Hyolithida is an extinct order of lophotrochozoans, one of the two of hyoliths, the other being the Orthothecida. Most information on the hyolithids comes from studies on the Hyolitha. Both orders had an operculum that was not hinged to the conch. However, the Hyolithida are distinct from the Orthothecida in having additional paired, curved, tusk-like appendages. The Hyolithida were probably bottom feeders living in shallow water, and had tentacles.
Families Known families of the Hyolithida are: Aimitidae Sysoiev, 1968 Aladracidae Geyer, 2017 Altaicornidae Sysoiev, 1970 Amydaicornidae Valkov, 1975 Angusticornidae Sysoiev, 1968 Australothecidae Malinky, 2009 Carinolithidae Marek in Valent, Fatka, Szabad & Vokáč, 2011 Crestjahitidae Sysoiev, 1968 Galicornidae Valkov, 1975 Hyolithidae Nicholson, 1872 Nelegerocornidae Meshkova, 1974 Notabilitidae Sysoiev, 1968 Pauxillitidae Marek, 1967 Pterygothecidae Sysoiev, 1958 Similothecidae Malinky, 1988 Trapezovitidae Valkov, 1975
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