Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan

Abstract

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.

Peripheral and central employment of acid-sensing ion channels during early bilaterian evolution

“…Perhaps most noticeable, however, is the absence of ASICs in Ecdysozoa, a broad lineage including pan-arthropods, nematodes, and priapulids, each of which we considered in our phylogenetic analysis. Ecdysozoa split from Spiralia ~650 Mya ( Kumar et al, 2017 ), adopting a chitinous cuticle utilizing rigid locomotory structures and requiring periodic molting for growth ( Howard et al, 2020 ; Schmidt-Rhaesa et al, 1998 ). Concomitantly, Ecdysozoa lost the ectodermal, motile ciliated cells inherited from the last common ancestor of Cnidaria and Bilateria that mediate locomotory, feeding, secretory, and sensory functions in most other bilaterians ( Ringers et al, 2020 ; Valentine and Collins, 2000 ).…”

Peripheral and central employment of acid-sensing ion channels during early bilaterian evolution

“…Perhaps most noticeable, however, is the absence of ASICs in Ecdysozoa, a broad lineage including pan-arthropods, nematodes, and priapulids, each of which we considered in our phylogenetic analysis. Ecdysozoa split from Spiralia 600-800 Mya [55–57], adopting a chitinous cuticle utilizing rigid locomotory structures and requiring periodic molting for growth [79, 80]. Concomitantly, Ecdysozoa lost the ectodermal, motile ciliated cells inherited from the last common ancestor of Cnidaria and Bilateria that mediate locomotory, feeding, secretory, and sensory functions in most other bilaterians [81, 82].…”

Neuroanatomy in a middle Cambrian mollisoniid and the ancestral nervous system organization of chelicerates

“…With a few exceptions 21,26,27 , most instances of exceptionally preserved nervous systems in Cambrian BST deposits are restricted to members of total-group Euarthropoda, including gilled lobopodians 20 , radiodonts 17 , fuxianhuiids 15,19,28 , megacheirans 16,22,24 , and bivalved forms 18,29,30 . These records encompass a broad phylogenetic sampling, which allows reconstructing the complex evolutionary history of the central nervous system (CNS) in this phylum, clarifying the segmental origin of euarthropod head structures, and illuminating the ancestral neuroanatomical organization of extant representatives 28,31 .…”