The early permian branchiosaurids (Amphibia) of Sardinia (Italy): Systematic palaeontology, palaeoecology, biostratigraphy and palaeobiogeographic problems

摘要

The branchiosaurid fauna from the Is Alinus-lake of Rio su Luda Formation (lithofacies c) in the Perdasdefogu Basin of Sardinia (Italy) includes three species: Melaneipeton eisfeldi, Apateon kontheri and Apateon flagrifer The same three species are known from Gottlob-lake in the Thuringian Forest Basin of Germany and are interpreted as valid in the sense of the "biospecies" concept, not closely related to each other and most probably reproductive in the same lake. That is a very surprising and convincing conformity of both branchiosaurid faunas: parallel evolution of three different amphibian species in two different areas seems impossible. Gene flow must have existed between the populations of the northern and southern parts of the Variscan mountain chain. Additional findings of three-dimensionally preserved but isolated amphibian bones, such as jaw fragments with pleurodent dentition, have been discovered in limestone of the Rio su Luda Formation (lithofacies d) from the Ortu Mannu section. They belong very probably to those branchiosaurids. Occurrences in both Sardinia and Thuringia require corresponding migration routes for the branchiosaurs, possibly via basins in southern France, the French Massif Central and southern Germany. Possibly all branchiosaurids of these three species are neotenic and referable to the ecomorphological "stream-type" with favourable migration capabilities, as in modem urodelans such as newts and salamanders. The Perdasdefogu Basin of Sardinia is presently the southernmost known occurrence of branchiosaurid amphibians in Europe, and its palaeo-position is of special interest. Sardinia was obviously part of a hydrographical system at the southern flank of the Variscides. No insurmountable migration barrier, such as a marine sound or seaway, existed between Sardinia and Palaeo-Europe, because the same species of obligatorily non-marine amphibians are known from these two areas. The Sardinian occurrence widens significantly the amphibian migration routes described before. On the basis of these amphibians, it is possible to correlate the Rio su Luda Formation of Sardinia with the Goldlauter to Lower Oberh of Formation of the Thuringian Forest reference section by combined use of Melanerpeton eisfeldi and Apateon flagrifer as index species of phylo(morpho)genetic lineages. In synthesis with isotopic ages, this time span is equivalent to the Asselian/Sakmarian transition of the global scale; most likely the Rio su Luda Formation correlates directly with the Upper Goldlauter Formation (Late Asselian). (c) 2007 Elsevier B.V. All rights reserved.