pliocene to the Early Pleistocene, represents a good example of contribution to the knowledge of past climates.

The archaeo-palaeontological sites studed in this work, are well as their ages, are indicated in the French version (fig. 1 and 2).

Squamate faunas: During the first half of the Middle Pliocene (Late Ruscinian, Biozone MN15 and MN15/MN16 boundary), at least 13 squamate families are present (tab. 1). The fauna is similar to that from the Miocene, although becoming slightly impoverished but still retaining a tropical character, as shown by the presence of Varanidae, Aniliidae s.l. and Elapidae. Today, these families inhabit tropical areas and their northern boundary is essentially situated between the 20 degrees C and 25 degrees C mean annual temperature isotherms (fig. 3). The great majority of modern representatives of these families basically live in and or semi-desert regions. Nevertheless, such an arid character is somewhat mitigate in the South-East of France by the presence of some amphibian species confined to humid and aquatic environments. During the second half of the Middle Pliocene (Early Vyllanian, Biozone MN 16), the squamate fauna becomes more impoverished (only 9 families, fig. 4)) and more thermophilous families disappear (tab. 2). Only families that are still currently present in the Mediterranean area and whose northern boundary is situated between the 15 degrees and 20 degrees C mean annual temperature isotherms persist (fig. 3). The arid character of the climatic seems to be increasing. During the Late Pliocene (Late Vyllanian, MN17, tab. 3, fig. 4), scolecophidian snakes, some Gekkonidae different from the current species, the Scincid Mabuya, and oriental vipers no longer live in the region, whereas other thermophilous groups (Agamidae, Blanidae and some Colubridae) are affected by a southward withdrawal. At the same time, some species coming from Central Europe arrive in France (fig. 4). Apparently, the establishment of the present fauna take place during the Early Pleistocene (tab. 4 and fig. 4). At that time, exotic family (Agamidae) is no longer present and the last Ophisaurns (Anguidae) has found refuge, before final local disappearance, in the South of the Iberian Peninsula (Barranco Leon 5 and Fuente Nueva 3).

Climatic data (fig. 6): During the first half of the Middle Pliocene, some families with unquestionable tropical tendencies are well represented. The climate may still have hold some tropical character (mean annual temperature > 20 degrees C with a poor contrasting seasonality) and aridity. During the second half of the Middle Pliocene, the families with basically tropical affinities are no longer represented. This faunal scarcity seems to point out a significant climatic degradation, with a decrease of the temperature (mean annual temperature < 20 degrees C) and the establishment of climate with contrasted seasons (probably of Mediterranean type). At the same time, the aridity seems to increase. During the Late Pliocene, the drop in temperature becomes more pronounced and some Centro-European species arrive in France. The mean annual temperature do as not seem to exceed 15 degrees C at Montousse 5, 16 degrees C at Vallirana and 18 degrees C at Casablanca-Almenara 3. The Early Pleistocene seems to begin with cooler climatic conditions. The disappearance of the last Agamidae corresponds to a fall of the mean annual temperature below 15 degrees C.

Biogeographical affinities : We made a Principal Components Analysis (PCA) (fig. 5) from the number of determined species for each period and from the number of species currently present in the various biogeographical regions of the Western Paleartic. It shows that the Western European squamate fauna has acquired a modern character only since the Late Pliocene (about 2,6 Ma).