Yolkia

Yolkia is a genus of small, extinct marine mollusks that lived during the Jurassic period, approximately 183 to 145 million years ago. The genus is characterized by its small, discoidal shells, which are typically less than 10 millimeters in diameter. Yolkia shells are smooth, with a rounded margin and a central umbilicus, which is a small depression around the apex of the shell. The shell is composed of aragonite, a form of calcium carbonate.

Yolkia is known from fossils found in Europe, North America, and Asia. The genus is named after the yolk-like appearance of the shell's interior, which is filled with a calcareous mass that resembles the yolk of an egg. This unique feature is thought to have been used for buoyancy or as a food source by the animal.

The taxonomy of Yolkia is somewhat controversial, with some researchers suggesting that it may be a junior synonym of the genus Pseudohastula. However, recent studies have supported the distinctiveness of Yolkia, based on differences in shell morphology and the presence of unique internal structures.

Yolkia is an important fossil genus for understanding the evolution of marine mollusks during the Mesozoic era. Its small size and simple shell structure make it a useful index fossil for dating sedimentary rocks. Despite its small size, Yolkia provides valuable insights into the diversity and ecology of Jurassic marine ecosystems.