06 Pavia.Pmd

Bollettino della Società Paleontologica Italiana, 45 (2-3), 2006, 217-226. Modena, 15 gennaio 2007217 Lissoceras monachum (Gemmellaro), a ghost Ammonitida of the Tethyan Bathonian Giulio PAVIA G. Pavia, Dipartimento di Scienze della Terra, via Valperga Caluso 35, I-10124 Torino (Italy); [email protected] KEY-WORDS - Systematics, Ammonitida, Lissoceras, Bathonian, Tethys. ABSTRACT - L. monachum has been frequently recorded in the Upper Bajocian and Lower Bathonian, but references in literature differ in morphological details as they are based on a juvenile and poorly-preserved holotype. In addition, the type of L. monachum comes from a bed affected by taphonomical condensation mixing fossils from Early and Middle Bathonian times, i.e. the biochronological meaning of Gemmellaro’s taxon cannot be specified with biostratigraphic criteria from the type-locality. These references are here revised and refused on the basis of two topotypes recently sampled at Monte Erice in western Sicily, which allow a precise morphological definition of L. monachum by means of architectural and sutural characteristics. The only acceptable biostratigraphic datum comes from a Lower Bathonian specimen from southern France. The differences from L. ferrifex, L. magnum, and L. ventriplanum are discussed. Finally, the suture-line of topotype PU111502 of L. monachum and those specimens from the Upper Bajocian of the Venetian Alps, here named L. aff. monachum, support the phyletic relationships between L. ferrifex and L. magnum through transitional forms of L. monachum aged for the Tethyan Early Bathonian. RIASSUNTO - [Lissoceras monachum (Gemmellaro), un Ammonitida fantasma del Batoniano Tetideo] - L’ammonite Lissoceras monachum, della famiglia medio- e tardo-giurassica Lissoceratidae, risulta ripetutamente citata nella letteratura sistematica relativa al Baiociano superiore e al Batoniano inferiore. Quasi tutte le citazioni sono però alterate dalla diversa interpretazione con cui gli autori hanno descritto il taxon; la causa è da ricercare nell’inconsistenza dell’olotipo rappresentato da un esemplare giovanile affetto da un cattivo stato di conservazione. Recenti ricerche sulla località-tipo di L. monachum, a Monte Erice in Sicilia Occidentale, hanno fornito due topotipi che rendono possibile la definizione morfologica del taxon; essi sono conservati presso il “Museo di Geologia e Paleontologia” dell’Università di Torino con numeri di catalogo PU111501 e PU111502. Il materiale topotipico, così come il tipo, proviene da uno strato che presenta forte condensazione tafonomica con mescolamento di fossili del Batoniano inferiore e medio; tale situazione stratigrafica non permette quindi di chiarire il significato biocronologico della specie. L’unico dato biostratigrafico accertato per L. monachum (Gemmellaro) si riferisce a un esemplare del Batoniano inferiore della Francia sud-orientale, mentre tutte le citazioni in letteratura sono da riferire ad altri lissoceratidi, come L. ferrifex (Zittel), L. magnum Galácz, L. ventriplanum Wendt. Nel dettaglio delle differenze architetturali e lobali, L. ferrifex presenta ventre largo e arrotondato, sezione dei giri regolarmente ovata, linea lobale con L stretto e sella laterale asimmetrica; L. ventriplanum ha parete ombelicale corta con bordo arrotondato, sezione dei giri subtriangolare, ventre stretto e appiattito, linea lobale con L largo, U2 profondo, LS tozza e asimmetrica; L. magnum differisce per le maggiori dimensioni assolute, la sezione subtriangolare allargata, l’ombelico relativamente aperto. Per quanto riguarda la collocazione cronostratigrafica, L. monachum risulta limitata al momento al Batoniano inferiore. Tuttavia le caratteristiche della linea di sutura del topotipo PU111502, comparate con quelle di esemplari del Baiociano superiore delle Alpi Venete qui determinate come L. aff. monachum, sostengono l’ipotesi di un collegamento filetico con L. ferrifex attraverso forme di transizione ferrifex-monachum da rinvenire in livelli del Baiociano superiore tetideo. Su tale traiettoria filetica potrebbe inserirsi L. magnum, verosimilmente dopo uno stadio morfologico intermedio rappresentato da L. monachum. INTRODUCTION 55), which would further enlarge the stratigraphic range up to the Kimmeridgian. However, this topic is not The macroconchiate genus Lissoceras Bayle, 1879, relevant to the aims of the present study and could among the Middle Jurassic haploceratids, is a typically only be solved after careful analysis of the Oxfordian Tethyan ammonoid; its dimorphism with the haploceratids. microconchiate Microlissoceras Sturani, 1971 was Many species of Lissoceras are commonly recorded regularly confirmed by authors. The highest taxonomic from both Bajocian and Bathonian Stages, although their biodiversity is recorded