| Vicissicaudata | |
|---|---|
 | |
| Sidneyia minor, a vicissicaudatan unplaced in any subgroup | |
 | |
| Cheloniellon, acheloniellid | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| (unranked): | †Artiopoda |
| (unranked): | †Vicissicaudata Ortega-Hernández, Legg & Braddy, 2013 |
| Subgroupings | |
Vicissicaudata is an unranked group ofartiopodan arthropods, containingCheloniellida,Aglaspidida and several other genera outside these groups likeSidneyia andEmeraldella.
Vicissicaudatans are similar to most other artiopods in shape, with rounded bodies consisting of many widetergites, sometimes with a distinguishable axis along the midline. This clade is defined by a differentiated posterior region,[2] often ending in atelson and caudal appendages. The morphology of this posterior region varies between clades. Aglaspidids, alongsideEozetetes[3] andCarimersa,[4] have longstyliform telsons and relatively small, flap-like caudal appendages while cheloniellids andTardisia have much longer caudal appendages similar to the caudal furcae of various taxa, alongside much smaller telsons more resembling the postabdominal segments. The paraphyletic "xenopods" are more similar to those of aglaspidids, also with long telsons and flap-like caudal appendages, however inSidneyia[5] the caudal appendages have become large with broad, lobed tips. In terms of bodily morphology, aglaspidids andCarimersa closely resembletrilobites, with cheloniellids having an unusual radial arrangement of their tergopleurae. The various basal taxa are mostly less derived in body shape with rounded tergite edges and large head shields, although the first appendage pair ofKodymirus vaguely resembles the frontal appendages ofradiodonts.[6] Most have eyes, although some later aglaspidids and cheloniellids, alongsideTardisia andEtainia,[7] seem to lack them. However, the placement of eyes varies throughout Vicissicaudata, with some having eyes on the underside of their carapace (such asEmeraldella),[8] so the apparent lack of eyes may simply be due to them not being visible on fossils. Unlike trilobites, most vicissicaudatans are not biomineralised, therefore they are much rarer and mainly appear inlagerstätten.
Vicissicaudatans seem to have a wide distribution both in time and space, with fossils being recorded from the Cambrian to the Carboniferous. Fossils have been found in locations including but not limited to theBurgess Shale,[8]Wheeler Shale,[9]Chengjiang biota,[5]Paseky Shale,[6]Emu Bay Shale,[3]Fezouata Formation,[10]Llanfallteg Formation,[7]Coalbrookdale Formation,[4]Hunsrück Slate[11] andMazon Creek.[1]
A close relationship betweenSidneyia,Emeraldella,aglaspididans andcheloniellids had long been hypothesised, but the grouping lacked a formal name until 2013. Vicissicaudata is often considered to be closely related to the clade containingtrilobites and their close relatives (Trilobitomorpha) as part ofArtiopoda,[12] though some studies hypothesise that Artiopoda is paraphyletic, with some studies hypothesising that Vicissicaudata is more closely related to other arthropod groups likechelicerates than to trilobitomorphs.[13]
Cladogram after McCoy et al. 2025:[14]
Cladogram after Lerosey-Aubril et al. (2017).[12]
| Vicissicaudata | |
Cladogram after Jiao et al. 2021.[15]
| Vicissicaudata | |
Vicissicaudata translates to "changed tail", in reference to the differentiation of the posterior body region in the group.[2]
