|Drahomira - a monoplacophorans from the Silurian Period, showing paired muscle scars. Image is under CC BY of Národní Muzeum.|
These ancient organisms belong to phylum Mollusca and (similarly to snails) have a single shell composed of calcium carbonate. The shell is bilaterally symmetrical and oriented with its tip forward. This was probably the original orientation of molluskan shell. Other unusual features of monoplacophorans include for example hint of body segmentation, which is well reflected by the arrangement of paired muscle scars (see image).
|Pragamira - a Silurian monoplacophorans. Image is under CC BY of Národní Muzeum.|
Paleozoic monoplacophorans were probably substrate feeders at the sea bottom, just like their living relatives. Some genera such as Drahomira from the Silurian Period could have lived as filter feeders at shells of dead nautiloid cephalopods. Unlike recent monoplacophorans, the Paleozoic ones inhabited mainly shallow and well-oxygenated water. Fossils of Paleozoic monoplacophorans are known from Central Europe, Scandinavia , China, United States and Russia.
You can read interesting studies on fossil monoplacophorans here:
Horný Radvan J. (2005): Muscle scars, systematics and mode of life of the Silurian family Drahomiridae (Mollusca, Tergomya) - http://www.nm.cz/publikace/archiv.php?id=4&rok=61&kcislu=1%E2%8%932&f_=Zobrazit
Horný Radvan J. (2009): Patelliconus Horný, 1961 and Mytoconula gen. n. (Mollusca, Tergomya) from the Ordovician of Perunica. - http://www.nm.cz/publikace/archiv.php?id=4&rok=65&kcislu=1-2&f_=Zobrazit