About the Jurassic Solnhofen Lagerstätte

(Blurb for Main Page) The Jurassic Solnhofen Limestone is known for its exquisite fossils, which show exceptional preservation of soft and delicate parts. What conditions led to the accumulation of the limestone, the inclusion of dead organisms in its layers, and the remarkable quality of fossilization?

The Solnhofen Limestone is an Upper Jurassic geological formation found in northwestern Bavaria, Germany. It is considered an important 'conservation Lagerstätten'—a deposit containing fossils with exceptional preservation (Seilacher et al., 1985). Solnhofen fossils are usually complete and may even preserve impressions of the original soft tissue. Although the fossils are not very common (Barthel et al., 1990), intensive quarrying of the Solnhofen Limestone over the centuries (Fig. 1) has brought to light thousands of beautiful specimens which have been sold to museums around the world.

The fossiliferous limestone extends over an area of ~80 x 30 km, with many specimens coming from quarries around the towns of Solnhofen and Eichstätt. The best fossils are found in planar, laterally continuous medium to very thin (30-0.5 cm) beds of very finely grained, light-colored limestone (Fig. 2). This is the stone (known as Plattenkalk) which has been quarried in the region for construction and lithography, and has even been designated by the International Commission on Geoheritage (IUGS) as a Geoheritage Stone.

It is thought that the Solnhofen Plattenkalk formed in coastal lagoons protected from the open sea by sponge and coral reefs (Barthel et al., 1990). There is some ambiguity, however, about the process of deposition of the lime mud. The limestone beds are generally interpreted as accumulating in calm water conditions, because of their fine grain size and thin, laterally extensive lamination. However, the lime mud is also interpreted as being transported into the lagoon from the open sea during storm events (Barthel et al., 1990), each bed representing a distinct event of fast sedimentation (Etter, 2002; Gerschermann et al., 2021).

Another challenge lies in explaining the reason for limited decay and excellent preservation of dead organisms, documented by most Solnhofen fossils. One suggestion is that stagnation of bottom waters in the lagoonal basins caused hypersalinity, preventing scavengers from colonizing the lagoon floor. There is, however, no clear evidence of evaporitic beds or minerals in the Solnhofen Plattenkalk, making the inference for salinity stratification essentially speculative (Barthel et al., 1990; Etter, 2002; Gerschermann et al., 2021). It has also been suggested that rapid burial in sediment (obruption), existence of bacterial films on the substrate, or even thixotropic behavior of the calcite ooze may have played a role in favoring articulated and soft-parts preservation (Seilacher et al., 1985; Gerschermann et al., 2021).

Fossils of the Solnhofen Limestone represent an unusual combination of diverse habitats within the same deposit. Marine habitats are represented mostly by echinoderms, mollusks, crustaceans, and fish. Freshwater fauna are predominantly insects. Terrestrial biota include reptiles, flying insects, and pterosaurs. There is no clear evidence (e.g., channels, terrigenous sand) for fluvial input into the lagoon, yet land animals and plant remains must have been somehow transported into the basin. Strong offshore winds have also been suggested as a possible mechanism (Barthel et al., 1990; Etter, 2002).

It is believed that most of the animals that fossilized in the Solnhofen Plattenkalk were brought into the lagoonal basins during storms, and were already dead or died shortly thereafter (Fig. 3). No living macrobenthos is thought to have been present in the basin throughout sedimentation of this limestone unit (Etter, 2002).

In summary, although the Solnhofen fossils are typically interpreted as originally deposited in a very calm setting, several factors (e.g., allochthonous provenance of the organic remains, mixture of habitats, distinctive bedding) point to the importance of episodic, high energy events for the formation of this unit. Episodic pulses of sedimentation also point to a shorter time frame for deposition of the limestone, challenging the unrealistic scenario of uniform conditions being maintained in the Solnhofen lagoons for hundreds of thousands of years (Etter, 2002).

Figure 1: Historical lithography of a Solnhofen quarry (public domain; accessed from https://de.wikipedia.org/wiki/Solnhofener_Plattenkalk#/media/Datei:Solnhofen_II.jpg)

Figure 2: Thinly bedded Plattenkalk from a Solnhofen Limestone quarry (Image credit: Adobe stock, #71416285)

Figure 3: The fossil of a horseshoe crab preserved at the end of its death track. Several examples of this type of association are known from the Solnhofen Limestone. Specimen GRI #454.

References:

Barthel K.W., Swinburne N.H.M., Conway Morris, S., 1990. Solnhofen: A Study in Mesozoic Palaeontology. Cambridge: Cambridge University Press.

Etter, W., 2002. Solnhofen: plattenkalk preservation with Archaeopteryx. In Bottjer, D.J., Bambach, R.K., Etter, W., Hagadorn, J.W., and Tang, C.M. (eds.), Exceptional Fossil Preservation: A Unique View on the Evolution of Marine Life. Columbia University Press, New York, pp. 327-352. Available at: books.google.com.

Gerschermann, S., Ballhaus C, Gäb F (2021) Rheological properties of calcite oozes: Implications for the fossilisation in the plattenkalks of the Solnhofen-Eichstätt lagoons in the Franconian Alb, Germany. PLoS ONE 16(6): e0252469. DOI: 10.1371/journal.pone.0252469.

Seilacher, A., Reif, W.E., and Westphal, F., 1985. Sedimentological, ecological and temporal patterns of fossil Lagerstätten. Philosophical Transactions of the Royal Society of London. B, Biological Sciences, 311(1148), pp.5-24. https://doi.org/10.1098/rstb.1985.0134.