English: Morphology and evolution of the posterior trunk region within the Vicissicaudata. (a) morphology of the posterior trunk regions of aglaspidids (ancestral condition), Emeraldella, and cheloniellids; a morphologically-distinct, pretelsonic segment bearing appendage derivatives (in yellow) occurs in front of the telson (in pink) in all these taxa. (b–d) evolutionary scenarios describing the fusion of the pretelsonic segment and telson into a tailspine, and the relationship between pretelsonic appendages (furcal rami) and postventral plate (pvp) in aglaspidids. (b) aglaspidid ancestor possessed pretelsonic appendages (in yellow) and postventral plate (in purple), but the former structures disappeared in most descendants. (c) aglaspidid ancestor possessed pretelsonic appendages only (in yellow), which were lost in most descendants – the postventral plate (in purple) evolved independently. (d) aglaspidid ancestor possessed pretelsonic appendages only, which later evolved into the postventral plate in most descendants (both structures are in yellow).