14-626: The Somasteroidea , or Stomasteroidea , is an extinct order of asterozoan echinoderms first defined in 1951 by W. K. Spencer. Their first appearance in the fossil record was in the Early Ordovician ( Tremadocian ) and they had become extinct by the Late Devonian ( Famennian ). They are similar to the asteroids in that their bodies are flattened dorsoventrally and they have five petaloid arms with broad bases. The ambulacral plates in somasteroids are simple and unspecialized, and
28-400: Is a monophyletic clade ; its sister group is Echinozoa . Members of the class Asteroidea are characterised by a star-shaped body plan consisting of a central disc and multiple radiating arms. They usually exhibit pentamerous radial symmetry, but some species typically have a symmetry based on a number other than five. The arms have very broad bases and their skeletal support is provided by
42-500: Is a feature of Somasteodea. However, Villebrunaster lacks axially oriented ossicles along the lateral margins of the arms, which are found in somasteroids. This suggests that it is a primitive member of the group. The radial water channel are large and run close to the ventral side of the body, while the transverse water channels small and difficult to recognise. The ossicle series are larger in V. fezouataensis than in V. thorali. Phylogenetic analysis indicate that Villebrunaster
56-436: Is an extinct taxon that has been given class rank by some authors. It is believed to be paraphyletic . Villebrunaster Villebrunaster is an extinct genus of starfish-like animal belonging to Asterozoa that lived around 480 million years ago during Early Ordovician Period in modern-day southern France and Morocco . As of 2022, it contains two species, namely V. thorali and V. fezouataensis . V. thorali
70-435: Is that of a typical starfish having five radiating arms. Mouth is at the centre of the body. The mouth region is composed of three types of endoskeletons called ossicles , such as half-cylinders or the ambulacral, virgal that form skeleton between the ambulacral, and a pair of mouth plates that radiate into the mouth opening. The arms are broad and evenly arranged to form pentagonal structure. The presence of virgal ossicles
84-475: The class Asteroidea (the starfish ), the class Ophiuroidea (the brittle stars and basket stars ), and the extinct taxa Somasteroidea and Stenuroidea . Originally proposed in the late 1800s, Asterozoa was not used in F.A. Bather 's two-subphylum echinoderm taxonomy in 1900. However, it was adopted as part of a four-subphylum taxonomy by the Treatise on Invertebrate Paleontology in 1966. Asterozoa
98-512: The arms were thought to be not flexible and were unable to assist in feeding, but the oral mouth parts were more complex. This prehistoric echinoderm -related article is a stub . You can help Misplaced Pages by expanding it . Asterozoa The Asterozoa are a subphylum in the phylum Echinodermata , within the Eleutherozoa . Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes
112-513: The calcareous plates called ossicles or ambulacral plates in the body wall. These are joined with muscular and connective tissue giving flexibility. The mouth is on the lower or oral surface, with a corresponding anus on the upper or aboral surface; though the anus is absent in Paxillosida , wherein the mouth serves also as an anus, otherwise leading to a blind gut, similar to their Ophiuroid relatives. Their large coelomic cavity can extend from
126-408: The disc into the arms. The paired gonads are also located in the arms and release gametes via the gonoducts. The lateral flexion of the arms is limited by how the ambulacral plates are arranged. On the oral surface of the disc and radiating arms are four rows of ambulacral plates which are associated with two or four rows of tube feet, which are used in locomotion. On the upper or aboral surface of
140-425: The disc is an opening, the madreporite . This is connected to the water vascular system which assists in respiration and provides hydraulic pressure for the tube feet . Members of the class Ophiuroidea are characterised by having the gut and internal organs confined to the central disc. The arms are clearly separated from the disc. The tube feet do not have adhesive properties, but serve a sensory function. In all
154-409: The extant genera , the ambulacral plates are fused in pairs and form joints in the arms, known as vertebrae. In some extinct genera, they are not fused in this way. The ambulacral grooves function as internal epineural canals. All members of the order Somasteroidea (or Stomasteroidea) are extinct. Their first appearance in the fossil record was in the early Ordovician and they had probably died out by
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#1732776724827168-430: The late Devonian. They are similar to the asteroids in that their bodies are flattened dorsoventrally and they have five petaloid arms with broad bases. The ambulacral plates in somasteroids are simple and unspecialized, and the arms were thought to be not flexible and were unable to assist in feeding, but the oral mouth parts were more complex. An example for Somasteroidea is Villebrunaster fezouataensis . Stenuroidea
182-518: Was described by Aaron W. Hunter and Javier Ortega-Hernández at the University of Cambridge as Cantabrigiaster fezouataensis in 2021. The specimens, originally collected from Fezouata Shale Formation in Morocco, were reanalysed by American palaeontologist Daniel B. Blake and Frederick H.C.Hotchkiss who moved to taxonomic position the genus Villebrunaster in 2022. The body of Villebrunaster
196-731: Was described in 1951 and V. fezouataensis was described in 2021. Villebrunaster represents one of the oldest members of asterozoans, and perhaps, according to a description in 2021, the earliest divergent stem-group (ancestral members) of Asterozoa. The first species was discovered and described by British palaeontologist William Kingdon Spencer in 1951. The fragmentary specimens were collected from Saint-Chinian Formation in southern France. Spencer recognised it as among "the earliest starfish known." Another British palaeontologist Juliette Dean Shackleton identified new specimens as Ampullaster ubaghsi in 2005, which were later reclassified as V. thorali. The second species, V. fezouataensis,
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