NEW INTERPRETATIONS REGARDING THE DEPOSITIONAL ENVIRONMENT OF THE LATE JURASSIC SOLNHOFEN FORMATION – EVIDENCE FROM PALYNOLOGY

The Solnhofen Formation is renowned as one of the richest fossil lagerstätten in the world. It is perhaps most famous as the source of all ten specimens of the earliest fossil bird Archaeopteryx. Solnhofen Formation has produced thousands of specimens of exquisitely preserved marine fish, reptiles, invertebrates, algae, and fewer numbers of terrestrial plants and animals. Despite repeated attempts to recover palynomorphs from the Solnhofen Formation, no reports have been published of organic-walled microfossils. Some previous reports have categorically stated that the Solnhofen is “barren” of palynomorphs as the result of extensive post-depositional alteration.

We collected samples from limestone quarries in southeastern Germany, and processed those that appeared to be least oxidised. Most were barren or “practically barren”. However, a few samples produced well-preserved palynomorphs, including acritarchs, dinoflagellates, pollen, and spores.

The climate during Solnhofen limestone deposition has been described as semi-arid, based in part on the "total absence of ferns". However, since ferns are present as microfossils, the climatic conditions were almost certainly less dry than semi-arid.

The depositional environment of Solnhofen limestones is not fully understood. Hence, a variety of depositional models have been proposed (Munnecke et al., 2008). Barthel (1978) suggested that the finely laminated beds represent “normal” deposition, and the purer and thicker limestone beds represent storm deposits (tempestites). Others have proposed that the entire sequence was built up slowly and gradually as the result of recurrent coccolithophorid blooms (de Buisonjé 1985) or direct precipitation of calcium carbonate trapped by cyanobacterial mats (Keupp 1977, 1993).

The remarkably low concentration of palynomorphs in most Solnhofen limestones suggests Barthel's hypothesis is more likely, with carbonate ooze being periodically being deposited leeward of coral reefs impacted by storms. Carbonate ooze stirred up by these storms would form a fine carbonate mud that rapidly buried organisms. The low concentration of both terrestrial and marine palynomorphs suggests rapid sedimentation. The few limestone beds with a high concentration of palynomorphs probably represent “normal” deposition.