Some genera like Myophoria and Costatoria are useful index fossils. Also characteristic associations of genera and species that are not index fossils are suitable for a more detailed stratigraphic division.
Bivalves are very good facies indexes. While the shells of dead ceratids drifted along with currents and winds for weeks and even month (this wide and facies-independent distribution makes ceratids the better index fossils), benthic pelecypods often sedimented at or near they place where they lived. But this statement is not valid e. g. for the tempestites that can be found throughout the basin. The large tropical storms that caused the tempestites could have brought the components from a wide area. Storms are also responsible for the exhumation of remains of endobenthic organisms e. g. shallowly burrowing bivalves that later then can be found together with epibenthic organisms on one bedding plane - which gives, at first glance, a completely wrong picture about the density of organisms that lived on the sea floor.
But bivalves show a strong adaption of their shells to their preferred habitat. As especially Muschelkalk and Keuper nearly only yield steinkerns, the reliable methods of concluding from the valve's inner sides (muscle attachment points, mantle line) to the bivalve's preferred habitat are useless. But there's also a lot to learn from the general form of the valves and from the sculpture:
Marine epibenthic, lying freely or byssally attached, partially actively swimming:
Only a few bivalves lie freely on the sediment. They usually have a shell where one half is more convex than the other. Those that attach themselves with byssus threads to a hard ground often have two asymmetric "ears" on their hinge and are ventrally flattened to ensure a firm contact with the substrate. Forms that are capable of temporary active swimming usually have two slightly, but differently curved valves and symmetric "ears", enabling them to create a directed water flow. As the valves of epibenthic bivalves usually got seperated from each other by decay and water movement before getting covered by sediment, the valves are usually found isolated. Byssate forms can attach themselves to drifting wood logs, perhaps even to cephalopod shells, and use them as a transport vehicle to leave their original habitat (pseudoplanctic). They can cut their byssus threads anytime to stay at a suitable place.
Marine epibenthic, cemented
Bivalves that attach themselves to a hard ground by cementing one valve onto it always show adaptions to the specific substrate in the shape of their valves. Suitable hard grounds can be a single sand grain, other valves or cephalopod shells. The cemented valve often copies the substrate's structure, e. g. the sculpture of an ammonoid or another bivalve. The two valves are therefore often found isolated from each other in different places (the cemented one at the original living place, the free one can get drifted away by storms or currents). Their shell is very thick compared to their overall size - as they can't escape from predators or other dangers by creeping or swimming, the shell is required to offer good protection. Cemented bivalves like Placunopsis were the most important reef builders in the Muschelkalk sea. They preferred facies with little sedimentation and died when the sedimentation rate exceeded the reef growth (as the reef was the required and only available hard ground for new larvas to attach). With their contribution to the formation of hard grounds cemented bivalves also play an important role in the development of complex biocenosis, like the bivalve/echinoderm reefs of the Trochitenkalk where valves fomed the initial hard grounds that allowed crinoid growth and also later on played an important role as structure builders.
Marine endobenthic, shallowly burrowing
A large number of Muschelkalk bivalves belongs to this group. They mostly have round or heart-shaped, strongly curved valves with well-distinguished ridges. The commissure (line that separates one valve from the other) is mostly even and only rarely gaping. Examples are Myophoria and Lima. They often get exhumed by storm events which also leads to a separation of the valves.
Marine endobenthic, deeply burrowing
Deep burrowers do mostly have just sligthly ridges or none at all. They have elongated shells, especially at the back end where the siphons leave the shell they are often gaping. The valves are comparably thin. As deep burrowers can go as deep as more than 1 m below the sediment surface they are comparably safe from exhumation by storms and therefore often can be found with both valves still together in living position (the elongated back end pointing upwards). Example: Pholadomya.
Limnic, lacustrine, fluviatile, brackish
Bivalve remains in the brackish to fluviatile Keuper sediments are usually so badly preserved that it is often impossible to assign them to a genus or even a species. Fresh water bivalves generally show only little specific adaptions as their environment may change within comaparbly short times. They are characterized by a sculpture of more or less distinct growth lines without ridges or nodes. Their shell is thin in relation to their size. Most forms lived epibenthic and their valves got damaged and fragmented before they were covered by sediment. It's typical for terrestric sedimentation environments that bivalve shells get dissolved due to the little carbonate content in the sediments, before the diagenetic hardening of the rock has reached the point where the valve sculpture could have been permanently preserved. So, it's common to find only a deformed hollow space or steinkern with little details. Compared to marine ones, fresh water bivalves have a much thicker periostracum (a skin-like tissue that covers the outside of the valves and is, among other functions, responsible for shell repair) that can often be found in sediments that formed under anoxic conditions as a black coal coating on the surface of steinkerns. Unionites, a relative of the modern pond shell (Unio), is the only non-marine Keuper bivalve to occur in large masses in certain beds, mostly as an indicator for brackishness during sea transgressions over a limic facies area, or the other way round when isloated sea basins decreased in salinity and reached a brackish state for a short time.
Lower Muschelkalk, Rüdersdorf
Lower Muschelkalk, Krautheim
Upper Muschelkalk (mo 2), Héming (F)
Myophoria sp.
Upper Muschelkalk, Stuttgart
Lower Keuper (Anoplophora-Dolomit), Rielingshausen
The carbonate shell got completely dissolved, but the organic periostracum was preserved as a coal coating in the anoxic swamp sediment.
Lower Keuper, Zwingelhausen
Lower Keuper (Anthrakonit- bank), Kirchberg/Jagst
Karnian, Gorenja Vas (Slowenia)