Crustei

Crustei is a provisional term used in some discussions of crustacean evolution to denote a basal or stem group within Crustacea. It is not recognized as a formal taxon in major classification schemes, and its circumscription varies among authors. The concept is used to describe fossil lineages from the Cambrian to the Devonian that display a mix of primitive and derived features linking early forms to later crustaceans. In several reconstructions, Crustei is treated as a paraphyletic assemblage rather than a clade with a single common ancestor, reflecting uncertainty about early branching events in Crustacea.

Morphologically, proposed members are described as having segmented bodies with a flexible exoskeleton, simple biramous limbs, and a tendency toward a shallow carapace, but with notable variation between lineages. The fossil record of putative Crustei material is primarily marine and often small in size, complicating taxonomic assignment. Modern descendants, if any survive, are not well defined; most discussions of Crustei focus on historical relationships rather than contemporary diversity.

The term is mainly of interest to researchers studying early crustacean evolution, ontogeny, and the interpretation of incomplete fossil data. Because Crustei is not universally adopted, its usage may be limited to specific papers or debates; other researchers may prefer alternative classifications that place the included forms within established crustacean groups. See also Crustacea, Malacostraca, fossil crustaceans.