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]
Paleoecology
Cheloniellon may have been a benthicpredator,[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]
^ abcdefghijklmStürmer, Wilhelm; Bergström, Jan (1978-06-01). "The arthropod Cheloniellon from the devonian hunsrück shale". Paläontologische Zeitschrift. 52 (1): 57–81. doi:10.1007/BF03006730. S2CID87725308.
^ abcWendruff, Andrew James, et al. "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA." BioRxiv (2018): 407379. [1]