Calcichordates
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The stylophorans are an extinct, possibly
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
group allied to the
Paleozoic Era The Paleozoic ( , , ; or Palaeozoic) Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma at the start of ...
echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s, comprising the prehistoric cornutes and
mitrate Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates. Along with the cornutes, they form one half of the Stylophora. Morphology The organisms were a few millimetres long. Like the echinod ...
s. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a variety of very different reconstructions of their anatomy, how they lived, and their relationships to other organisms. Stylophorans have played a major role in debates over the origin of
chordates A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
, as under the
calcichordate hypothesis The calcichordate hypothesis, formulated by British Museum paleontologist Richard Jefferies, holds that each separate lineage of chordate ( Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoder ...
they were interpreted as being
stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
chordates. However, multiple lines of evidence argue against the calcichordate hypothesis, and stylophorans are now widely agreed to belong to the echinoderm
total group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
. Debate remains over whether they are stem-group echinoderms which predate the origin of radial symmetry, or highly modified descendants of radially symmetric echinoderms.


Description

The general stylophoran body plan consists of a flattened
theca In biology, a theca (: thecae) is a sheath or a covering. Botany In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common ar ...
and a single jointed appendage which extends from it. Stylophoran tests are composed of stereom calcite plates, which has traditionally been the basis for assigning them to Echinodermata. However, they also lack the
radial symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symme ...
characteristic of most other echinoderms, with the earlier members of the group being flattened and asymmetrical, and the later ones closer to bilateral symmetry. In '' Mitrocystites'' and perhaps in other forms the appendage does not end in an attachment organ, and may have served the organism for movement. ''
Cothurnocystis ''Cothurnocystis'' is a genus of small enigmatic echinoderms that lived during the Ordovician. Individual animals had a flat boot-shaped body and a thin rod-shaped appendage that may be a stem, or analogous to a foot or a tail. Fossils of ''Cothu ...
'' is asymmetrical and boot-shaped, and '' Mitrocystites'' is bilaterally symmetrical and more streamlined. There were three major hypotheses about the anatomy and systematics of stylophorans. The earliest idea, proposed by Ubaghs was that the group was aberrant echnoderms with a single starfish-like feeding arm. In the
calcichordate hypothesis The calcichordate hypothesis, formulated by British Museum paleontologist Richard Jefferies, holds that each separate lineage of chordate ( Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoder ...
proposed by Richard Jeffries, it was suggested that some or all stylophorans might have had
gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In c ...
s like
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s, and the appendage was a tail that contained a
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
. This reconstruction led to the hypothesis that some or all of the stylophorans may have been ancestral to the
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
branch of the
deuterostomes Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...
, rather than being within the echinoderms. An alternative hypothesis (proposed by D.G. Shu,
Simon Conway Morris Simon Conway Morris (born 1951) is an English palaeontologist, evolutionary biologist, and astrobiologist known for his study of the fossils of the Burgess Shale and the Cambrian explosion. The results of these discoveries were celebrated ...
, and others) suggested that stylophorans were ecologically stalked echinoderms. The appendage was reconstructed as a stalk to attach the animal to the sea floor, but the hypothesis proposed the group was close to the ancestor of both echinoderms and
hemichordate Hemichordata ( ) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and incl ...
s, and the stalk contained a notochord-like element or neural tube. Both hypotheses of chordate relationships were later disproven by the discovery of fossil soft tissues in a study published in 2019. Scanning electron microscopy of preserved soft tissues in the single appendage of two genera of stylophorans showed the appendage contained an
ambulacral Ambulacral is a term typically used in the context of anatomical parts of the phylum Echinodermata or class Asteroidea and Edrioasteroidea. Echinoderms can have ambulacral parts that include ossicles, plates, spines, and suckers. For example, sea ...
canal with tube feet covered by mobile plates, and was not a tail or stalk with fixed plates covering a notochord or neural tube. The appendage is therefore interpreted as a starfish-like feeding arm. The enlarged base of the arm contains an extension of the body cavity rather than muscle blocks to move a tail; the structure at the base of the arm is most parsimoniously reconstructed as the mouth. Stylophorans are therefore reconstructed as true echinoderms of uncertain affiliations, without radial symmetry but with stereom and a water vascular system. The body was oriented with the arm as the anterior end of the animal, which lay on the substrate; food would have been captured by tube feet and moved down the arm to the mouth. Some genera may have also used the water vascular system for locomotion. Rather than swimming with a muscular post-anal tail as in chordates, mobile genera would have crawled "arm-first" using a water vascular system, like starfish and sea cucumbers.


Classification

There are over 120 known species of stylophoran. Stylophora is conventionally divided into two
orders Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...
, Cornuta and Mitrata. Mitrates may have evolved from cornutes, which would render the cornutes paraphyletic. An alternate taxonomy regards the mitrates and mitrate-like cornutes as forming the order Ankyroida, while maintaining the order Cornuta for the less mitrate-like cornutes. The most basal known stylophoran is ''Ceratocystis''.


See also

* List of echinodermata orders * Calcichordate Theory * Edrioasteroidea *
Helicoplacoidea Helicoplacoidea is an extinct class (biology), class within the Echinodermata. All known taxa were discovered in sediments dating back to the Cambrian.
*
Blastozoa Blastozoa is a subphylum of extinct echinoderms characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. It ranged from the Cambrian to the Permian. Biserial, triradiate, and pentaradiate ambulacra ...


References


External links


Taxonomicon.nlBorntraeger-cramer.de: "A calcichordate interpretation of the new mitrate ''Eumitrocystella savilli'' from the Ordovician of Morocco"Functional morphology of stylophoran echinoderms
{{Taxonbar, from=Q3323202 Paleozoic echinoderms Cambrian echinoderms Prehistoric animal classes