Original paper

Ammonites, taphonomical analysis and biostratigraphy of the Aalenian (Middle Jurassic) from Capo San Vigilio (Lake Garda, N Italy)

Dietze, Volker; Cresta, Stefano; Martire, Luca; Pavia, Giulio


The Aalenian succession of Capo San Vigilio shows high historical value by the studies of palaeontologists of the 19th century, who instituted many species of molluscs, mainly ammonites, which are still used as reference for the Aalenian biochronostratigraphy. Two sections have been sampled, Monte Pomo and Capitello quarry, the former a bit more expanded. In both sections, the lithostratigraphical succession is made up of three units: the Oolite di San Vigilio Fm (Toarcian-Aalenian), Calcari a Skirroceras del Capitello Fm (latest Aalenian-Bajocian) and Rosso Ammonitico Veronese Fm (late Bajocian-Tithonian). Our research concentrates on the uppermost part of the "San Vigilio Oolite" that shows a characteristic lithology of pinkish to yellow grainstones with mineralized ooids, and on the lower part of the " Skirroceras Limestone" with pink, bioclastic, fine-grained grainstones. The palaeontological study deals with the Aalenian ammonites. Due to the evidence of stratigraphical condensation, a taphonomical analysis has been carried out in view to distinguish between resedimented fossils, coeval to the encasing bed, and reelaborated fossils firstly buried in an older sediment. The criteria of their distinction are discussed. The fossil assemblages are affected by recurrent taphonomical reelaboration, except for the scattered resedimented fossils present at the topmost Monte Pomo section. The use of "indirect biostratigraphy" made possible to build up and correlate the biochronostratigraphical successions of the two sections in which each fossil assemblage has been referred to the chronostratigraphical scale of the Aalenian Stage. In the taxonomical part some selected ammonite taxa are analyzed in detail: Spinammatoceras Schindewolf, 1964, Malladaites Linares & Sandoval, 1996, with M. sandovali n. sp., Haplopleuroceras Buckman, 1892, Zurcheria (Parazurcheria) Fernández -López, 1988, Abbasitoides Géczy, 1966, Riccardiceras Westermann, 1995, and Westermannites Dietze et al., 2001. The resedimented specimens of Haplopleuroceras subspinatum represent the first record of this species in the Mediterranean Province, and allow dating the upper beds of the Monte Pomo section into the latest Aalenian Concavum Chron, Formosum Subchron. Malladaites sandovali n. sp. resembles Spinammatoceras tenax in its general morphology and links the two genera. The publication of de Gregorio (1886a), "forgotten" in nearly all later publications, was published in January 1886, prior to vacek (1886), which was published in July 1886. However, regarding the taxa here under investigation, the taxa described by de Gregorio (1886a) are either nomina oblita or nomina dubia. de Gregorio (1886b) was published after vacek (1886) so that vacek 's species claim priority.


taphorecordsammonite taxonomytaphonomybiostratigraphyaalenian stage