stemarthropod

Stem arthropod is a term used in evolutionary biology to describe organisms that lie on the stem lineage leading to the crown group of Arthropoda. In other words, stem arthropods are more closely related to modern arthropods (insects, crustaceans, myriapods, and chelicerates) than to other living animals, but they are outside the last common ancestor of all living arthropods. The concept helps scientists study the origin and early evolution of arthropods by focusing on transitional forms rather than defining a formal clade.

Morphology of stem arthropods is typically mosaic, combining primitive and derived features. They often show segmentation and some form of protective exoskeleton, but they may lack the highly specialized tagmata and diverse appendages seen in crown-group euarthropods. Some lineages possess lobopod-like limbs or distinctive frontal appendages, while others display more articulated but still primitive limbs. Eyes, when preserved, range from simple to more complex structures. Because many of these fossils are short-lived or poorly preserved, their exact anatomy and relationships are frequently debated, and reconstructions remain subject to revision.

Most known stem arthropods come from Cambrian deposits and include a range of organisms such as radiodonts (including groups related to Anomalocaris), as well as other early enigmatic forms often discussed in relation to Hallucigenia, Aysheaia, and Fuxianhuiida. While not a single cohesive group, stem arthropods collectively illuminate the sequence of changes that culminated in the diversity of modern arthropods.