Resumen de Morphological and morphometric variations in middle and late pleistocene microtus arvalis and microtus agrestis populations: chronological insight, evolutionary trends and palaeoclimatic and palaeoenvironmental inferences
This PhD thesis is focused on the study of Middle and Late Pleistocene populations of the common vole Microtus arvalis and the field vole Microtus agrestis. These two species were chosen because they are widespread and consistently present in the fossil record of late Middle and Late Pleistocene (ca. 420 – 11 ka) in Europe therefore providing excellent samples for comparison between different chronologies and different geographic areas. Furthermore, due to their wide geographic distribution, their rapid evolution, and their well-documented dental patterns, they can function as chronological and biochronological markers. Also, they can be used as proxy for palaeoclimatic and palaeoenvironmental reconstructions, due to their high sensitivity to the changes of the ecosystem they live in.
In the study of fossil populations of Microtus, the first lower molars (m1s) often are the best diagnostic element. On the grinding surface, they present dentine fields surrounded by enamel strips. These fields usually assume a triangle shape (T), except for the fields at the anterior and posterior end of the tooth, called the anterior cap (AC) and the posterior lobe (PL). Based on the number of triangles (numbered 1 to 7, from PL to AC), their relative shapes and proportions, the degree of confluence or separation between them and with the AC, it is possible to assign the m1 to the appropriate species.
The m1s were analysed from a morphometric and morphological point of view. The measurements were taken following those described and vastly used in the study of fossil and extant populations of Microtinae: L=total length of the tooth; a=length of the anterior part of the tooth (anteroconid); W=width of the tooth; e=width of anteroconid; d=degree of closure of the AC; La=width of T4; Li=width of T5. Only complete m1s of adult specimens were taken into account for measurements. The morphotypes were identified following the criteria used in current paleontology and biology and based on classic literature. Based on the degree of opposition or alternation of T6 and T7; their degree of confluence or separation between them and with the AC and the shape of AC, m1s are be assigned to different morphotypes. We propose the use of a new index, Lagr/Larv, to quantify the variations in size of M. agrestis and M. arvalis in relation to each other. This index proved to be an excellent indicator of the general humidity level that characterized environmental conditions in the surrounding of a given site.
For this thesis, we studied populations of M. arvalis and M. agrestis coming from 13 sites of the Iberian Peninsula, 14 sites of the Italian Peninsula, nine sites of the Carpathian Basin, three of Croatia and two of Belgium.
The study of the morphological and morphometric variations of Microtus arvalis and M. agrestis during late Middle and Late Pleistocene allowed us: - to refine the chronological attributions of palaeontological and archaeological sites of the Mediterranean region; - to identify divergent lineages, early signs of speciation, immigration events and displacement of populations; - to detect oscillations and variations in the environmental conditions in the surroundings of a given sites.
