Fig. 3
From: Palaeontological and Molecular Evidence Linking Arthropods, Onychophorans, and other Ecdysozoa
Relationships of panarthropods including extinct taxa (names in italics, lower in the tree) and living taxa (names in upper case at the top of the tree). The series of fossils from Megadictyon to Leanchoilia represent stem group Arthropoda, whereas all species descended from the most recent common ancestor of living chelicerates and insects or crustaceans are crown group arthropods. The letters A–G represent evolutionary events in the arthropod stem group, including: A paired, segmentally arranged midgut glands; B trunk appendage flaps in the form of lateral lobes; C trunk without annulation; D appendages having stiffened regions separated by soft membranes (=arthropodization); E hard, articulated tergal exoskeleton; F limb bases with spiny, gnathobasic endites; G locomotory limbs with only five to seven articles exclusive of terminal claw