The Moltrasio Formation also known as the Lombardische Kieselkalk Formation is a geological formation in Italy and Switzerland. This Formation mostly developed in the Lower or Middle Sinemurian stage of the Lower Jurassic, where on the Lombardian basin tectonic activity modified the current marine and terrestrial habitats.[2] Here it developed a series of marine-related depositional settings, represented by an outcrop of 550–600 m of grey Calcarenites and Calcilutites with chert lenses and marly interbeds, that recovers the Sedrina, Moltrasio and Domaro Formations.[2] This was mostly due to the post-Triassic crisis, that was linked locally to tectonics.[3] The Moltrasio Formation is considered a continuation of the Sedrina Limestone and the Hettangian Albenza Formation, and was probably a shallow water succession, developed on the passive margin of the westernmost Southern Alps.[3][4] It is known due to the exquisite preservation observed on the Outcrop in Osteno, where several kinds of marine biota have been recovered.[5]
| Moltrasio Formation | |
|---|---|
| Stratigraphic range: Middle Sinemurian-Lower Pliensbachian ~ | |
 Outcrop | |
| Type | Geological formation |
| Unit of | Monte Generoso Basin |
| Sub-units | Osteno Outcrop |
| Underlies | Domaro Formation |
| Overlies | Sedrina Limestone |
| Thickness | 200 m |
| Lithology | |
| Primary | Limestone |
| Location | |
| Coordinates | 46°00′N 9°06′E / 46.0°N 9.1°E |
| Approximate paleocoordinates | 33°12′N 15°36′E / 33.2°N 15.6°E |
| Region | Lombardy |
| Country | |
| Type section | |
| Named for | Moltrasio |
| Named by | Antonio Stoppani[1] |
| Year defined | 1857 |
  Moltrasio Formation (Italy)  Moltrasio Formation (Lombardy) | |
Fossils
Exceptional fossilization
Apart from the Eocene of Monte Bolca, the Sinemurian of Osteno is the only fossil deposit in Italy in which soft bodies are preserved. The Osteno site was discovered in 1964. It was recovered from a series of 6 metres (20 ft) package of fine laminated, gray, spongiolitic, micritic limestone.[6] Coroniceras bisulcatum allowed to date the outcrop as the Bucklandi zone, lower Sinemurian.[6] The outcrop is a good documentation of a particularly complete fauna and flora of the Lower Jurassic which is not exactly common in the Southern Alps.[6] The Osteno outcrop, part of the formation, is worldwide known due to the exceptional preservation of mostly marine biota, including rare fossilized components, helping to understand the ecosystems of the local Sinemurian margin of the Monte Generoso Basin.[7] The high local variety of fossils found is most likely due to unique conditions of preservation, where phosphatized soft tissues have not been observed in any fish or polychaetes, but they are common in crustaceans (33%) and also occur in a smaller percentage of teuthids (14%).[8] Soft part preservation through phosphatization in this deposit includes the muscles and branchia of Crustaceans, fish tissues, and the digestive tracts of coleoids, polychaetes, and nematodes. These fossils are interpreted as having been preserved in a stagnant, restricted basin with anoxic conditions likely within the sediment pore waters.[5]
Nematoida
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Eophasma jurasicum |
| Osteno Outcrop | A nematode, type member of the family Eophasmidae inside Desmoscolecida.[10] A rare find, since nematode fossils are absent in most Mesozoic marine rocks.[9] |
|
Annelida
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Melanoraphia maculata |
| Osteno Outcrop | A polychaete, member of the group Phyllodocemorpha with incertae sedis assignation. This polychaete was probably an open swimmer more than a subtrate dweller.[11] |
|
Crustacea
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
| Multiple specimens fairly preserved | Osteno Outcrop | A decapod, type member of the family Aegeridae. This genus is the most abundant local crustacean, and was a shrimp-like creature that was probably necrophagous.[13] |
| |
| Multiple specimens fairly preserved | Osteno Outcrop | A decapod, type member of the family Coleiidae. Probably a bottom dweller predatory crustacean.[14] |
| |
| 41 specimens, fairly preserved | Osteno Outcrop | A decapod, type member of the family Erymidae. The species was originally identified as Pustulina sinemuriana.[19] | ||
| 137 complete and fragmentary specimens | Osteno Outcrop | A decapod, type member of the family Erymidae. The species was originally identified as Pustulina sinemuriana.[19] | ||
| 81 complete and fragmentary specimens, | Osteno Outcrop | A decapod, member of the family Mecochiridae. Rather rare compared to other local crustacean genera. | ||
