Comparative morphological study between Cambrian lobopodians and extant tardigrades

Abstract

Background: Order Arthrotardigrada includes marine heterotardigrades which have been known to retain more plesiomorphic conditions than the Eutardigrada. Especially, Parastygarctus was considered as the most primitive genus, based on the morphological comparison with a Cambrian lobopodian, Aysheaia pedunculata from the Burgess Shale, Canada, which was one of the two known Cambrian lobopodian until the 1980’s. However, since the 90’s thirteen more Cambrian lobopodian species have been discovered from the Chengjiang, China, which have significantly promoted our knowledge on the Cambrian lobopodian morphology, necessitating novel morphological comparison between tardigrades and the Cambrian lobopodians. Results: We have compared morphological characters between tardigrades and the Cambrian lobopodians. Among several species of Cambrian lobopodians, Aysheaia pedunculata displays six peri-buccal papillae around the mouth opening, which are reminiscent of those of apochelan Milnesium tardigradum. Eutardigrades and the Cambrian lobopodians have terminally-opened mouth which is an important plesiomorphic character of panarthropods, whereas most heterotardigrade show a ventrally-opened mouth. The Cambrian lobopodians had claws at the end of their limbs, lacking digits or toes which are present in heterotardigrades. The Cambrian lobopodians and eutardigrades show lobe-like limbs, while heterotardigrades have telescopic limbs. Presence of circum-oral elements and pharyngeal teeth are plesiomorphic for Ecdysozoa, but only eutardigrades have these structures. In addition, the elongated pharyngeal bulb and parallel piercing stylets of apochelan tardigrade reminds the elongated pharynx and parallel rostral spines of Kerygmachela kierkegaardi respectively. Conclusions: Based on the morphological comparison with the Cambrian lobopodians, it is suggested that eutardigrades retain more plesiomorphic morphologies than heterotardigrades. Particularly, apochelans display many similarities with Cambrian lobopodians.