Two species behind the same mask The palaeontological approach caught out by two European species of Trivia Mollusca, Gastropoda Deux especes sous un meme masque Le point de vue paleontologique piege par les coquilles de deux especes europeennes de Trivia Mollusca, Gastropoda

Dommergues, E.; Dommergues, J.; Dommergues, C.H.gues

Revue de Paleobiologie 252: 775-790

2006


ISSN/ISBN: 0253-6730
Accession: 038859877

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Abstract
The experiment reported in this paper was devised to assess to what extent loss of information as a result of fossilization might affect species discrimination in palaeontology. The test consisted in first conducting a "palaeontological-style" study of a set of recent mollusc shells and then reviewing those results in the light of our biological (neontological) knowledge of the group. The study species and their geographical range were selected (1) so as to provide a plentiful set of finely preserved shells collected in a clearly defined spatio-temporal framework and (2) to allow "manipulation" of information usually lost during fossilization. Trivia arctica and T. monacha, two closely related extant species of marine gastropods living in the infratidal zone of the European and North-African coasts, meet these requirements perfectly. The two species can be readily and conclusively identified from numerous biological traits. In particular, T. monacha shells can be easily recognized by three obvious dark coloured spots on their dorsal side whereas T. arctica shells display no such spots. The study was based on a set of 957 shells collected from 19 separate sectors along coasts from Norway to Sicily. Shell morphology was quantified by a Fourier-type analysis (DFT, Discrete Fourier Transform) of the dorsal, lateral and apical contours of each shell. Ribbing density was also considered. The "palaeontological-style" study concentrated exclusively on features usually available on a well preserved fossil shell. For example, the coloured pattern of the shell (e.g. dark spots) was deliberately ignored. The results obtained did not indicate the existence of separate sympatric species although the wide range and/or complex structure of variability was locally difficult to interpret. Even so, a clinal trend from the Norwegian Sea to the Mediterranean Sea was apparent. This trend was most obvious for ribbing density and less pronounced for shell morphology. The neontological study was based on the same set of 957 shells but the species T. arctica and T. monacha were distinguished by the absence vs. presence of dark dorsal spots. Initial identification of the species revealed that T arctica and T. monacha both display a broadly similar clinal variation but that the pattern is more obvious for T. arctica than for T. monacha. So, although the two species contribute to overall disparity in different proportions, the discrepancy is nevertheless insufficient to infer the existence of two separate sympatric species unless the coloured patterns are taken into account. In the "palaeontological-style" study, the cline observed is a coherent but composite image, masking the true underlying biodiversity.