# Calcare di Sogno

The Calcare di Sogno ("Sogno Limestone"; also known as the Sogno Formation) is a geological formation in Italy, dated to roughly between 182-169 million years ago and covering the Lower Toarcian-Late Bajocian stagess of the Jurassic Period in the Mesozoic Era. Thalattosuchian remains are known from the formation, as well as fish and other taxa.

## Description

During the Early Jurassic, concretely towards the Toarcian, the Lombardy Basin became a relatively deep, fully pelagic area, located between the so called Lugano High, at the west, and the Trento Plateau to the east, with several troughs and palaeohighs (West to east: Monte Nudo Trough, Lugano High, Generoso Trough, Corni di Canzo High, Albenza Plateau, Monte Cavallo High, Sebino Trough and Botticino High). The formation is characterized by a disposition of regional deposition equivalent to the German Posidonia Shale, with a benthic setting and deposition trends, mostly populated by marine fauna. The environment of the formation was related to a marginal marine deposit, with probably epicontinental deposition from near land environments, being connected to the central European seas and the North African currents of the Toarcian. The formation is linked with the Toarcian Anoxic Event, that is measured in the “Fish Level”, that is also the most fossiliferous section.

## Environment

Two cores, the Colle di Sogno and Gajum are among the best sections that recovered the ecological changes in the Pliensbachian-Toarcian Lombardy Basin. Carbon-and oxygen-isotope data calibrated against nannofossil biostratigraphy has shown that the palaeobathymetry of the deposits was about 1000 and 1500 m, being the deepest records of the T-OAE in the western Tethyan region. As the Sogno Formation was deposited mostly in a pelagic setting, influenced by both the European and African bioregions, taxa of several provenances mix in this layer. The Nannofosil assemblage, that ranges from moderate/poor to good decreasing in the Toarcian AOE (drastic decrease in total abundance is observed in the Fish Level), includes the taxa Lotharingius (L. hauffii, L. sigillatus, L. crucicentralis, L. velatus), Discorhabdus ignotus, Diductius constans, Carinolithus (C. poulnabronei, C. superbus), Mitrolithus jansae and Watznaueria sp.1 in the Gajum Core, while the Sogno Core shows abundance of the genera Biscutum, Calyculus, Carinolithus and Crepidolithus, whereas Bussonius, Diductius, Similiscutum, Parhabdolithus and Tubirhabdus are extremely rare. The overall structure of this microtaxa assemblage trends to suggest a correlation with the biohorizon seen in coeval layers in the Lusitanian Basin, where a common trend is observed in the Western Tethys of north-south migration pathway for several organisms, including calcareous nannoplankton and ammonites.
A local index genus for environment evolution is Schizosphaerella spp. (specially S. punctulata), showing a lower valve size than in coeval layers on connected basins (Lusitanian and Paris Basins), as local result of the Lower Toarcian Jenkyns Event, indicating changes in ocean acidification and fertility rather than temperature.

## Fossil content
### Ichnofossils

| Genus | Species | Location | Material | Type | Origin | Notes | Images |
| Planolites | P. isp. | Colle di Sogno | Cylindrical burrows | Pascichnia | Polychaetes | Burrow-like ichnofossils referred to vermiform deposit-feeders. Sometimes considered a junior synonym of Palaeophycus. | Planolites fossil |

