"Rock of the Month #168, posted for June 2015" ---

Echinoderms

This month's specimen (Figure 1) is a brittle star, an ophiuroid, part of the phylum Echinodermata. This large marine grouping includes echinoids (sea urchins) and crinoids (sea lilies). The echinoderms also include stelleroids, e.g., starfish and ophiuroids such as the Jurassic Ophiurella (Black, 1972, p.140), also from Solnhofen. The diversity of physical form in echinoderms is very strong, though a five-fold symmetry is a common element, at least in most well-preserved specimens (Black, 1972; Turek et al., 1988, pp.360-415).

Jurassic strata in Bavaria

The Solnhofen limestone of Bavaria is noted for its extremely fine-grained micritic ** limestone, capable of supporting the reproduction of fine illustrations and text (Fig. 3). The purity and even texture that renders this the best-known lithographic stone also makes the Solnhofen limestone a wonderful medium for the preservation of fossils. These strata in the drainage basin of the river Danube were laid down in upper Jurassic time, some 150 million years ago. Chilingar et al. (1967) edited a volume reviewing the physical properties and other qualities of carbonate rocks from many areas, including Solnhofen and sites in the U.S.A. (Ohio, Tennessee, Pennsylvania), southern Ontario in Canada and elsewhere. The limestone is extraordinarily pure in bulk, but is also known for the development of dendrites of iron and manganese oxides across exposed surfaces (e.g., pyrolusite dendrites, in Von Straaten, 1978; Desautels, 1968, p.48; Chopard et al., 1991).

** Micrite is a term used in carbonate sedimentology, referring to a very fine-grained limestone, formed from limy mud, composed of minute crystals of calcite less than 5 microns (0.005 mm) in diameter.

The Solnhofen limestone is the most famous Lagerstätten (assemblage of superbly preserved fossils) in the world. The oldest-known fossil bird (Archaeopteryx) was discovered here in the late 19th century. Archaeopteryx represents the early phase of the very successful and long-lived aerial reign of the pterosaurs, across Jurassic and Cretaceous times, and six continents (Monastersky et al., 2001). The Teylers museum in Haarlem, Netherlands, is an historical museum of science, and a unique showcase of late 18th century museum curation, with samples and equipment, labels, and eclectic displays and display cabinets of wood and glass. The exhibits include excellent fossil fish from the Solnhofen lithographic stone. Smaller fossils include Saccocoma from Solnhofen (viewed 17 September 2014).

The very fine-grained, uniform, platy limestone (plattenkalk) formed during calcareous deposition in a set of semi-isolated basins in a late Jurassic back-reef complex (Barthel et al., 1990). The fossils include many genera that are but seldom preserved, including jellyfish, crustaceans and insects. Seilacher et al. (1985, pp.11-15) note the unusual aspects of the Solnhofen fauna, including fossil birds and fossil fish, as well as pelagic crinoids (Saccocoma, a stemless crinoid, is the most common Solnhofen fossil). had a stemless calyx and porous skeleton, adaptations to a pelagic existence (Turek et al., 1988, pp.386-387). Obrution (rapid burial) is a key feature of Lagerstätten. In this case, the Solnhofen organisms were swept into salty basins where they sank into oxygen-poor deeper layers, where few scavengers could survive, and were entombed in a limy mud. The depth of the Tithonian lagoons has been estimated at 30-60 metres.