Middle Triassic Ammonoid Fauna from the Recoaro and Tretto Areas (NE Italy) and Its Stratigraphic and Paleobiogeographic Evidence
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TO L O N O G E I L C A A P I ' T A A T L E I I A Bollettino della Società Paleontologica Italiana, 57 (3), 2018, 217-250. Modena C N O A S S. P. I. Middle Triassic ammonoid fauna from the Recoaro and Tretto areas (NE Italy) and its stratigraphic and paleobiogeographic evidence Paolo Mietto, Stefano Manfrin & Manuel Rigo P. Mietto, Dipartimento di Geoscienze, Via Gradenigo 6, I-35131 Padova, Italy; [email protected] S. Manfrin, Dipartimento di Geoscienze, Via Gradenigo 6, I-35131 Padova, Italy. M. Rigo, Dipartimento di Geoscienze, Via Gradenigo 6, I-35131 Padova, Italy; [email protected] “Questo lavoro sul Triassico del Recoarese, argomento che è stato alla base di molte nostre comuni ricerche, è dedicato a Vittorio De Zanche, indimenticabile Maestro e Amico che troppo prematuramente il destino ha voluto sottrarre alla famiglia e alla comunità scientifica” “This paper on the Triassic of the Recoaro Area, which was the starting topic of our common research, is dedicated to Vittorio De Zanche, unforgettable Maestro and Friend, who was prematurely stolen by destiny from his family and scientific community” KEY WORDS - Anisian, Ladinian, Recoaro area, Ammonoids, Upper Muschelkalk, Paleobiogeography. ABSTRACT - Between the mid 19th and the beginning of the 20th centuries, several Austrian and German geologists such as Beyrich, Mojsisovics and Tornquist, studied the geology of the Recoaro area (NE Italy). They described important ammonoid faunas, the majority being collected from a stratigraphic unit known as the Nodosus Formation. Among these faunas are problematic species that have been the focus of long discussion both from a biostratigraphic and taxonomic point of view. Unfortunately, most of the historical material has been either destroyed or lost, and recent studies in the field provided only a few scarce specimens thus making taxonomic revision difficult. One of the unresolved issues is related to the possible occurrence of the ceratitids peculiar to the Upper Muschelkalk of the Central Europa (Germanic) Basin and the Sephardic Triassic province among the Alpine ammonoids. An unexpected discovery of an old ammonoid collection stored at the Museo Geologico e Paleontologico of Padova University (MGP-PD) permitted confirmation of the presence of a distinctive ammonoid species Alloceratites tornquisti (Philippi) within the Triassic Recoaro domain. Two other species of Alloceratites have also been found in the Val Gola section near Trento. These occurrences are significant taxonomically, biostratigraphically and in relation to Middle Triassic sequence stratigraphy. RIASSUNTO - [Le faune ad ammonoidi medio triassici di Recoaro e Tretto (Italia nord orientale) ed il loro significato stratigrafico e paleobiogeografico] - Fra la seconda metà del 1800 e gli inizi del secolo successivo, vari geologi di lingua tedesca (fra i quali Beyrich, Mojsisovics e Tornquist) studiarono la geologia del Recoarese, descrivendo fra l’altro importanti faune ad ammonoidi, quasi totalmente provenienti da una unità stratigrafica nota come Formazione a Nodosus. Si tratta di un insieme di specie assai problematico, costituito prevalentemente da forme di difficile collocazione tassonomica e biostratigrafica. Purtroppo il materiale storico andò quasi completamente distrutto o disperso, rendendo così impossibile una completa revisione tassonomica; le ricerche condotte in tempi recenti nel Recoarese, peraltro, hanno fornito solo scarsi elementi faunistici di confronto. Uno dei problemi aperti riguardava la possibile presenza, fra le forme alpine, di ceratitidi caratteristici del Muschelkalk superiore dei Bacini dell’Europa Centrale (bacini Germanico e Sefarditico). Tuttavia, il ritrovamento inaspettato di una piccola e antica collezione di ammonoidi presso il Museo Geologico e Paleontologico dell’Università di Padova ha permesso di confermare la presenza nel Triassico del Recoarese di una specie tipica del Muschelkalk superiore, Alloceratites tornquisti (Philippi), presenza che assume particolare significato non solo dal punto di vista tassonomico ma anche biostratigrafico oltre che in termini di stratigrafia sequenziale, anche perché confortato dal rinvenimento di altri ceratitidi riferibili al genereAlloceratites nella sezione della Val Gola presso Trento. INTRODUCTION in particular with his monograph about the Triassic of the Recoaro and Tretto areas (Tornquist, 1901c). Successively, The Triassic of the Recoaro area (NE Italy) has been other authors, among which Fabiani (1920a, b), Rosenberg studied by many paleontologists since the 19th century (1959, 1961), De Boer (1963), Epting et al. (1976), (Arduino, 1769; Buch, 1848; Schauroth, 1855a, b, 