NEW EXCAVATIONS IN THE CASSINA BEDS (MONTE SAN GIORGIO, MIDDLE TRIASSIC)

PRELIMINARY REPORTS


Silvio Renesto*, Cristina Lombardo** & Rudolf Stockar***


*Dipartimento di Biologia Strutturale e Funzionale, Università degli Studi dell'Insubria, Via Durant 3, 21100, Varese, Italy

**Dipartimento di Scienze della Terra "A. Desio" - Università degli Studi di Milano, via Mangiagalli, I-20133, Milano, Italy

***Museo Cantonale di Storia Naturale, Vi aC. Cattaneo 4, Lugano, Swizerland

 

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In 2006 the Museo Cantonale di Storia Naturale (Lugano) began a new excavation at Monte San Giorgio (UNESCO WHL), in order to redescribe bed by bed on a surface of about 40 m² the so-called Cassina Beds (Lover Meride Limestone, Early Ladinian). In the new site the fossiliferous beds represent an almost 3m. thick interval of mainly interbedded finely laminated, organic-rich shales and limestone with intercalated thicker bituminuos micritic and marly limestones.


Normal graded calcarenites, shoving erosional surfaces and bearing scattered clearly reworked fossil fragments, suggest the instability of the basin margin and occasional influence of turbidity currents. Volcaniclastic layers (tuffs and bentonites) are frequent througout the section. These new excavation brought to light an interesting vertebrate fauna.

In the Cassina beds both neopterygians, as Archaeosemionotus and Eosemionotus, and paleopterygian, as Saurichthys and Peltopleurus are represented.

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The medium sized Arcaeosemionotusis characterized by the mosaic-like covering of the cheek and well-developed dentition, probabily adapted to an hemi-durophagous diet

 

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Eosemionotus, with its typical thin and elongate teeth on premaxillary and dentary bones and the small fins with large fulcra, is reported for the first time in these levels

 

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Peltopleurus is widely represented throughout the different levels of Monte San Giorgio, with a surprising intra- and interspecific variability

 

The most abundant findings, however, belong to the large predator actinopterygian fish Saurichthys. Many complete and well preserved specimens (mainly S.curionii, but S.macrocephalus is also present). Among yet prepared specimens, four specimens of Saurichthys contained several small skulls of thiny specimens that can confidently be considered as embryos.

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In one of the specimens each skull of the embryos is associated to a narrow and elongate, curled cylinder which are segmented and made of phosphate. Structure and chemical composition suggest that the narrow cylinders may represent some post cranial structure of the embryos. The size and length of these structures, with respect of the associate skulls, is compatible with that of the musculature of the embryos and each segment could correspond to a somite.

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If this interpretation is correct, this represents the first case of preservation of soft tissues of embryos of Saurichthys.



I diritti dell'articolo, delle foto e delle illustrazioni sono del © Museo Cantonale di Storia Naturale di Lugano - Si ringrazia Rudolf Stockar e il Museo Cantonale di Storia Naturale di Lugano per averci concesso la pubblicazione di questo studio preliminare

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