First data on the shell structure of the Early Cretaceous (Berriasian) rhynchonellids from Crimea

Abstract

The shell structure of the Early Cretaceous rhynchonellids from Crimea has not been previously studied. First data on the shell structure of Berriasian rhynchonellids from the family Praecyclothyrididae Makridin, 1964 from southwestern and central Crimea are presented. Sulcirhynchia semenovi (Moisseev, 1939), S. berriasensis (Lobacheva, 1980), S. gracilis (Lobacheva, 1977), Belbekella airgulensis Moisseev, 1939, B. mutabilis Lobacheva, 1983, B. minor Lobacheva, 1983 and partly Lamellaerhynchia rectimarginata (Smirnova, 1972) are studied. The shell wall of most species consists of three layers: external (finely or coarsely crystalline), fibrous, and prismatic layers. The layers are usually strongly recrystallized, especially external and prismatic layers. New fibers were formed by repeated dichotomy of ridges in different areas of the fibrous layer independently of the distance from the anterior margin.