Cheloniellon

Cheloniellon range about 20 centimeters in body length (excluding appendages).[1] The flattened, ovoid body compose of 11 tergites (dorsal exoskeleton), all but the posteriormost are laterally expanded and covered the appendages underneath each of them. The boundaries between tergites have a radiated appearance. Dorsal surface of the first tergite have a pair of kidney-shaped eyes.[1] Based on the differentiation of corresponding appendages, the first 2 tergites and the remaining 9 tergites were interpreted as those of cephalon (head) and trunk, respectively.[1] Contray to the widely-referred reconstruction by Stürmer & Bergström (1978),[1] but as most of the Cheloniellids, There is no evidence of telson (a medial tail-like terminal structure) in any described fossil materials.[2]

Ventral structures of the anterior region of Cheloniellon calmani, showing all cephalic appendages and first two trunk appendage pairs. Dark grey (first diagram) and coloured region (second diagram): appendicular structures, Ant (yellow): antenna, Pap (green): 2nd cephalic appendage, Est: esthetasc-like brush, Lbr (red): labrum, Gap (cyan): gnathobasic appendage, Gn: gnathobase, Tap (blue): trunk appendage, Ex: exopod, Te: tergite.

Underneath the tergites were numerous pairs of appendages. The anteriormost appendage pair were antennae, followed by a pair of specialized second appendages that bore spines and esthetasc-like brushes, and 4 pairs of leg-like appendages that have overlapped gnathobases (jaw-like structure on leg base).[1] The mouth was covered by a labrum with spiny surface, situated between the second appendages and the first gnathobasic appendages.[1] The last gnathobasic appendage pair and the remaining 5 appendage pairs anterior to it were interpreted as corresponding to the second tergite and first tergite, respectively.[1] Appendages posterior to the cephalon were 8 pairs of biramous appendages (each compose of a leg-like endopod and a lobe-like exopod) and a pair of furcae, corresponding to the remaining 9 trunk tergites.[1]

Cheloniellon may have been a benthic predator,[3] using its gnathobases to crush prey while the spiny labrum may have helped lead food items toward the mouth opening.[1] The specialized second appendages are too fragile to be raptorial, instead they may have played a tactile role, sensing the property and position of food items.[1] The endopod and exopod of the trunk appendages may have had a locomotory and respiratory function, respectively.[1][2]

Cheloniellon is a genus of Cheloniellida, an extinct arthropod taxon that have a controversial phylogenetic position. It was previously thought to be chelicerate-related,[1][2] but later studies repeatedly suggests it as a member of Artiopoda, forming the clade Vicissicaudata with Aglaspidida and related genera.[4][5][6] Within Cheloniellida, Cheloniellon branched next to Neostrabops but basal to Triopus and Duslia.[4][5]