echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
s that lived only in the
Middle Cambrian
Middle or The Middle may refer to:
* Centre (geometry), the point equally distant from the outer limits.
Places
* Middle (sheading), a subdivision of the Isle of Man
* Middle Bay (disambiguation)
* Middle Brook (disambiguation)
* Middle Creek ...
epoch. Homostelea is a junior synonym. The classification of cinctans is controversial, but they are probably part of the echinoderm
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. ...
.
Cinctans were sessile, asymmetrical animals with a skeleton made of stereom plates and a racquet-shaped body composed of a theca and stele. They may have had a lifestyle similar to modern tunicates, filter-feeding by pumping water through gill slits in their pharynx.
Description
Cinctans were asymmetrical animals, though some species were nearly bilaterally symmetrical. Like all echinoderms, cinctans have a skeleton made of plates of stereom. The body of cinctans was divided into two sections, the main body, called the theca, and a posterior appendage called a stele. The overall shape of cinctans has been compared to a tennis racquet. The theca of cinctans was surrounded on its margins by a frame of large stereom plates called a cinctus, and the dorsal and ventral surfaces were covered in a tessellated arrangement of small plates. The stele was essentially an extension of the cinctus, rather than a discrete appendage, and would have been fairly stiff side-to-side but possibly more flexible up and down. The stele was not a holdfast, but may have served to stabilize the animal. In most cinctans, the overall shape of the theca was only mildly asymmetrical, but in the unusual genus ''Lignanicystis'' the theca was highly asymmetrical, convergent on the stylophorans in some respects. The species ''Graciacystis'' could reach a theca length of .
The theca of cinctans contained three major openings. The mouth was located on the right side of the anterior end, in the cinctus, and was associated with marginal grooves. All species had a left marginal groove, but the right marginal groove was sometimes absent and always shorter than the left one. The anus was located near the anterior end, on the right side, indicating that the gut was U-shaped. It was surrounded by a cone of plates equivalent to the
periproct The periproct is the final body segment in annelid worms. The anus is located on this segment. The term also refers to the small region surrounding the anus of the sea urchin.
See also
*Prostomium
*Earthworm
*Sea urchin
Sea urchins () are s ...
of modern echinoderms. The largest opening, called the porta, was located at the anterior end and was covered by an operculum. It was likely an atrial opening like that of tunicates.
The asymmetry of the marginal groove likely indicates that cinctans had a
water vascular system
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms m ...
comprising two hydrocoels, with the left hydrocoel larger than the right. In species with only a left marginal groove, the right hydrocoel may have been absent as in modern echinoderms.
Biology
Cinctans have been hypothesized to be
epibenthic
Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.suspension feeders, with a lifestyle similar to modern
tunicates
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one tim ...
. They likely rested on the seafloor parallel to the current, with their mouth and porta oriented downstream. They likely fed by actively pumping water through their pharynx.
Cinctans grew largely by the expansion of the plates. A small number of plates were added to the stele, and somewhat more to the dorsal and ventral surfaces, during growth, but the number of plates making up the cinctus remained constant throughout growth. Because of the asymmetric anatomy of cinctans, they likely underwent torsion when metamorphosing from a larva into an adult.
Classification
The classification of cinctans, like that of other carpoid echinoderms, is contentious. They have been hypothesized to be
blastozoa
Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. This subphylum ranged from the Cambrian to ...
ns, stem-group hemichordates, and stem-group echinoderms.
Phylogenetic analyses have found cinctans to be stem-group echinoderms, intermediate between ctenocystoids and solutans.
The internal phylogenetic relationships within cinctans have been difficult to study, due in part to their highly specialized anatomy making it difficult to determine their ancestral anatomy. However, three families are recognized, the Trochocystidae, Gyrocystidae, and Sucocystidae, as well as some basal genera which do not fit into any of the families.
Distribution
All cinctan fossils are from the Middle Cambrian. The earliest cinctan is ''Protocinctus'', which dates to Cambrian Stage 5, now known as the Wuliuan. Their diversity peaked during the
Drumian
The Drumian is a stage of the Miaolingian Series of the Cambrian. It succeeds the Wuliuan and precedes the Guzhangian. The base is defined as the first appearance of the trilobite '' Ptychagnostus atavus'' around million years ago. The top i ...
. The youngest cinctans are in the genera ''Undatacinctus'' and ''Sucocystis''. Cinctans died out just prior to the beginning of the
Furongian
The Furongian is the fourth and final epoch and series of the Cambrian. It lasted from to million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocian Stage. It is subdivided into three ...
epoch, during the
Guzhangian
The Guzhangian is an uppermost stage of the Miaolingian Series of the Cambrian. It follows the Drumian Stage and precedes the Paibian Stage of the Furongian Series. The base is defined as the first appearance of the trilobite '' Lejopyge laevig ...
age. The decline of cinctans was associated with a marine regression, and cinctans may have struggled to migrate as sea levels changed or run out of habitat due to the regression and cooling.
Cinctan fossils are found in the former continents of Siberia, Avalonia, and western Gondwana. They have been found in the Czech Republic, France, Germany, Italy, Morocco, Russia, Spain, and the United Kingdom.
History
The first cinctan named, ''Trochocystites'', was described in 1887. Cinctans were originally considered to be
cystoids
Cystoidea is a class of extinct crinozoan echinoderms, termed cystoids, that lived attached to the sea floor by stalks. They existed during the Paleozoic Era, in the Middle Ordovician and Silurian Periods, until their extinction in the Devo ...
, but in the early 20th century they were recognized as a distinct group. Because Cincta was originally described as an order, they were later assigned to their own subclass Homostelea, but Homostelea is now considered a junior synonym of Cincta.
Under the calcichordate interpretation of carpoids, cinctans were initially interpreted as basal stem-group echinoderms. Later, however, they, as well as ctenocystoids, were interpreted as possible stem-group
hemichordates
Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and includ ...