| Ostenosculda[20] | Ostenosculda teruzzii | Single Specimen fairly preserved | Osteno Outcrop | ||
| Multiple specimens fairly preserved | Osteno Outcrop | A decapod, member of the family Erymidae. The species was originally included in the genus Eryma as E. meyeri, although the specimens reveal morphological characters diagnostic of Palaeastacus.[19] | ||
Phlyctisoma[17] |
| 10 specimens, in a fairly good state of preservation | Osteno Outcrop | A decapod, type member of the family Erymidae. The species was originally identified as Pustulina sinemuriana.[19] | |
| 7 complete specimens, in a fairly good state | Osteno Outcrop | A decapod, member of the family Litogastroidae. Probably a bottom dweller predatory crustacean.[14] | ||
| Teruzzicheles[21] | Teruzzicheles popeyei | Various specimens fairly preserved | Osteno Outcrop | A decapod belonging to the family Polychelidae |
Xiphosurida
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Ostenolimulus[22] |
|
| Osteno Outcrop | A horseshoe crab, member of the family Limulidae. This genus represents the oldest Jurassic limulid and only the second xiphosuran known from Italy.[22] Also indicates that this family were a diverse and widespread clade during the Jurassic.[22] |
Thylacocephala
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Ostenocaris cypriformis |
| Osteno Outcrop | A thylacocephalan, type member of the family Ostenocarididae inside Conchyliocarida. Originally Interpreted as a bizarre Cirriped. Ostenocaris is the most common fossil of the formation, and the main identified thylacocephalan from the formation. In the first interpretations, the genus was shown as a filter-feeding organism, which used the cephalic sac as a burrowing organ to ensure adhesion to the substrate. Based on the presence of Coprolites associated to the genus, with abundant masses of alimentary residues (hooks of cephalopods, vertebrates, remains of Crustacea) in the stomach of these organisms, Ostenocaris cypriformis was probably a necrophagous organism, and the cephalic sac can be tentatively interpreted as being a burrowing organ employed during the search for food, or as an organ of locomotion with intrinsic motility.[25] |
|
Cephalopoda
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
| Shells | Osteno outcrop | An ammonitidan, type member of the family Arietitidae inside Psiloceratoidea. The main ammonite identified locally. | ||
| Shells | Osteno outcrop | An ammonitidan, member of the family Schlotheimiidae inside Psiloceratoidea. It was not the most abundant ammonite, but was rather common. | ||
| Shells | Osteno outcrop | An ammonitidan, member of the family Arietitidae inside Psiloceratoidea. It was less common than other ammonites, although index for some locations. | ||
| Shells | Osteno outcrop | An ammonitidan, member of the family Lytoceratidae inside Lytoceratina. It was not the most abundant ammonite, but was rather common. | ||
| Several specimens with decent preservation | Osteno outcrop | A coleoid, type member of the family Ostenoteuthidae. A unique group of coeloids, only found at this location. Coleoids with ten arms each bearing pairs of hooks, Ostenoteuthis measure between 18 and 30 cm length.[27] |
| |
| Shells | Osteno outcrop | An ammonitidan, type member of the family Oxynoticeratidae inside Ammonitida. It was not the most abundant ammonite, but was rather common. | ||
| An incomplete specimen consisting of the arm crown only | Osteno outcrop | A coleoid, member of the family Ostenoteuthidae. A unique group of coeloids, only found at this location.[27] |
|
Echinoderms
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Hispidocrinus[28] |
| Multiple ossicles | Pradalunga | An Crinoidean, member of the family Isocrininae inside Isocrinida. | |
Isocrinus[28] |
| Multiple ossicles | Ubiale - Clanezzo | An Crinoidean, member of the family Isocrininae inside Isocrinida. | |
Millericrinus[28] |
| Multiple ossicles | M. di Grone | An Crinoidean, member of the family Millericrinidae inside Millericrinida. | |
| Multiple specimens | Ubiale - Clanezzo | An ophiuridan, member of the family Ophiodermatidae inside Ophiodermatina. Extant tropical species like Ophioderma are benthic predators and scavengers that show the same short spines seen in Palaeocoma.[6] |
|
Hemichordata
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Megaderaion[29] | Megaderaion sinemuriense |
