Echinoderms

Home

Echninoderms prefer a marine environment with a constant amount of salt and low sedimentary load. Too high sedimentation rates or sudden sedimentary events impair (clog) the ambulacral system echinoderms use for respiration and nutrition. Both necessary conditions were rarely given in the shallow, sometimes stormy and often salinar Muschelkalk sea.

Barrel-shaped ossicles of the crinoid Encrinus appear for the first time in the dolomites of the basal Lower Muschelkalk and can also be found in the Schaumkalkbänke. Due to a hypersalinar environment during the Middle Muschelkalk, crinoids where not able to establish themselves in the basin. Just with the beginning of the Upper Muschelkalk conditions changed. In the so-called Trochitenbänke of the lower Upper Muschelkalk, rocks can consist entirely of crinoid ossicles. The Trochitenbänke show the characteristic structure of a pelecypod-crinoid bioherm: A tempestite layer, mostly made up of shell fragments, is the initial hard ground that crinoids need to attach. Together with shells the holdfasts build up a stable platform that continues to grow upwards, while cavities are filled with sediment and ossicles. Finally, during a heavy storm, the entire bioherm gets covered by a mud layer that suffocates the sessile crinoids and pelecypods. The cycle starts from the beginning as soon as new tempestite layer develops.

Especially the areas around Crailsheim-Gammesfeld (Gammesfelder Barre) and Neckarwestheim are famous for the wide abundance of extremely well preserved crinoid specimens. While also many holdfasts can be found there, in other areas crinoids are far rarer, mostly incomplete, and holdfasts still attached to the hard ground are seldom found. The Gammesfelder Barre and the area of Neckarwestheim were highs within the Muschelkalk Basin, so it's likely that the crinoids preferred such locations because of a higher oxygen level, lesser sedimentary load and a constant current that simplified the filtration of organic particles. Crinoids that are ocassionally found at locations far away from the highs have probably been dislocated by tropical storms which also explains their incomplete preservation.

While within the Trochitenbänke only crinoid and shell fragments can be found, the conservation deposits that were caused by storms on top of the beds offer a far more complete image of the ancient fauna. Apart from completely preserved crinoids, including meter-long stems and still attached holdfasts, and common bivalves there are rare remains of sea urchins, esp. stings, and - as a great exception - starfishes.

Holocrinus from the Upper Muschelkalk is the first crinoid to have pentahedral ossicles instead of round ones, a feature that makes Holocrinus the ancestor of todays crinoid order Isocrinida. The limitation of Holocrinus findings to a single bed (Holocrinus-Bank) of the Upper Muschelkalk shows that this crinoid was only able to immigrate into the Germanic Basin under very special living conditions, and that it was unable to survive when conditions changed again.

Other not so rare echinoderms of the Upper Muschelkalk are brittle stars, represented by the genuses Aspidura and the larger Acruora. Aspidura has an average diameter of 0.5 to 1 cm, Acruora can grew up to 2 cm. 100s of specimens can usually be found in conservational deposits that formed during storms. Brittle stars that tried to hide from the suffocating mud are sometimes found under pelecypod shells or inside nautiloid or ceratitid shells.

The abundance of crinoid fragments lowers and lowers the more one approaches the upper boundary of the Muschelkalk, even ossicles of Encrinus liliiformis disappear. No crinoids occur in the Keuper, obviously due to the clastic sedimentation and rapidly changing salt concentration in large basin areas.

The transgression that finally lead to the re-establishment of a fully marine environment in the lowermost Jurassic started in the Upper Keuper (Rheatian). The Rhätsandstein represents a clastic delta, influenced by marine transgression events. Large brittle stars, resembling more to the later jurassic ones as to their small triassic ancestors, left their five-armed feeding burrows or resting traces Asteriacites, that sometimes cover entire bedding planes.


Encrinus liliiformis SCHLOTHEIM

Open crown with well-preserved pinnulae

Upper Muschelkalk, Crailsheim


Encrinus liliiformis SCHLOTHEIM

Partially open crowns with pinnulae visible

Upper Muschelkalk, Crailsheim


Encrinus liliiformis SCHLOTHEIM

(Crown closed for protection against sediment)

Upper Muschelkalk (Hassmersheimer Schichten), Zwingelhausen


Aspidura scutellata BLUMENBACH

Upper Muschelkalk (evolutus zone), Gundelsheim

The arms show an obvious positioning according to the current direction (from the upper right to the lower left). If the specimen was found in an undisturbed bed, it would have been possible to figure out the absolute direction of the mud flow that burried the brittle star.


Acruora squamosa PICARD

(Detail of a slab with about 50 Aspidura specimens and one Acruora)

Upper Muschelkalk (evolutus Zone), Gundelsheim


Aspidura scutellata BLUMENBACH

(Detail of a slab with about 50 Aspidura specimens and one Acruora)

Upper Muschelkalk (evolutus Zone), Gundelsheim


Brittle star resting/feeding trace Asteriacites lumbricalis SCHLOTHEIM (Negative plate)

(Detail of a 50x50 cm slab with more than 100 traces)

Upper Keuper (Rhätsandstein), Tübingen


Scale lenght, if not otherwise stated: 1 cm