Palaeontological and Molecular Evidence Linking Arthropods, Onychophorans, and other Ecdysozoa

Abstract

Membership of Arthropoda in a clade of molting animals, the Ecdysozoa, has received a growing body of support over the past 10 years from analyses of DNA sequences from many genes together with morphological characters involving the cuticle and its molting. Recent analyses based on broad phylogenomic sampling strengthen the grouping of cycloneuralian worms and arthropods as Ecdysozoa, identify the velvet worms (Phylum Onychophora) as the closest living relatives of arthropods, and interpret segmentation as having separate evolutionary origins in arthropods and annelid worms. Determining whether the water bears (Phylum Tardigrada) are more closely related to onychophorans and arthropods or to unsegmented cycloneuralians such as roundworms (Nematoda) is an open question. Fossil taxa such as the Cambrian anomalocaridids provide a combination of arthropod and cycloneuralian characters that is not observed in any living ecdysozoan. Fossils break up long branches and help to resolve the sequence of character acquisition at several critical nodes in the arthropod tree, notably in a suite of Cambrian lobopodians that may include the stem groups of each of the major panarthropod lineages.

Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods

“…Based on this finding, the contralateral commissures have been reinterpreted as a derived feature (=synapomorphy) of Onychophora and Arthropoda to the exclusion of Tardigrada [33]. However, our immunocytochemical and electron microscopic data clearly show contralateral fibres and bundles of neurites in all four trunk ganglia in specimens of Macrobiotus cf.…”

Sophisticated digestive systems in early arthropods

“…Numerous Cambrian arthropods from exceptional biotas have well-preserved digestive systems with biserial midgut glands that display a variety of shapes from simple, short, bunch-like or lobe-like diverticula to extensive branching features (for example, naraoiids 49 ). They have been interpreted as digestive glands 49,50 in stem lineage arthropods such as Kerygmachela 22,51 ( Supplementary Fig. 3), Opabinia 52,53 ( Fig.…”

“…Most recent phylogenies of Ecdysozoa based on molecular and morphological data [1][2][3][4][5][6][7][8]51 place priapulid worms within the Cycloneuralia 58 , a sister group of Panarthropoda (Arthropoda, Tardigrada, Onychophora), and suggest a divergence time preceding the rise of panarthropods 3 . No Cambrian priapulid 45,47 has a complex digestive system.…”

“…Nonetheless, the ability to exploit diverse food sources via sophisticated digestive systems was likely a driving factor in the evolution of arthropods and the further elaboration into complex trophic webs. [1][2][3][4]6,7,51 . Blue dots indicate lineages in which digestive glands are lacking, red dots indicate lineages that have sophisticated digestive systems.…”

Early Terrestrial Animals, Evolution, and Uncertainty

“…However, they also effectively demonstrate a major cause of uncertainty amongst arthropod workers-the traditional basis for assessing evolutionary relationships is morphology (Darwin 1859;Skelton 1993). Thus, assessing whether structures are homologous (share a common origin) or homoplasious (the result of convergence) is vital to understanding a group's relationships (Edgecombe 2009). This is rarely clear-cut, however.…”