Cheloniellida is a taxon of extinct Paleozoic arthropods. As of 2018, 8 monotypic genera of cheloniellids had been formally described, whose fossils are found in marine strata ranging from Ordovician to Devonian in age. Cheloniellida has a controversial phylogenetic position, with previous studies associated it as either a member or relative of various fossil and extantarthropod taxa. It was later accepted as a member of Vicissicaudata within Artiopoda.
Morphology
The flattened, ovoid body of cheloniellid comprises an eye-bearing cephalon and segmented trunk region, dorsallydivided bya series of tergites. The cephalon could be divided into procephalon and gnathocephalon. Compared to other members of Artiopoda, the head shield of cheloniellid is relatively reduced. The trunk is wider than the cephalon and is made up of 8-13 tergites. The pleural tips of first few tergites are directed anterolaterally, becoming increasingly posterolaterally directed rearward, giving the segmental boundaries between tergites a radiated appearance. The last trunk segment, also known as postabdomen, is tiny and laterally encompassed by the pleural regions of previous tergite. yus'', a cheloniellid with a pair of raptorial cephalic appendages. Based on available materials, the cephalon comprises a pair of antennae and 5 pairs of uniramus appendages, with the posterior 4 pairs bore gnathobases. There are evidences that the non-gnathobasic second cephalic appendages are specialized or even raptorial in some species. Each of the trunk segments has a pair of biramous appendages each consisting of a leg-like endopod and a shorter exopod. The last trunk segment has a pair of spine/whip-like appendages referred as furcae or cerci, some species bore a medial spine between it which may or may not be a telson.
Classification
Phylogenetic position
While Boudreaux regarded Cheloniellida as a class, further studies usually treat Cheloniellida as an order. Cheloniellida has a controversial phylogenetic position within arthropod higher classifications, with studies mainly around 20th century suggested it as a relative/member of either Crustacea, Trilobita, Chelicerata or Aglaspidida. Some species even had been misidentified as polyplacophoranmollusks when being first described. Originally, the iconic cheloniellid Cheloniellon was believed to be a crustacean similar to trilobites. Subsequent authors suggests that it occupied a position intermediate between trilobitomorphs and chelicerates, while some also interpreted it as a sister group of crustaceans or chelicerates as well. The suggested close relationship between cheloniellids and chelicerates was inferred by the gnathobasic appendages similar to those of merostomes, and the hypothesis that the chelicerates arose from trilobitomorphs through the loss of deutocerebral antennae and specialization of tritocerebral appendages into chelicerae, a scenario which is not supported by gene expression, neuroanatomical and developmental evidences. As of 21th century, Cheloniellida was mostly found to be form a clade with Aglaspidida and Xenopoda. The clade was formally named Vicissicaudata in 2013, united by a differentiated terminal trunk area that bears a pair of non-leg-like appendages. Numerous phylogenetic analysis also retrieved Vicissicaudata within Artiopoda, a diverse arthropod taxon comprise of trilobites and similar fossil taxa that may or may not closely related to chelicerates.
