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Nomenclatural suitability in ammonoid classification: generic versus subgeneric status of dimorphic pairs

Pavia G.

Volumina Jurassica

2006

Vol. 4, no. 1

241-242

Dimorphism in ammonoids is fully accepted, since many decades, on the basis of both the fossil record and the biology of modern cephalopods. Anatomy, ecology, genetics support definition of coupling among living animals. Ammonoids offer only indirect evidences for dimorphic interpretation; actually the possibility of coupling macro- and microconch counterparts bases on parameters controlled by autecologic, environmental and palaeobiogeographic constraints which are in turn filtered by taphonomic factors, so that the institution or the delimitation of a dimorphic taxon never is an easy practice. Callomon (1981) summarized the conceptual and technical factors that could guide such analyses: comparison of individuals that show signs of maturity, study of assemblage variability, and consequently of the dimorphic size ratio, evaluation of the phylogenetically stepped development of dimorphism. Of course comparison of fossils of the same age is fundamental; it is worth noting that a careful taphonomic analysis is needed to avoid any mixing of heterochronous palaeobiologic entities. As often, of course, the evidence does not help in the dimorphic practice, the definition of a pair of corresponding morphs does not occur frequently in literature. Nevertheless, in Middle and Late Jurassic Ammonitida, dimorphic coupling constitutes nearly the rule. The presence of micro- and macroconchs in the same bed or set of beds let to reconstruct a dimorphic species which approaches to biospecies: cf. Westermann & Riccardi (1979) for Bajocian Otoitidae of Argentina and Pavia (1983) for Bajocian Stephanoceratidae of S France. But in most circumstances, evidences are not conclusive in favour of a single binomen, and the dimorphic counterparts are classified in different morphospecies which are arranged in separate supraspecific taxonomic levels, usually with subgeneric relationship. The subgeneric rank, though most popular, is not applied by all authors (e.g. Fernandez-Lopez 1985) who prefer to keep dimorphic counterparts distinct at generic level. Different reasons support this position. I would like to stress a single practical aspect which takes into account the nomenclatural tie of subgenera: the homonymy. In fact, if two homonym morphospecies are supposed to be dimorphs, and then have to be referred to the same genus, according to priority it is necessary to change the younger binomen. It is easy to imagine what confusion could derive, mainly if later such a homonymy becomes inconsistent. This possibility is real: within Bajocian Stephanoceratidae (Pavia 1983, p. 83) a single specific name is repeated two-times or even more in binomens which could be coupled: e.g. "Normannites" hoffmanni Westermann vs. Teloceras hoffmanni (Schmidtill & Krumbeck), "Itinsaites" helveticus Maubeuge vs. "Polystephanus" helveticus Maubeuge, and others in different ammonite families too. The separation of dimorphs at generic level would avoid this danger without decreasing the palaeobiologic sense.