### Molluscs

| Genus | Species | Stratigraphic position | Material | Notes | Images |
| Bositra | B. buchii | Most of the formation | Shells | A posidoniid ostreoidan. The habitat and mode of life of Bositra has been debated for more than a century. There have been different interpretations, such as a pseudoplanktonic organism, a benthic organism living on the open marine floor, where it was the main inhabitant of the basinal settings, and a hybrid mode, where it has a life cycle with holopelagic reproduction controlled by changes in oxygen levels, and even a chemosymbiotic lifestyle, related to the large crinoid rafts, being the main "Safe conduct" to evade anoxic events. All the opinions along the years led to a large study in 1998, where the size/frequency distribution, the density of growth thanks to the lines related to the shell size and the position of the redox boundary by total organic carbon diagrams has revealed that Bositra probably had a benthic mode of life. | Thousands of specimens in one matrix |
| Collina | C. gemma | Mount Brughetto Mount Cornizzolo | Shells | A Dactylioceratidae ammonite. Present and abundant on the Mediterranean Toarcian realm. | |
| Cornaptychus | C. lythensis | Mount Brughetto Mount Cornizzolo | Shells | An indeterminate ammonite. Some of the specimens found are very fragmentary, making its identification complex. | |
| Dactylioceras | D. polymorphus | Mount Brughetto Mount Cornizzolo | Shells | Type member Dactylioceratinae family of Ammonites. A common Mediterranean genus, found in deposits across all of Europe. | |
| Harpoceras | H. sp. | Mount Brughetto Mount Cornizzolo | Shells | Type genus of the Harpoceratinae ammonite family | |
| Hildaites | H. sp. | Mount Brughetto Mount Cornizzolo | Shells | A Hildoceratidae ammonite | |
| Mesodactylites | M. sapphicus M. sp. | Mount Brughetto Mount Cornizzolo | Shells | A Nodicoeloceratinae ammonite | |

### Arthropods

| Genus | Species | Stratigraphic position | Material | Notes |
| ?Antrimpos | ?A. sp. | Mount Cornizzolo | 1 complete specimen, MSNM i10852 | A Penaeidae Decapodan. |
| Archaeopalinurus | A. cfr. A. levis | Mount Cornizzolo | Various specimens | A Palinuroidean Decapodan. |
| Coleia | C. cf.banzensis | Mount Cornizzolo | 15 specimens, complete and incomplete | An Erymidae Decapodan. |
| ?Etallonia | ?E. sp. | Mount Cornizzolo | Single Isolated chelae, MSNM i10855 | An Axiidae Decapodan. |
| Gabaleryon | G. garassinoi | Mount Brughetto Mount Cornizzolo | Various specimens | A Coleiidae Decapodan. Was confused with Proeryon hartmanni specimens. |
| Proeryon | P. hartmanni | Mount Brughetto Mount Cornizzolo | Various specimens | An Erymidae Decapodan Crustacean, common in Mediterranean formations. |
| Uncina | U. cf.posidoniae U. alpina | Mount Cornizzolo | Isolated chelae, MSNM i10851, il0863, i10864 | An Astacidean Decapodan of the family Uncinidae. A large decapodan, with sizes up to 40 cm. |

### Fish

| Genus | Species | Stratigraphic position | Material | Notes |
| Leptolepis | L. coryphaenoides L. sp. | Monte Cornizzolo | +100 specimens | Type member of the family Leptolepidae inside Leptolepiformes. It is the most abundant fish found in the formation. |
| Pachycormus | P. sp. | Monte Cornizzolo | Several specimens | The main member of the family Pachycormidae inside Pachycormiformes. Large sized fish, able to reach near 1.4 m long. |
| Pholidophorus | P. sp. | Monte Cornizzolo | Several specimens | Type member of the family Pholidophoridae inside Pachycormiformes. A small sized fish, mostly related to marine deposits, associated with various predatory behaviours, including coleoids and Crocodylomorphs. |

### Crocodyliformes

| Genus | Species | Location | Material | Notes | Images |
| cf.Pelagosaurus | cf. P. sp. | Monte Cornizzolo | Various specimens MSNM V4012, MSNM V4013. | A Thalattosuchian marine crocodylomorph of the family Teleosauridae.The specimens found were of small size, with several characters such as opened neurocentral vertebral sutures and non sutured caudal pleurapophyses, that show a possible juvenile or subadult status. | |

### Flora

Several plant leaves and fragments of wood were not identified.
| Genus | Species | Stratigraphic position | Material | Notes | Images |
| Ginkgo | G. digitata | Mount Brughetto Mount Cornizzolo | Leaves | Affinities with the Ginkgoaceae. Arboreal plants related to the modern Ginkgo species. | |
| Pagiophyllum | P. kurri | Mount Brughetto Mount Cornizzolo | Leaves | Affinities with the Cheirolepidiaceae and Araucariaceae. Arbustive to arboreal plants with several leaf morphotypes, probably from nearshore environments. | |