Jurassic ammonites from the Swabian Alps

jurassic ammonites found in a clay quarry (Swabian Alps)
jurassic ammonites found in a clay quarry (Swabian Alps)

Which is my favourite rock or fossil? Which is connected with a story as well? I pondered the question for a while. But I no longer own those with the best stories, like the rock I picked up while waiting to dry at least a little bit after falling into the Koenigssee when 'walking' a plank between two jetties. Without a nice story to accompany any rock or fossil, I simply settled for some special fossils despite the fact that they are incomplete and in poor condition.

Where did I find these fossils?

For many years I attended a meeting of fossil hunters organised by the youth hostel Lochen (in Baden-Wuerttemberg, Germany) around Easter. In this area the fossils are Early to Late Jurassic depending on the height of the location. Our leaders were two geological taxidermists, one of them lived nearby. He always knew where to find foundation pits and their spoils and if they would contain fossils. But they took us to quarries as well, especially the oil shale quarry near Dormettingen, the abandoned limestone quarry in a protected area near Nusplingen, a Quaternary sand pit near Mengen, and even once to Holzmaden.

Only once we have been to an obviously abandoned clay quarry near Schoemberg. The ground of the clay quarry swayed softly while we crossed it to reach a higher level. Up there the rocks looked like shale but were much softer and crumbled to 'dust' when raked with the hammer. They contained lots of ammonites - nothing special in that area - but these ammonites still had their outer shell (at least part of it), though in a poor state of conservation! Only where they had been packed tightly they were pressed flat like they would have been in oil shale. Since they had been moist for millennia we were advised to pack them in tinfoil for weeks to prevent their disintegration during desiccation. Normally you would at least clean your fossil but I didn't dare to do anything with these - and so it stayed for years until I took them out for photographs. Now I thought the tiniest one may be expendable and soaked it in water for a few hours and then cleaned it with an old toothbrush. Since it did no harm, the others were cleaned as well.

What exactly are these fossils?

The ammonites obviously belong to the family Hildoceratidae. Due to the imperfection and poor conservation state it's hard to determine the exact genus, but for the number of ribs and the profile I suppose it to be Polyplectus or Hildaitis. That they are preserved in clays indicates that they lived in a shallow marine basin not too far from the coast (clays are weathering products deposited in low-energy environment).

The rock in the clay quarry is dark grey to black, in this area a sure indicator of Lower Jurassic (the Middle Jurassic is always brown and the Upper Jurassic always whitish) but I could find no informations about the deposition time of the quarry's clay. Clay has been deposited in this area in Pliensbachian and Toarcian. I suppose the clay to be Toarcian since two species of the Hildoceratidae family are index fossils for the nearby Toarcian Thetys Ocean and Hildoceratidae generally are not especially prominent in this area in Pliensbachian. When I added up the facts correctly, the ammonites' age is about 180 Ma.

Gisela Lunkwitz