1859; Mostler (1976), Barbieri et al. (1977), further described Benecke, 1866, 1868; Beyrich, 1875a, b; Mojsisovics, the geology of the whole Recoaro area. A deep revision 1876; Bittner, 1879a, b, 1880, 1881a, b, 1883, 1884; of the stratigraphy and geology of this area is referable to Tommasi, 1904; Trevisan, 1932; Selli, 1938), because Barbieri et al. (1980, 1982), De Zanche & Mietto (1981) of the large amount of fossils yielded by the Anisian and Gianolla et al. (1998), who also described in detail (Middle Triassic) stratigraphic units, such as the Gracilis the Triassic volcanism of the Recoaro area. Formation, the Voltzia beds, and the Recoaro Limestone. Beyrich (1875a) first described the occurrence The first relevant studies on the Triassic stratigraphy of this of ammonoids in a basinal stratigraphic unit of the area were conducted by Tornquist (1898b, 1899, 1900c), Recoaro area, originally considered equivalent to ISSN 0375-7633 doi:10.4435/BSPI.2018.16 218 Bollettino della Società Paleontologica Italiana, 57 (3), 2018 the “Buchensteiner Schichten” of the Balaton and originally described in the Recoarese area by Tornquist Dolomites. However, Mojsisovics (1882) was the (1898a) as Ceratites sp. ind. aff. nodosus Bruguiere. For first author who described these ammonoid faunas, these two species documented both in the Recoarese area establishing new and biostratigraphically important and in the Upper Muschelkalk, Wenger (1957) established taxa such as the ammonoid “Trachyceras recubariense” the subgenus Ceratites (Austroceratites), which was and “Trachyceras margaritosum”. Later, Tornquist accepted as a real genus by Tozer (1980b). Nevertheless, (1896, 1898a) described new ammonoids from the same Urlichs (1997) considered dubious Austroceratites stratigraphic unit, apparently recognizing the ammonoid thuringiacus and also the presence of Austroceratites (now species Ceratites nodosus (Bruguiere), peculiar of the Alloceratites) in the Germanic Basin. Germanic Muschelkalk, along with a not well defined Along with these debatable taxa, Tornquist (1898b) species, probably related to the “nodose Ceratiten” of the described new problematic species, most of which came Germanic Basin. It has to be mentioned that at this time from the Tretto area. Unfortunately, the whole collection only three valid species of Ceratites were known from the by Tornquist stored at the Strasbourg Museum was Germanic Upper Muschelkalk, C. nodosus (Bruguiere, destroyed by fire on the 8th February 1967. 1792), C. semipartitus (Buch, 1849), and C. enodis Also the holotype of Trachyceras recubariense by (Quenstedt, 1845). For this reason, many authors used the Mojsisovics (1882: tav. VII, fig. 1), which was deposited name C. nodosus for almost all Ceratites species that were in the Museum of the Berlin University, has been named in subsequent years following Philippi (1901b) and lost. Fortunately, the type specimens of Trachyceras Riedel (1916). Since this finding, the basinal stratigraphic margaritosum and Arpadites trettensis, described by unit of Recoaro has been named “Nodosus Schichten” Mojsisovics (1882), and stored at the Geologische or Nodosus Formation. The Illyrian to Longobardian Bundesanstalt of Wien (GBA) are still available. age of this formation has been constrained by conodont We invested many years of work on the revision biostratigraphy (Mietto & Petroni, 1979, 1980). of the Anisian ammonoid faunas from Recoaro. We The unexpected record of Upper Muschelkalk looked for fossil localities, but we also tried to identify ammonoids in the Alps arose bewilderments among specimens from this area in the collections of several other contemporary research workers (Arthaber, 1896a, museum in Berlin, Königsberg, Jerusalem, and Strasbourg. b; Diener, 1900; Philippi, 1901a), including Tornquist Only few specimens belonging to species described by (1900a, b, 1901a, b), who referred his specimens to the Mojsisovics and Tornquist were found. Unexpectedly, ammonoid species Ammonites subnodosus Münster, 1831 a small collection of ammonoids from the Tretto area and Ammonites nodosus Bruguiere, after a comparison was recovered at the Museo Geologico e Paleontologico between his ammonoid collection from the Recoaro dell’Università di Padova (MGP-PD). The name of area and similar specimens from the Romania and the collector and the year of collection are not known, Sardinia terrains (Tornquist 1900a, b, 1901a, b). In fact, probably this collection was deposited in the Museum Tornquist (1898a, b) classified the ammonoid specimens at the beginning of the 20th century. A well-preserved he collected from the Recoaro and Germanic basins as specimen of “ammoniti nodose” from the Recoaro area Ceratites subnodosus (Mstr) Tornq. and Ceratites sp. ind. was identified among this fauna. This specimen, referred aff.nodosus Bruguiere, respectively. During