Neoichnology of rare biogenic structure Koilosoma (Hitchcock 1858) in fluvial sediments of the Abhar-rud River, Zanjan Province, NW Iran

Two polygonal biogenic sedimentary rare structures of Koilosoma (Hitchcock 1858) have been found due to neoichnology of the Abhar-rud river fluvial sediments، northwest Iran. One of them is polygon holes، with about one square meter area. Polygons are same in size and depth، and are hexagonal in shape. They have distinct boundaries like the honeycomb. This sample is like the reported specimens from United States of America and may be referred to tadpole nests or holes. Other specimen is similar to later in shape، arrangement and size، but has been found with its trace-makers، Nemacheilus sp. Cobitidae fishes. All fish had same arrangement in holes، heading upstream current and each hole was occupied by one fish. It seems that holes are feeding structures، and produced during axial undulatory movements by fish school.