Solnhofen lithographic limestone fauna

Solnhofen lithographic limestone fauna The Solnhofen lithographic limestones are Late Jurassic in age and occur beneath a large part of Bavaria, southern Germany. They consist of extremely fine-grained evenly bedded limestone, formerly quarried for use in printing. The environment in which the limestones were deposited was a series of marine lagoons restricted by barrier islands and a coral–hydrozoan barrier reef. High rates of evaporation coupled with restricted exchange of water with the open sea resulted in hypersalinity and the development of density stratification. The bottom waters of these lagoons were probably stagnant and provided an environment hostile to life. More than 600 species of organisms are preserved in the limestone. They represent a variety of different environments, including open marine, reef, lagoon, and terrestrial habitats. Most of the specimens were probably washed into the hypersaline lagoons during storms and then preserved by rapid burial as a result of suspension fallout of carbonate particles. Soft parts are often preserved, including the wing membranes of pterosaurs (flying reptiles) and the muscle tissues and intestines of fish. Many of the specimens show contraction and coiling that is probably a post-mortem feature caused by dehydration in the brine and subsequent contraction of ligaments.

The invertebrate fauna includes many planktonic (floating) forms, particularly jellyfish and the stemless crinoid Saccoma, and also nektonic (swimming) organisms such as cephalopods. Many arthropods are also present including the horseshoe crab Mesolimulus and the crayfish Mecochirus. In some instances these arthropods were still alive when they were washed into the lagoons and were sufficiently tolerant of the salinity change to survive for a short while before being preserved at the end of their trails. Many species of flying insects such as dragonflies are also present, apparently washed into the lagoons from the land by seasonal rainwater floods.

The vertebrate fauna consists mostly of fish, including sharks and rays, but also terrestrial organisms such as dinosaurs, pterosaurs, and the earliest bird, Archaeopteryx. The small theropod (carnivorous dinosaur) Compsognathus is known only from a small amount of skeletal material. Some pterosaurs, such as the small form Pterodactylus, are preserved with the outlines of the wing membranes and tail rudders, which have been of great value in their reconstruction. The most important fossils are those of Archaeopteryx, an animal that shows a mixture of bird-like and reptilian features. Although the skeleton is essentially reptilian and features a long bony tail, a theropod pelvis, hands with functional fingers, and a reptilian skull with teeth, its feathers were preserved in the fine-grained lithographic limestone, leaving no doubt as to its avian affinities; study has shown that these were flight feathers. Only six specimens have been collected since the first was found in 1861, making this the rarest and certainly most valuable fossil in the Solnhofen Lagerstätte (fossil accumulation). As the Archaeopteryx and pterosaur skeletons are generally complete, it seems unlikely that they were washed into the lagoon from the land. A more probable explanation is that they were caught in flight by storms, blown into the ocean and drowned. Sinking to the bottom they would have been rapidly covered by the fine-grained carbonates that preserved the details of their soft parts.

David K. Elliott