STUDY OF THE PALEONTOLOGICAL RECORD OF THE BESANO FORMATION (MIDDLE TRIASSIC) AT ¿SASSO CALDO¿, VARESE, UNESCO WHL MONTE SAN GIORGIO

The Besano Formation consists of an alternation of laminated dolomitic banks and bituminous shales, and sparse cineritic tuffs that are dated as Late Anisian–Early Ladinian. It is one of the richest fossil-bearing formations from the Monte San Giorgio area; on the Italian side of Monte San Giorgio, the Sasso Caldo site is the one from which the greatest part of the Besano collection housed at the Museo di Storia Naturale di Milano is from. This Ph.D. thesis aims to the study of the Besano Formation macrofauna, through biochronostratigraphic zonation of the Sasso Caldo Site, revision of the large ichthyosaur Besanosaurus leptorhynchus), and study of the most important specimens, chosen for their preservation and rarity, but also to test the hypothesis of variations in the influence of open sea on the Besano basin. All the available ammonoids and bivalves from the Sasso Caldo site (Besano Formation), housed in the collections of the Museo di Storia Naturale di Milano, were examined and determined. The systematic study led to the recognition of 15 ammonoid taxa belonging to 10 genera, and five species belonging to the bivalve genus Daonella. The study of bed-by-bed collected specimens allowed the biochronostratigraphic classification of the Sasso Caldo section and the time-calibration of invertebrate and vertebrate bioevents. Results evidence that at Sasso Caldo site crops out almost the entire middle to upper Besano Formation. corresponding the Nevadites secedensis ammonoid zone. The trend of distribution of specimens reflects the establishment of an intraplatform basin with discontinuous open-marine influence in the middle Besano Formation, while the upper Besano Formation corresponds to a shallower subtidal restricted platform environment. Among the terrestrial taxa recovered at Sasso Caldo from the upper Besano Formation, a remarkably well-preserved fossil scorpion (BES SC 1973) is described in this thesis. This finding corroborates the hypothesized existence of a near shoreline during the deposition of the upper Besano Formation. BES SC 1973 is assigned to a new taxon gen. et sp. nov., included in the family Protobuthidae. This finding represents the first arachnid recorded from the Besano Formation, and the second genus attributed to the family Protobuthidae. This specimen is also the first reported Italian Mesozoic fossil scorpion. Regarding vertebrates, MSNM V927 and 928, a portion of the axial skeleton of a large diapsid, is attributed to Helveticosaurus zollingeri, a rare diapsid known only from the Besano Formation. This animal was recovered in association with the ammonoid Ticinites, at the base of the N. secedensis Zone, in coincidence with the establishment of the intraplatform basin of the middle Besano Formation. This specimen is the first record of skeletal remains and the second specimen assigned to the taxon in Italy. In this work the niche occupied by this animal in the Middle Triassic coastal ecosystems and its swimming style are also revised and discussed. MSNM V926, and SMNS 50010, respectively a portion of ribcage and an isolated partial forefin of a large ichthyosaur, were attributed to Cymbospondylus buchseri. MSNM V926 represents the first specimen attributed to this taxon and recovered on the Italian side of Monte San Giorgio. A great part of this thesis is dedicated to the revision of Besanosaurus leptorhynchus. The specimens studied and attributed to Besanosaurus leptorhynchus preserve a remarkably complete cranial and postcranial anatomy so that this taxon can be now accounted among the best-understood Middle Triassic ichthyosaur taxa. The revision of the skull morphology of this taxon clarified long-standing controversies regarding its cranial anatomy and the taxonomy of shastasaurids from Monte San Giorgio. The six specimens here described represent a potential ontogenetic series composed of an embryo (inside the maternal cavity of BES SC 999), likely two subadults, and four adults. They can be ordered by increasing size as follows: embryonic material of BES SC 999, PIMUZ T 4376, PIMUZ T 1895, BES SC 999, BES SC 1016, GPIT 1793/1, PIMUZ T 4847. Also, Besanosaurus resulted the largest Middle Triassic ichthyosaur taxon of the Western Tethys to date, since a full adult size is confidently estimated to be almost 8 m in PIMUZ T 4847. Besanosaurus is characterized by a long, slender, and gracile snout, representing an ecological specialization never seen before the Anisian in a large-sized diapsid. The study of the postcranial anatomy of Besanosaurus leptorhynchus is based on four specimens: PIMUZ T 4376, PIMUZ T 1895, BES SC 999, PIMUZ T 4847. The results suggest that this taxon possesses a peculiar bauplan, which in its proportions fits in between Cymbospondylus and the shastasaur-grade ichthyosaurs. Swimming capabilities of Besanosaurus leptorhynchus were tested and compared with Cymbospondylus and Mixosaurus. Among the ichthyosaurs from the Besano-Monte San Giorgio fauna (Cymbopondylus, mixosaurids, and Besanosaurus), different hunting strategies, demonstrated by different morphologies and dimensions of the rostra, as well as different body proportions and swimming styles, should have led to niche partitioning. The key phylogenetic position occupied by Besanosausurus leptorhynchus in the ichthyosaurian phylogeny was investigated: the analysis shows that this taxon represents the basalmost member of shastasaur-grade ichthyosaurs, recovered to be a paraphyletic group. Eventually is addressed a study of the embryonic material preserved in BES SC 999. We deem the material in the body cavity of BES SC 999 unambiguously embryonic and attributable to Besanosaurus leptorhynchus. Here the embryonic material is described in detail and qualitatively compared with the maternal specimen and to other known ichthyosaur prenatal specimens.