The position of the Arthropoda in the phylogenetic system

@article{SchmidtRhaesa1998ThePO,
  title={The position of the Arthropoda in the phylogenetic system},
  author={Andreas Schmidt‐Rhaesa and Thomas Bartolomaeus and Christian Lemburg and Ulrich Ehlers and James R. Garey},
  journal={Journal of Morphology},
  year={1998},
  volume={238}
}
Traditionally, Panarthropoda (Euarthropoda, Onychophora, Tardigrada) are regarded as being closely related to Annelida in a taxon Articulata, but this is not supported by molecular analyses. Comparisons of gene sequences suggest that all molting taxa (Panarthropoda, Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera) are related in a monophyletic taxon Ecdysozoa. An examination of the characters supporting Articulata reveals that only segmentation with a teloblastic segment formation… 

Current Understanding of Ecdysozoa and its Internal Phylogenetic Relationships.

Twenty years after its proposal, the monophyly of molting protostomes-Ecdysozoa-is a well-corroborated hypothesis, but the interrelationships of its major subclades are more ambiguous than is

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

Analysis of DNA sequences from many genes together with morphological characters involving the cuticle and its molting strengthen the grouping of cycloneuralian worms and arthropods as Ecdysozoa, identify the velvet worms (Phylum Onychophora) as the closest living relatives of arthropoda, and interpret segmentation as having separate evolutionary origins in arthropod and annelid worms.

The Articulata hypothesis – or what is a segment?

Based on numerous similarities not shared with other bilaterian taxa it is suggested that segmentation of annelids and arthropods is homologous and apomorphic for a monophyletic Articulata.

Proposing a solution to the Articulata–Ecdysozoa controversy

Morphological, palaeontological and molecular implications of this theory suggest that segmentation has been lost in phyla such as Nematoda and Priapula, but the Kinorhyncha may show a ‘reduced segmentation’ with serially arranged muscles associated with a ringed cuticle.

Polychaeta, Annelida, and Articulata are not monophyletic: articulating the Metameria (Metazoa, Coelomata)

Annelida and Polychaeta are non-monophyletic, even when including Echiura, Clitellata, and Pogonophora, and Articulata; and Owenia plus Enterocoela form a monophyletics group related to the tubicolous polychaetes.

The clade Ecdysozoa, perplexities and questions

Armored kinorhynch-like scalidophoran animals from the early Cambrian

The fossil record confirms that all three scalidophoran phyla diverged no later than the Cambrian Period, including the new species Eokinorhynchus rarus and two unnamed but related forms.

Origin and differentiation of nephridia in the Onychophora provide no support for the Articulata

Information studied in the onychophoran Epiperipatus biolleyi provided no evidence for nephridia of Annelida and Arthropoda being a synapomorphy of these taxa, and weakened the traditional Articulata hypothesis.
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