Pleistoannelida is a group of annelid worms that comprises the vast majority of the diversity in phylumAnnelida. Discovered through phylogenetic analyses, it is the largest clade of annelids, comprised by the last common ancestor of the highly diverse sister groupsErrantia and Sedentaria (Clitellata and related polychaetes) and all the descendants of that ancestor.[1][4] Most groups in the Clade find their ancestors within the Cambrian explosion when Annelid diversity expanded dramatically.[5] The Pleistoannelida clade covers a variety of traits. However, the evolution of simple to complex eyes, developed papillae for burrowing, and for some specialized radioles for feeding can be seen universally across every species.[6][5] New findings have discovered the range of Annelid diversity have led to uncertainty if groups with developed ancestral traits should remain within the clade. Furthermore, there's been a lack of recently discovered Annelid traits being used in the categorization of groups within the clade, leading to many hypothesis on how to do so and which should remain within the clade.[7] Currently three smaller clades that were originally a part of the groups Errantia and Sedentaria have been proven to fall outside while still being connected to the basal groups.[6]
Half of a fossil containing the species of worm Esconites zelus.
(A) Anterior end of Magelona mirabilis, showing prostomium, palps, achaetous first segment and first five chaetigers (dorsal view)
There has been research conducted to help determine the taxonomy of the clade Pleistoannelida. The research includes the morphological characteristics of the clade as well as the phylum Annelida as a whole. For example, there are studies of gene expression in annelids. It was supported that larval eyes of annelids are considered homologous to pigmented eyes of bilaterians. It is believed that annelid adult eyes evolved in a common ancestor of Pleistoannelida.[11] Besides the eyes, it has also been studied that commissures in the brain, glanglia, and nuchal organs were to have also evolved in the lineage of Pleistonannelida (Errantia and Sedentaria) by observing Magelonamirabilis.[12] At a cellular level, it was found that the highly conserved mitochondrial gene order could be only depicted to the clade Pleistoannelida.[13]
Genetic
There has been much research done on the genetics of Pleistoannelida and Annelida as a whole to determine the phylogeny of the clade, because many taxa do not share ancestral characteristics. Genetic studies have led to the current phylogenetic tree, which separates the clade into the two sister groups of Errantia and Sedentaria.[14] Genetic data was also used in the placement of groups of Myzostomida, Nerilliade, and Aberranta within Pleistoannelida. These groups were previously thought to be anywhere from annelids to flatworms, but genetics studies have found strong evidence to place them solidly in Annelida, with the closest relations to groups in the clade Pleistoannelida.[15][16] The most current research puts Myzostomida in Errantia, however the data is not strong enough to make this a certainty.[13] Research places Nerillidae closest to Eunicida (supported by certain morphology) and places Aberranta near Nerillidae or syllids and nereidids.[15]
^ abcStruck TH (2019). "Phylogeny". In Purschke G, Böggemann M, Westheide W (eds.). Handbook of Zoology: Annelida. Vol. 1: Annelida Basal Groups and Pleistoannelida, Sedentaria I. De Gruyter. pp. 37–68. doi:10.1515/9783110291582-002. ISBN9783110291469.
^Weigert, Anne; Connard, Helm; Meyer, Matthias; Birgit, Nickel; Arendt, Detlev; Hausdorf, Bernhard; Santos, Scott R.; Halanych, Kenneth M.; Purschke, Günter; Bleidorn, Christoph; Struck, Torsten H. (2014). "Illuminating the Base of the Annelid Tree Using Transcriptomics". Molecular Biology and Evolution. 31 (6): 1391–1401. doi:10.1093/molbev/msu080 – via Oxford academy.
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