| Osteno Outcrop | An acorn worm, member of the family Harrimaniidae inside Enteropneusta. It was a large sized member of its family. This genus probably lived linked to sand bottoms, where it made burrows to hide and find food.[29] |
Chondrichthyes
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Hybodontiformes indet | Teeth | Osteno Outcrop | A shark, member of the family Hybodontiformes. | ||
Myriacanthidae[30] | Gen et sp. nov | Virtually complete skull with associated parts of the branchial skeleton | Osteno Outcrop | A chimaerid, member of the family Myriacanthidae inside Chimaeriformes. | |
Ostenoselache stenosoma | One complete articulated specimen | Osteno Outcrop | An euselachiian, type member of the family Ostenoselachidae inside Elasmobranchii. A unique genus of shark, only found in the Osteno Formation. With an elongated eel-shaped morphology, the specimens found measure at least 240 mm in length, with a neurocranium with a long, complex rostrum.[16] It has been suggested it was an animal able to produce electric shocks, probably to kill prey.[16] |
| |
Palaeospinax pinnai | A single specimen in four contiguous pieces and including parts of the dentition and squamation | Osteno Outcrop | A Shark, type member of the family Palaeospinacidae inside Synechodontiformes. Open water swimmer. | ||
Squaloraja sp. | One complete articulated specimen | Osteno Outcrop | A chimaerid, type member of the family Squalorajidae inside Squalorajoidei. This genus belongs to a lineage of ray-like chimaeras. |
|
Actinopterygii
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Cosmolepis ornatus |
| Osteno Outcrop | A primitive actinopterygiian, member of the order Palaeonisciformes. A late surviving representative of the family. |  | |
Dapedium sp. |
| Osteno Outcrop | A primitive actinopterygiian, type member of the family Dapediidae inside Holostei. Probably a Coral Environment wanderer. |  | |
Furo ("Eugnathus") sp. |
| Osteno Outcrop | A primitive actinopterygiian, type member of the family Furidae inside Ionoscopiformes. An enlongated Gar-alike taxon. |  | |
Pholidophorus cf.bechei |
| Osteno Outcrop | A primitive actinopterygiian, type member of the family Pholidophoridae inside Teleostei. Open water swimmer, probably forming schools. It is the main fish recovered on the formation. |  | |
Pholidolepis sp. |
| Osteno Outcrop | A primitive Actinopterygiian, member of the family Pholidophoridae inside Teleostei. Open water swimmer, probably forming schools. |  | |
Peripeltopleurus sp. |
| Osteno Outcrop | A primitive actinopterygiian, member of the family Peltopleuridae inside Neopterygii. Probably represents the only known Jurassic survivor of the order Peltopleuriformes |  | |
Cf.Pteroniscus sp. |
| Osteno Outcrop | A primitive actinopterygiian, member of the order Palaeonisciformes. A complicated genus know from younger-aged freshwater deposits, whose modern status is rather dubious. |
Sarcopterygii
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
Undina (Holophagus) cf. gulo |
| Osteno Outcrop | A Coelacanth, member of the family Latimeriidae inside Coelacanthiformes. Isolated 20 mm scales of coelacanths analogous to the genus Holophagus are quite frequent on the Osteno deposits, suggesting a taxon up to 75 cm long. |
|
Plants
| Genus | Species | Material | Location | Notes | Images |
|---|---|---|---|---|---|
|
| Osteno Outcrop.[33] | A coniferophytan, member of the family Cheirolepidiaceae inside Pinopsida. | ||
Equisitites bunburyanus |
| Osteno Outcrop.[33] | A Horsetail, member of the family Equisetaceae inside Equisetales. A freshwater-related plant, that grew as fast as modern bamboo on tropical settings. | ||
|
| Osteno Outcrop.[33] | A spermatopsidan, member of the family Bennettitales inside Bennettitopsida. Cycad-like flora, common on dry settings. | ||
Pachypteris cf. rhomboidalis |
| Osteno Outcrop.[33] | A tree fern, member of the family Umkomasiaceae inside Peltaspermales. Fronds of large tree ferns, related to humid ecosystems. | ||
|
| Osteno Outcrop.[33] | A coniferophytan, member of the family Araucariaceae inside Pinopsida. | ||
Williamsonia sp. |
| Osteno Outcrop.[33] | A spermatopsidan, member of the family Bennettitales inside Bennettitopsida. Cycad-like flora, common on dry settings. |
| |
Zamites sp. |
| Osteno Outcrop.[33] | A spermatopsidan, member of the family Bennettitales inside Bennettitopsida. Cycad-like flora, common on dry settings. |
|
See also
- List of fossiliferous stratigraphic units in Italy
- Calcare di Sogno, Toarcian fossiliferous formation of Lombardy
- Saltrio Formation, Sinemurian fossiliferous formation of Lombardy
- Rotzo Formation, Pliensbachian fossiliferous formation of Veneto