semilunus

Semilunus is a genus of fossil cephalopods belonging to the order Nautilida. These extinct marine mollusks are characterized by their distinctive, crescent-shaped shells. The genus was first described by German paleontologist Friedrich von Ammon in 1897. Semilunus fossils are primarily found in the Jurassic period, with specimens discovered in various locations across Europe, including Germany, France, and the United Kingdom.

The shell of Semilunus is typically involute, meaning that the whorls are coiled so tightly that they cover the earlier whorls. The outer lip of the shell often exhibits a pronounced sinus, contributing to its crescent or semi-lunar appearance. This shape is thought to have been an adaptation for swimming or maneuvering in its ancient marine environment. The precise function of this morphology is still a subject of scientific study.

While Semilunus shares some characteristics with modern Nautilus species, it is distinguished by its unique shell form and specific geological age. The discovery and study of Semilunus fossils provide valuable insights into the evolution of cephalopods and the marine ecosystems of the Jurassic period. Classification within Nautilida places it alongside other ancient lineages of shelled cephalopods, offering clues about their phylogenetic relationships.

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