Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Panarthropoda - Arthropoda Evolution¹

On morphological criteria 3 crown phyla, Onychophora, Tardigrada, and Arthropoda, appear to be closely related, so were grouped as Panarthropoda in an early classification (Nielsen, Scharff & Eibye-Jacobsen, 1996). According to the authors¹ Onychophora and Arthropoda are indicated by molecular data to be sister groups, though Tardigrada may be the sister group of those 2 (Campbell et al., 2011). A more complex picture is produced by including some of the more flamboyant animals of the Cambrian Explosion Fauna in the phylogenetic analysis. The authors¹ suggest many of these unusual forms are suggested by their morphologies to have affinities with onychophorans, while others have characteristics similar to arthropods. Included among these animals there are a significant number that do not fit within any crown phylum, which includes some interesting forms such as Opabinia. Among these forms from the Early Cambrian the phylogenetic relationships have not yet been fully resolved. The earliest known trace fossils of Panarthropoda first appear in Stage 2 of the Cambrian. Virtually all the clades of stem arthropods make their first appearance in Cambrian Stage 3, though some lobopod body fossils appear in Stage 2, as well as some reports of arthropod body fossils.

Arthropod Origins and Early Evolution²

An arthropod tree reconstructed from phylogenomics in which an Arthropoda splits into Pycnogondia + Euchelicerata and Myriapodia + Pancrustceae. Morphological data sets also produce the same chelicerate-mandibulate groups; including most taxa that are known from Konservat-Lagerstätten of Palaeozoic age. With respect to the interrelationships among the extant Panarthropoda clades, between Onychophora and Arthropoda there is a sister group relationship that is endorsed by transcriptions and microRNAs, though homoplasy is forced by this hypothesis in the characters of the segmental ganglia that are shared by tardigrades and arthropods. Lobopodians, dinocaridids, bivalve arthropods, fuxianhuiids document the successive appearance of features characteristics of arthropods in the stem lineage of Euarthropoda (crown group arthropods). It is suggested by molecular dating that arthropods originated and diversified initially in the Ediacaran, though the earliest known evidence of Panarthropoda is from the Early Cambrian.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado

2. Edgecombe, Gregory D., and David A. Legg. "Origins and Early Evolution of Arthropods." Palaeontology  (2014): n/a-n/a.