Phyllocarids

Phyllocarids are a subclass of crustaceans. They are characterized by a bivalved carapace that completely encloses the body. This carapace is not calcified, unlike that of many other crustaceans, and it is articulated. Phyllocarids are considered the most primitive of the extant crustacean subclasses. The fossil record of phyllocarids extends back to the Cambrian period, making them among the oldest known arthropod groups.

The body of a phyllocarid is typically elongated and segmented. They possess a distinct head, thorax, and abdomen. The appendages are numerous and serially homologous, meaning they have a similar basic structure. Respiration in phyllocarids occurs through the gills, which are usually located on the thoracic appendages.

Phyllocarids are exclusively marine organisms. They inhabit a variety of marine environments, from shallow coastal waters to the deep sea. Their diet generally consists of detritus or small planktonic organisms, which they filter from the water or gather from the substrate.

There are two extant orders within the subclass Phyllocarida: the Leptostraca. These living phyllocarids are relatively rare and not well-known compared to other crustacean groups. Their primitive features and ancient lineage make them of significant interest to evolutionary biologists studying the history of arthropod diversification. Fossil phyllocarids are more commonly found and provide important insights into early marine ecosystems.