User:Manudouz/sandbox/Xenacoelomorpha

Etymology
The term "xenacoelomorph" derives from the four Ancient Greek words, meaning "strange, unusual",  , the alpha privative, expressing negation or absence,  , meaning "cavity", and  , meaning "form". This refers to the fact that this taxonomic group contains xenoturbellids, hence the prefix '', and acoelomorphs, which have a structure lacking a fluid-filled body cavity.

The term Xenoturbella derives from the Ancient Greek word, meaning "strange, unusual",  and from the Latin word turbella meaning "stir, bustle". This refers to the unusual taxonomic status of the animal, initially considered as related to turbellarians, a group of flatworms whose aquatic species stir microscopic particles close to their ciliated epidermis.

Phylogeny
In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.
 * Acoelomorpha is the sister group to Xenoturbellida, and both taxa consitute the so-called Xenacoelomorpha clade. The close evolutionary relationship between Xenoturbella and Acoelomorpha is supported by their morphology (structure of epidermal  cilia ), embryology (direct development without a feeding larval stage ), and by hundreds of proteins concatenates.
 * The phylogenetic placement of Xenacoelomorpha among bilaterian animals is not yet well defined, despite increased taxon and gene sampling, (re)-analyses of published data sets, and use of more sophisticated models of sequence evolution in phylogenomic studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to Ambulacraria within deuterostomes (i.e., the Xenambulacraria hypothesis) on the one hand, and Xenacoelomorpha as sister group to all other bilaterians (i.e., the Nephrozoa hypothesis) on the other. However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.

Philippe et al. (2019)
"Highlights:
 * Model violations hinder inference of the phylogeny of Bilateria
 * Methods predicted to cause errors place Xenacoelomorpha outside Nephrozoa
 * Xenacoelomorpha is the sister group of Ambulacraria (hemichordates and echinoderms)
 * Monophyletic deuterostomes (Chordata plus Xenambulacraria) are not clearly supported"

Martín-Durán et al. (2018)
Our findings restrict the use of molecular patterns to explain nervous system evolution, and suggest that the similarities in dorsoventral patterning and trunk neuroanatomies evolved independently in Bilateria.

About Xenambulacraria
"First is Reisinger’s proposed idea of an association between Xenoturbella and archicoelo- mate deuterostomes (5 Ambulacraria: hemichordates and echinoderms) (Reisinger, 1960). Westblad had al- ready noted detailed similarities of the epidermis of Xen- oturbella and of hemichordates. Reisinger emphasised this resemblance and further noted similarities of the sta- tocyst of Xenoturbella to that found in certain sea cucumbers (Holothuria, Echinodermata) and proposed that Xenoturbella might represent the sexually mature larva of an animal related to the echinoderms and hemi- chordates. He went further and even speculated that it might belong to the life cycle of a known type of archi- coelomate and subsequently develop into an animal sim- ilar to enteropneusts, which would later become sexu- ally mature for a second time."

About flatworms
The "traditional" view before the 1990s was that Platyhelminthes formed the sister group to all the other bilaterians, which include, for instance, arthropods, molluscs, annelids and chordates. Since then, molecular phylogenetics, which aims to work out evolutionary "family trees" by comparing different organisms' biochemicals such as DNA, RNA and proteins, has radically changed scientists' view of evolutionary relationships between animals. Detailed morphological analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree that Acoelomorpha, consisting of Acoela (traditionally regarded as very simple "turbellarians" ) and Nemertodermatida (another small group previously classified as "turbellarians" ) are the sister group to all other bilaterians, including the rest of the Platyhelminthes. However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians, although it agreed that both are more closely related to cnidarians (jellyfish, etc.) than other bilaterians are.

Xenoturbella, a bilaterian whose only well-defined organ is a statocyst, was originally classified as a "primitive turbellarian". Later studies suggested it may instead be a [[deuterostome], but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha.