| Altmühltal Formation | |
|---|---|
| Stratigraphic range:Tithonian[1][2] | |
 Outcrop of the Solnhofen Limestone | |
| Type | Geological formation |
| Underlies | Mörnsheim Formation[1] |
| Overlies | Rögling Formation[1] |
| Lithology | |
| Primary | Lithographic limestone[1] |
| Location | |
| Coordinates | 48°54′00″N11°00′00″E / 48.9000°N 11.0000°E /48.9000; 11.0000 |
| Approximate paleocoordinates | 40°06′N19°12′E / 40.1°N 19.2°E /40.1; 19.2 |
| Region | Bavaria |
| Country | Germany |
| Type section | |
| Named for | Solnhofen |
  Solnhofen Limestone (Germany) Show map of Germany Solnhofen Limestone (Bavaria) Show map of Bavaria | |
TheSolnhofen Limestone orSolnhofen Plattenkalk, formally known as theAltmühltal Formation, is aJurassicKonservat-Lagerstätte that preserves a rare assemblage offossilized organisms, including highly detailed imprints of soft bodied organisms such as sea jellies. The most familiar fossils of the Solnhofen Plattenkalk include the early featheredtheropod dinosaurArchaeopteryx preserved in such detail that they are among the most famous and most beautiful fossils in the world. The Solnhofen beds lie in theGerman state ofBavaria (Bayern), halfway betweenNuremberg (Nürnberg) andMunich (München) and were originally quarried as a source oflithographic limestone. TheJura Museum situated inEichstätt, Germany has an extensive exhibit ofJurassic fossils from the quarries of Solnhofen and surroundings, including marine reptiles,pterosaurs, and one specimen of the early birdArchaeopteryx.[3]
During theLate Jurassic, this area was anarchipelago at the edge of theTethys Sea. This included placid lagoons that had limited access to the open sea and where salinity rose high enough that the resulting brine could not support life. Since the lowest water was devoid ofoxygen, many ordinary scavengers were absent. Any organism that fell, drifted, or was washed into the lagoons from the ocean or the land became buried in softcarbonate mud. Thus, many delicate creatures avoided consumption by scavengers or being torn apart by currents. The wings ofdragonflies, the imprints of strayfeathers, and terrestrial plants that washed into the lagoons were all preserved. The fossils are not numerous, but some of them are spectacular, and their range gives a comprehensive picture of a local Jurassicecosystem.
At times, the lagoons almost dried out, exposing sticky carbonate muds that trapped insects and a few smalldinosaurs. Over 600 species have been identified, including twenty-nine kinds ofpterosaur ranging from the size of asparrow to 1.2 m (4 ft) in length.
The fine-grained texture of the mudsilt forming thelimestone from theSolnhofen area (which is composed mainly of the towns ofSolnhofen andEichstätt) is ideal for makinglithographic plates, and extensive quarrying in the 19th century revealed many fossil finds, as commemorated in the nameArchaeopteryx lithographica, all the specimens of which come from these deposits. The closest modern analogue to the Solnhofen conditions is said to beOrca Basin in the northernGulf of Mexico, though that area is much deeper than the Solnhofen lagoons.[4]
In respect of the locality being a 'first-class paleontological site displaying high-quality reference material for scientists worldwide, including all known specimens ofArchaeopteryx ', theInternational Union of Geological Sciences (IUGS) included the 'Jurassic Solnhofen-Eichstätt Archaeopteryx Serial Site' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'[5]
Significant members of the Solnhofen paleofauna includeArchaeopteryx, pterosaurs, and marine invertebrates.
Dendrites, branched dark brown to blackfractal tree-structure, are quite common in the limestone. Such formations are not plant fossil remains, butiron andmanganese oxides (psilomelane,(Ba,H2O)2Mn5O10) thatcrystallized out of solutions onbedding planes of the Solnhofen limestone. Dendrites have a chemical-mineralogical origin and are formed by adiffusion process.
