02861nas a2200265 4500008004100000022001400041245009300055210006900148260001500217300000900232490000700241520206800248653001302316653001802329653001402347653001802361653001802379653001802397100002402415700002702439700001702466700001902483700001502502856007802517 2020 eng d a1471-214800aAncestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan0 aAncestral morphology of Ecdysozoa constrained by an early Cambri c23/11/2020 a1-180 v203 a

Background: Ecdysozoa are the moulting protostomes, including arthropods, tardigrades, and nematodes. Both the molecular and fossil records indicate that Ecdysozoa is an ancient group originating in the terminal Proterozoic, and exceptional fossil biotas show their dominance and diversity at the beginning of the Phanerozoic. However, the nature of the ecdysozoan common ancestor has been difficult to ascertain due to the extreme morphological diversity of extant Ecdysozoa, and the lack of early diverging taxa in ancient fossil biotas.

Results: Here we re-describe Acosmia maotiania from the early Cambrian Chengjiang Biota of Yunnan Province, China and assign it to stem group Ecdysozoa. Acosmia features a two-part body, with an anterior proboscis bearing a terminal mouth and muscular pharynx, and a posterior annulated trunk with a through gut. Morphological phylogenetic analyses of the protostomes using parsimony, maximum likelihood and Bayesian inference, with coding informed by published experimental decay studies, each placed Acosmia as sister taxon to Cycloneuralia + Panarthropoda—i.e. stem group Ecdysozoa. Ancestral state probabilities were calculated for key ecdysozoan nodes, in order to test characters inferred from fossils to be ancestral for Ecdysozoa. Results support an ancestor of crown group ecdysozoans sharing an annulated vermiform body with a terminal mouth like Acosmia, but also possessing the pharyngeal armature and circumoral structures characteristic of Cambrian cycloneuralians and lobopodians.

Conclusions: Acosmia is the first taxon placed in the ecdysozoan stem group and provides a constraint to test hypotheses on the early evolution of Ecdysozoa. Our study suggests acquisition of pharyngeal armature, and therefore a change in feeding strategy (e.g. predation), may have characterised the origin and radiation of crown group ecdysozoans from Acosmia-like ancestors.

10aCambrian10aCycloneuralia10aEcdysozoa10aPalaeobiology10aPanarthropoda10aPhylogenetics1 aHoward, Richard, J.1 aEdgecombe, Gregory, D.1 aShi, Xiaomei1 aHou, Xianguang1 aMa, Xiaoya uhttps://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-020-01720-602745nas a2200313 4500008004100000022001400041245004600055210004300101260001500144300001600159490000700175520188000182653001302062653002102075653002602096653001502122653001702137653001702154653002602171653001902197653002802216100002402244700001902268700002702287700001802314700001702332700001502349856006702364 2020 eng d a0960-982200aA Tube-Dwelling Early Cambrian Lobopodian0 aTubeDwelling Early Cambrian Lobopodian c27/02/2020 a1529–15360 v303 a

Facivermis yunnanicus [1, 2] is an enigmatic worm-like animal from the early Cambrian Chengjiang Biota of Yunnan Province, China. It is a small (<10 cm) bilaterian with five pairs of spiny anterior arms, an elongated body, and a swollen posterior end. The unusual morphology of Facivermis has prompted a history of diverse taxonomic interpretations, including among annelids [1, 3], lophophorates [4], and pentastomids [5]. However, in other studies, Facivermis is considered to be more similar to lobopodians [2, 6, 7, 8]—the fossil grade from which modern panarthropods (arthropods, onychophorans, and tardigrades) are derived. In these studies, Facivermis is thought to be intermediate between cycloneuralian worms and lobopodians. Facivermis has therefore been suggested to represent an early endobenthic-epibenthic panarthropod transition [6] and to provide crucial insights into the origin of paired appendages [2]. However, the systematic affinity of Facivermis was poorly supported in a previous phylogeny [6], partially due to incomplete understanding of its morphology. Therefore, the evolutionary significance of Facivermis remains unresolved. In this study, we re-examine Facivermis from new material and the holotype, leading to the discovery of several new morphological features, such as paired eyes on the head and a dwelling tube. Comprehensive phylogenetic analyses using parsimony, Bayesian inference, and maximum likelihood all support Facivermis as a luolishaniid in a derived position within the onychophoran stem group rather than as a basal panarthropod. In contrast to previous studies, we therefore conclude that Facivermis provides a rare early Cambrian example of secondary loss to accommodate a highly specialized tube-dwelling lifestyle.

10aCambrian10aChengjiang Biota10aFacivermis yunnanicus10alobopodian10aluolishaniid10apanarthropod10aphylogenetic analyses10asecondary loss10atube-dwelling lifestyle1 aHoward, Richard, J.1 aHou, Xianguang1 aEdgecombe, Gregory, D.1 aSalge, Tobias1 aShi, Xiaomei1 aMa, Xiaoya uhttps://linkinghub.elsevier.com/retrieve/pii/S0960982220301196