in the Mediterranean and morphologic features are far from being univocally Submediterranean provinces, although its geographic defined and shared by authors. In this respect, it is distribution is wider, ranging from the Central Tethys worth noting that Lissoceras species usually represent to many places in the Pacific Realm (see Galácz, 1980 a small fraction, if not scattered components, of the for details). Biochronologically, Lissoceras encompasses Middle Jurassic ammonite assemblages, and this a long time span, from the Early Bajocian with L. scarcity hinders definition of convincing interspecific semicostulatum to the Middle Callovian with L. differences. To be precise, lissoceratids lie in a voultense. Synonymy with the Late Jurassic systematic limbo, as the consequence of the very simple Lissoceratoides is not yet defined (Galácz, 1980, p. morphology and the apparent (more or less real) wide ISSN 0375-7633 218 Bollettino della Società Paleontologica Italiana, 45 (2-3), 2006 stratigraphic range, which would hinder any depositional system. The type-level of Lissoceras biostratigraphic use and also the delineation of any monachum corresponds to a well-defined condensed phylogenetic trend. Such a situation is not dissimilar to interval of the middle part of this pelagic unit, which is that of phylloceratids which are essentially treated just informally named “Erice formation”. for “duty of chronicle”, apart from their paleoecologic Some details are needed to frame this type-level and paleobiogeographic implications (Westermann, within the succession. The Erice Fm. on the southern 1996). It is evident that the absence of a precise slope of Monte Erice shows great variation in thickness systematic revision of lissoceratids proceeds from the and facies, mainly in its lower part (Martire & Pavia, morphological characteristics of the platycone shell, 2002, fig. 41). In the Contrada Difali and Rocce del whose simplicity in architecture and ornament Calderaro sector, the formation reaches its maximum discourages any biostratigraphic and phylogenetic thickness, up to 180 m (Fig. 1). The lower part of the speculation. On the contrary, my opinion is that a careful succession is composed of alternating grey cherty study of large samples would improve definition of limestones and greenish marls; calcareous beds mainly morphological variability among lissoceratids of the consist of wackestones to packstones, occasional same age and of subsequent stages. grainstones with glauconite and phosphate grains and Bajocian Lissoceras are widely-known from the abundant filaments, echinoderm debris and sponge monographs of Sturani (1964, 1971) and Fernández- spicules. In the upper part the lithology is represented López (1985). However, the systematics of Late by cherty limestones or by nodular limestones with Bajocian lissoceratids actually seems to be slightly more bioclasts of the same types as below - except for complicated than their conclusions. With the goal of spicules, which are replaced by radiolarians; grainstones clarifying their biodiversity, the writer is revising the frequently show undulose laminae interpreted as Bajocian specimens from different European localities. hummocky cross-stratification. Martire & Pavia (2002) Bathonian Lissoceras have been less discussed than suggest a middle-outer ramp environment which could the older congeneric forms. The most up-to-date justify features such as grain-supported texture, intense studies have been made by Galácz (1980) and Sandoval bioturbation, abundant sponge spicules and storm beds. (1986). Some species seem to occur in both stages In the context of a ramp environment, the deposition (e.g., L. ferrifex, L. psilodiscus) but morphological of the coarse grainstone to rudstone megabed (150- analyses are far from uniform, whereas the 200 cm thick) at the top of the Erice succession may biochronologic value of other species needs be interpreted as the product of a single catastrophic confirmation, such as for L. magnum and L. event, such as a tsunami. ventriplanum. Among these species, L. monachum is In the Difali-Calderaro section, three fossiliferous one of the most frequently recorded taxon, although layers are described in literature. The first, some 65 m citations can not often be compared (e.g. Sturani, 1964; above the base, corresponds to the well-known locality Fernández-López, 1985). This is due to the fact that of Fontana Difali. In the past, a huge number of mollusc its morphology and biostratigraphic significance are remains were collected from here, including the undefined as the type is represented by a poorly Aalenian ammonites recorded by Gemmellaro (1886) preserved and juvenile specimen (holotype in Pavia & and Wendt (1971) and recently

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