Panarthropoda

Panarthropoda is a proposed animal superphylum that groups together the three phyla Onychophora (velvet worms), Tardigrada (water bears), and Arthropoda (insects, arachnids, crustaceans and their relatives). It is a widely used framework for understanding the early evolution of modular, segmented organisms with paired appendages, and it is accepted as a major lineage within Ecdysozoa, the group of molting animals.

The exact relationships among the three constituent phyla are debated; molecular studies frequently support close affinities among them but differ on the sister-group arrangement. A common scenario is that Tardigrada is the sister lineage to a clade formed by Onychophora and Arthropoda, placing the three within Panarthropoda, with origins in the early Cambrian or late Precambrian.

Anatomically panarthropods share features such as segmented bodies with repeated appendages and a developmental program that includes molting. The three phyla diverge in detail: arthropods have jointed limbs and hardened exoskeletons; onychophorans possess soft bodies with lobopodous limbs; tardigrades have compact bodies with short, unjointed limbs. The fossil record includes Cambrian lobopodians that are regarded as stem-panarthropods, illustrating transitional forms toward more derived body plans.

Panarthropoda remains a topic of active research, but it provides a framework for studying the origin of segmentation, appendage diversification, and the evolution of ecdysozoan molting across a broad group that includes most familiar animals.