First occurrence of an ophthalmosaurid ichthyosaur in the Upper Jurassic of the Northern Apennines (Marche, Central Italy): stratigraphic setting

An ophthalmosaurid ichthyosaur was discovered in Upper Jurassic deposits of the Umbria-Marche sedimentary succession near Genga (Ancona, Marche, Italy). While other Mesozoic marine reptiles have been found at different stratigraphic intervals in Italy, this specimen represents the first ichthyosaur ever recorded from the Upper Jurassic of the Apennines. It consists of an almost complete skeleton preserved on a slab, with an articulated vertebral column, and disarticulated skull and pectoral girdle. This paper describes the stratigraphy of the ichthyosaur-bearing deposits. The Umbria-Marche sedimentary succession is characterized by Meso-Cenozoic pelagites and hemipelagites overlying Lower Jurassic shallow-water carbonates (Calcare Massiccio Fm.). The rifting stage, which affected the Western Tethys in the Early Jurassic, produced a complex submarine architecture, resulting in a mosaic of variable facies and thickness differences in the syn- and post-rift succession. Extension linked with opening of the Liguria - Piedmont Ocean dismembered and drowned a huge carbonate platform (Calcare Massiccio paleoplatform), and converted it into a system with small horsts-and-grabens/semigrabens, where pelagic deposits capped the shallow-water carbonates. The original Early Jurassic paleobathymetric differences were levelled by Early Cretaceous times. The basin-fill deposits onlapped the (mostly pre-rift) Calcare Massiccio facies, exposed at the footwall of Jurassic faults in the form of paleoescarpments, and buried the structural highs while their margins were largely inactive. The area in which the ichthyosaur was found was characterized by numerous structural highs (Mt. Murano, Mt. Revellone, Mt. Scoccioni and Mt. Valmontagnola), onlapped by the Jurassic - Early Cretaceous basin-fill units. In the Middle Jurassic, a huge block (about 0,7 km longer axis) of Calcare Massiccio - characterizing the Il Sassone sector - collapsed from the western escarpment of the Mt. Scoccioni or from the Mt. Revellone high, due to tectonic/gravitative instability. The olistolith was embedded in the “Calcari e marne a Posidonia” Fm. (late Toarcian-?late Bajocian), and subsequently was onlapped by the “Calcari Diasprigni” (?late Bajocian-early Kimmeridgian) and “Calcari ad aptici e Saccocoma” (Kimmeridgian p.p.-early Tithonian) Fms. Additional stratigraphic evidence for tectonic instability across the study area is found in the Middle Jurassic with gravity flow deposits, slumps and breccias bearing Calcare Massiccio clasts. The specimen was discovered in the Upper Jurassic deposits onlapping the Il Sassone mega-olistolith. It was embedded in green and reddish limestones and cherty-limestones referable to the “Calcari ad aptici e Saccocoma” Fm. The faunal assemblage includes belemnites (Duvalia spp.), aptychi (Laevaptychus sp., ?Laevilamellaptychus sp.), rhyncholites (Leptocheilus sp.), shark teeth (hexanchiforms), trace fossils (Thalassinoides sp., Chondrites sp.), radiolarians, calcisphaerulids and abundant Saccocoma tenella. These elements, coupled with the absence of S. vernioryi, Chitinoidella spp. and calpionellids, constrain the age of the ichthyosaur to the late Kimmeridgian - earliest Tithonian.