Phylogeny Research Paper Example | Topics and Well Written Essays - 750 words

Phylogeny - Research Paper Example According to recent studies involving molecular analysis of 18S rRNA/DNA, paraphyly was evident in Porifera. It showed that Calcarea (calcareous sponges) appear to have closer affinities to other metazoans than fellow poriferans the siliceous sponges (both Hexactinellida and Demospongiae) which inevitably showed a great amount of radiation between these two groups (Adams, McInerney and Kelly 34). These new findings actually go a long way in supporting earlier calls for the subdivision of the phylum into two. More empirical data on molecular analysis in particular of 18S rRNA/DNA was provided which showed that there is a stronger affinity between calcareans and ctenophorans than that which is between calcareans and other poriferans. Poriferans (sponges) have always been regarded as the basal living metazoans that are monophyletic as far as phylogenetic studies are concerned. The various relationships of organisms at the metazoan tree base remain largely unknown despite the fact that n ew trends of triploblast systematics are emerging which provides a clear picture of the lineage. According to previously done classifications, these basal metazoan organism have been put in different relationships using several markers except one relationship; monophyly. Earlier analyses of the basal metazoans (sponges, placozoans, cnidarians and ctenophores) have almost unanimously agreed that cnidarians and ctenophores have more close ties or relations with triploblasts than they have with the sponges (poriferans) (Hooper and Willenz 11). In traditional phylogenetic schemes the ACANTHOCEPHALA, ENTOPROCTA, GASTROTRICHA, GNATHOSTOMULIDA, KINORHYNCHA, NEMATODA, NEMATOMORPHA, PRIAPULIDA and ROTIFERA were grouped together as aschelminths or pseudocoelomates. Discuss why we no longer support a taxon of ASCHELMINTHES and discuss how those phyla are now grouped. Aschelminthes was used to refer to assemblage of polyphyletic meiofaunal sized animals which included several phyla. However the legitimacy of Aschelminthes as a taxon was questioned on the basis of lack of morphological as well as molecular evidence. Currently, these organisms have been declassified into separate phyla. It is sometimes useful to use the term Aschelminthes to refer to all previously organisms that were classified under it. On top of this, it has not been agreed upon whether the formerly known organisms in this group make up a monophyletic group. To complicate matters further, it has not been decided on which phyla to place the Aschelminthes. According to recent morphological studies, the Aschelminthes was described as possibly having two clades (Aguinaldo 490). The two clades are gnathiferans that are hypothesized to contain a newly formed taxon Micrognathozoa while the other clade is Introverta. The second clade of Introverta is thought to be a possible link between Scalidophora and Nematoida. There is a possible remote relation between the introvertans and the panarthropods but for the gna thiferan clade, it has not yet been established if it falls within the bilateral organisms for certainty. Both gastrotrichs and chaetognaths phylogenetic placements are equally unsettled owing to unsettled issues in phylogenetic analyses. There has been a relation between Gastrotrichs and Nematoida, gnathiferans and introvertans. However, more close affinity between Gastrotrichs and introvertans than the others has been recognized and led to the formation of a clade called Nemathelminthes or