Acta Zoologica Cracoviensia, 39(1) 1996
Issue:
VOL 39(1) 1996
Keywords:
ml, Arvicolidae, Rodentia, biochronology, Pleistocene, Holocene
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Evolution of the first lower molar in the endemic vole Microtus (Tyrrhenicola) henseli (Arvicolidae, Rodentia, Mammalia) from Pleistocene and Holocene localities of Sardinia and Corsica
Abstract:
The first lower molars of the endemic vole Microtus (Tyrrhenicola) henseli from caves, karst fissures, and bone breccias of eight Sardinian localities of Middle Pleistocene and Holocene age, and from one Holocene cave deposit of northern Corsica have been studied. The material, consisting of a total of 198 ml’s, has been examined by means of morphometric methods, including multivariate techniques. A further preliminary analysis of the microstructural pattern of the enamel has been also carried out, on the basis of a small number of molars. The results of these analyses are fairly congruent. The morphological analysis has demonstrated the presence of correlations between some parameters (e. g., the length of the anteroconid complex and the anterior loop, or the elongation of T6 and the pinching of the neck) that strengthen the evolutionary significance of the observed trends, even though in the evaluation of the overall evolutionary level of each population it is necessary to realize that a certain degree of independence between the characters is present (mosaic distribution). The study of enamel microstructure has demonstrated the occurrence of great intraspecific variability that should be investigated in larger subsamples, but has also identified a general trend towards reduction of the tangential layer in the trailing edges, coupled with a reduction in thickness of the whole distal enamel walls of the dental triangles. On the basis of these results, the forms from the sites studied can be arranged in two main groups: a less derived group of Pleistocene forms from six localities, characterized by small size, a simpler and somewhat shorter anteroconid complex, and a broad neck, and a more derived group, composed of latest Pleistocene to early Middle Holocene samples, that includes forms with a more complex anteroconid, a more pronounced pinching of the neck, and a tendency towards larger size. The samples from the localities of Capo Figari and Grotta Dragonara show the least derived morphologies within the group of Pleistocene localities, with molars moderately differentiated in a microtine direction as concerns the pattern of enamel microstructure, while in this group the samples from Monte San Giovanni and Bonaria display intermediate characters. In the group including the Holocene forms, with more derived morphology and a reduced layer of tangential enamel and thinner trailing edges, the sample from the Corsican site of Fontaneddu is distinct in having highly derived features coupled with a slightly less elongated anteroconid complex. The evolutionary data resulting from our study may be useful for a more detailed biochronological subdivision of the Corsica-Sardinia faunal complexes, and suggest that a marked increase in evolutionary rates should have occurred at the end of the Pleistocene, possibly due to environmental changes related to climatic factors and to the impact of human civilization on the endemic mammal community.
