Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan
- PDF / 6,421,705 Bytes
- 18 Pages / 595.276 x 790.866 pts Page_size
- 76 Downloads / 138 Views
RESEARCH ARTICLE
Open Access
Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan Richard J. Howard1,3,4, Gregory D. Edgecombe1,4, Xiaomei Shi1,2, Xianguang Hou1,2* and Xiaoya Ma1,2,3*
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. Keywords: Ecdysozoa, Cambrian, Cycloneuralia, Panarthropoda, Palaeobiology, Phylogenetics Background Ecdysozoa are the moulting invertebrates, including arthropods, tardigrades and nematodes [1, 2]. Along with the Spiralia (e.g. molluscs, flatworms and annelids) and the Deuterostomia (e.g. chordates and echinoderms), *Correspondence: [email protected]; [email protected] 1 MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University, Chenggong Campus, Kunming 650500, China Full list of author information is available at the end of the article
the Ecdysozoa represent one of the major subdivisions of bilaterian animals. Ecdysozoa comprises the vast majority of this bilateral animal diversity (and indeed animals generally)—principally through the megadiverse arthropods. Together with Spiralia, the ecdysozoans comprise the Protostomia. Molecular clocks indicate the divergence b
Data Loading...
