oursins en loraine

During the entire Jurassic Lorraine was covered by a warm coastal sea, part of the Tethys, being situated at that time where we find the African Sahara desert today. So close by the continent - Pangea, now starting to break apart into today's continents - there were many shoals ideal for the development of reef communities with their immense bioderversity.

The development of these reefs was starting in the Middle Jurassic, to continue into the Cretaceous. Besides corals, sea-urchins are most prominent members of these reef communities, our region being benefited with in a very particular way.

This reef zone stretched from the Swiss Jura in the South until into Great Britain, crossing the entire East and Nord of France. In Lorraine, its Middle Jurassic strata are found between the valleys of the rivers Mosel and Maas, while later in the Upper Jurassic the reefs were located a little bit more to the West, around the river Maas valley.

There are often quite good possibilities for finding sea-urchins in the quarries at both sides of the river Maas valley as well as on the fields between both valleys. A very special situation occured during the construction of the new TGV-Est highspeed railroad across our region, at some places exposing strata extremely rich in sea-urchins of an extraordinary quality seldom known before. In this way, the title "The Ocean of Sea-Urchins" is justified - at least for some parts of our region.

Following some examples of sea-urchins that can be found in Lorraine:

Acrosalenia hemicidaroides	Acrosalenia spec.	Holectypus depressus
Clypeus spec.	Clypeus plotii	Nucleolites clunicularis
Nucleolites spec.	Paracidaris florigemma	Paracidaris florigemma (needle)

Sea-urchins can be divided into two major groups: regular and irregular ones. The first group shows a radial symmetry on a vertical axis, while the soncond one has a bilatral plain of symmetry. Their mouth is always on the lower side (the ventral or "oral" side), while the position of the anus can change. On the regular sea-urchins, it is located always vertically above the mouth on the top of the shell (on the dorsal or "aboral" side) - an example being the species of the group of the Cidarides.

But its position is changing on the irregular sea-urchins. In most species, it is still situated on the dorsal side, but nevermore on its top. It has shifted laterally, sometimes layered into a narrow furrow, as can bee seen on Clypeus plotii. In a little number of species, the anus even has migrated onto the lower side, being situated now near the mouth, but always in a more lateral position - as it is the case with Holectypus depressus.

Evolution of the sea-urchins, already beginning in the Devon (Paleozoic), was starting with regular forms, as can be found in the genus Archaeocidaris. Their fossils are extremely rare and expensive. But in the Middle and Upper Jurassic only they had their first large repartition, to endure more ore less well until recently - other times of vast diversification have been the Eocene and the Miocene - Pliocene periods of the Cenozoic.

Irregular sea-urchins appear for the first time at the beginnung of the Jurassic, descending from regular species. Such transitory forms can be found in the genus Acrosalenia: Although they are still perfectly regular sea-urchins, their anus has been shifted already a very little bit away laterally from the center of their dorsal side.

Another distinguishing feature is their spination: Regular sea-urchins have two different types of spines: The big primary spines, often club-shaped and thickened, attached to the big warts on the shell, and the small and acute secondary spines. Irregular sea-urchins have lost all primary spines, being covered only by a dense carpet of secondary spines. For this reason, their shells are always much more even than those of the regular species.

After their death, spines very quickly detached from the shells, the reason why fossil sea-urchins with some of their spines still attached are so very rare.

Reef fauna did not consist of sea-urchins alone, even if they occurred at some places in immense numbers. In the following some examples of corals and other animals living together with the sea-urchins. But where are the Ammonites? Jurassic, the Great Age of Ammonites, did it pass at these places without them?

Indeed, Ammonites are rare in reef communitites. More or less, almost all of them did prefer deeper (calmer) waters and avoided the reefs. If fossils of them are being found there, they almost always are preserved very badly, fragmentary and broken. Navigation was difficult for them in the agitated shallow waters of the reef zones, frequently resulting in injuries of their shells from collisions with corals.

Mostly all Ammonites found in these zones were lost animals or empty shells having been drifted into the reefs by marine currents.

Some examples of the fauna occurring together with the sea-urchins:

Isastrea bernhardina	Calamophylliopsis flabellum	Sponges
Pholadomya ovalis	Cossmannea spec.	Belemnites
Brachiopodes	Parkinsonia spec.	rib of a saurian