Brachiopod shill limestone

Brachiopod Schillkalk is a rock associated with brachiopods, also known as arthropods or armfoots. Brachiopods are a group of marine invertebrates with two-lobed shells that feed on microorganisms. They resemble bivalves externally, but differ in the arrangement of their shells. While bivalves have a left and right valve, brachiopods have an upper and lower valve. The brachiopod till contains the fossil remains of these organisms and is therefore of paleontological interest.

The two-valved shell of brachiopods is the most striking feature of these animals. The shell is divided into a dorsal and a ventral shell, with the ventral shell, also called the peduncle valve, usually larger than the dorsal shell, also called the arm valve. Unlike bivalves, where the plane of symmetry runs along the edge of the shell, in brachiopods it runs vertically through the single valve, dividing it into two symmetrical halves. The shape of the shells can vary in different species, from biconvex shells to concave-convex shapes. In some species, the shape changes from convex-concave to biconvex during development.

Brachiopod till consists of calcified shells covered with an organic layer called periostracum. Below the periostracum are layers of calcium carbonate (calcite) and protein cuticles. In some species, a layer of prismatic calcite crystals may still lie beneath. The shells may also have fine tubules formed by protrusions of the mantle containing secretions.

The brachiopod till is of geologic and paleontologic interest because it provides information about past marine ecosystems and the evolution of brachiopods. Fossils of brachiopods are widely distributed and serve as important guide fossils for determining the age of rock strata. The largest fossil brachiopod shells can be about 30 centimeters wide. By studying brachiopod fossils in the Schillkalk, scientists can draw conclusions about the environment, climate, and ecology of the time.