CORDIS - EU research results

Mathematical morphometry and continental stratigraphy: new methods to improve fossil rodents biochronology

Final Activity Report Summary - MMCS:IFRB (Mathematical morphometry and continental stratigraphy: new methods to improve fossil rodents biochronology)

Fossil small mammals, and some rodents in particular, are very important for the chronological attribution of the sediments they are found within, and for their correlation with past environmental and climatic conditions. The identification and systematics of fossil rodents is based on molars, whose morphology changes within the different species and during their evolution. In many cases, a simple observation and a qualitative approach are not enough to distinguish the different species of rodents, while a higher degree of precision is required for biochronological reconstructions. Mathematical approaches have been therefore investigated in order to analyse the shape variation of rodents' teeth and correlate such a variation with their evolution.

First of all, a cubic spline, i.e. a sequence of third degree polynomial equations, approach was used to represent the outline of the lateral boundary between enamel and dentine of molars, called linea sinuosa in rodents of the arvicolidae family. This method was then simplified to a series of Cartesian coordinates that represented the sequence of peaks and troughs, in which a linea sinuosa could be summarised.

The external contour of the masticatory surface, i.e. the occlusal contour, of first lower molars of the arvicolid genus ogmodontomys, which had a complex shape as those of all arvicolid rodents, was subsequently measured. 13 different measurements were taken on each specimen and were analysed with complex, multivariate, statistical tests. Multivariate tests were also used to analyse the harmonics obtained via decomposing the occlusal outline of the teeth in a Fourier series, by means of dedicated software. It was therefore possible to identify a complex pattern of evolutionary trends and a speciation event that gave rise to the evolved species o. poaphagus from the archaic o. sawrockensis.

These two species were also studied using the novel, at least for this field, approach of geometric morphometrics, which was a complex mathematical tool that acquired the coordinates of point on a contour and was able to compare them either statistically, using multivariate analyses, or visually, returning a deformation grid of all the analysed specimens. In this way, it was possible to correlate the morphological variation of arvicolid first lower molars with some climatic parameters, either in the fossil record, in the North American genus ogmodontomys, or with extant species, such as the Mediterranean species terricola savii.

Moreover, a study of the new rodents' findings in the site of Pirro Nord was elaborated. This rodent fauna was very important, since the recent discovery of archaic lithic artefacts in the karst fissures, which also contained a large vertebrate fauna that, thanks to the rodents, was approximately dated between 1.3 and 1.7 million years ago, rendered Pirro Nord the site of the oldest occurrence of human representatives in Europe.