facts that "Pan-Aves" and "Aves" are not the same group, the circumscription of the concept of "Pan-Aves" (synonymous with Avemetatarsalia) is only evident by examination of the above tree, and calling both groups "birds" is ambiguous. Stem mammals are those in the lineage leading to living mammals, together with side branches, from the divergence of the lineage from the Sauropsida to the last common ancestor of the living mammals. This group includes the synapsids as well as mammaliaforms like the morganucodonts and the docodonts; the latter groups have traditionally and anatomically been considered mammals even though they fall outside the crown group mammals. Stem tetrapods are the animals belonging to the lineage leading to tetrapods from their divergence from the lungfish, our nearest relatives among the fishes. In addition to a series of lobe-finned fishes, they also include some of the early labyrinthodonts. Exactly what labyrinthodonts are in the stem group tetrapods rather than the corresponding crown group is uncertain, as the phylogeny of early tetrapods is not well understood. This example shows that crown and stem group definitions are of limited value when there is no consensus phylogeny. Stem arthropods constitute a group that has seen attention in connection with the Burgess Shale fauna. Several of the finds, including the enigmatic Opabinia and Anomalocaris have some, though not all, features associated with arthropods, and are thus considered stem arthropods. The sorting of the Burgess Shale fauna into various stem groups finally enabled phylogenetic sorting of this enigmatic assemblage and also allowed for identifying velvet worms as the closest living relatives of arthropods. Stem priapulids are other early Cambrian to middle Cambrian faunas, appearing in Chengjiang to Burgess Shale. The genus Ottoia has more or less the same build as modern priapulids, but phylogenetic analysis indicates that it falls outside the crown group, making it a stem priapulid. Plesion-group The name plesion has a long history in biological systematics, and plesion group has acquired several meanings over the years. One use is as "nearby group" (plesion means close to in Greek), i.e. sister group to a given taxon, whether that group is a crown group or not. The term may also mean a group, possibly paraphyletic, defined by primitive traits (i.e. symplesiomorphies). It is generally taken to mean a side branch splitting off earlier on the phylogenetic tree than the group in question. Palaeontological significance of stem and crown groups