The calcichordate hypothesis holds that each separate lineage of

chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...

(

Cephalochordate A cephalochordate (from Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or prima ...

Urochordate 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 time cal ...

Craniate A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the muc ...

s) evolved from its own lineage of

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 echinoder ...

, and thus the

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 sea ...

s and the chordates are

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

s, with the

hemichordate 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 include ...

s as an out-group. It was formulated by

British Museum The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is among the largest and most comprehensive in existence. It docum ...

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

Richard Jefferies John Richard Jefferies (6 November 1848 – 14 August 1887) was an English nature writer, noted for his depiction of English rural life in essays, books of natural history, and novels. His childhood on a small Wiltshire farm had a great influ ...

. The hypothesis has been disproven as of 2019; exceptional preservation of soft tissues in the single appendage of the

stylophora The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a va ...

ns ''Thoralicystis'' and ''Hanusia'' revealed clear traces consistent with a water vascular system—an ambulacral canal with tube feet—covered by movable plates, where the calcichordate hypothesis would require the anatomy be a tail containing a notochord protected by fixed plates. The enlarged area at the base of the appendage, which in the calcichordate hypothesis would contain muscles to move the tail, contains an extension of the body cavity. The findings also disprove the hypothesis that the group were stalked echinoderms. Based on the internal anatomy, "calcichordates" (

ns) are revealed as echinoderms with a single, somewhat starfish-like arm that gathered food with tube feet and transferred it to a mouth at the base of the arm. The food-gathering arm may also have been used for locomotion, as in starfish. The fossils from the Bou Izargane ''

Lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

'' from the Lower Ordivician of Morocco were unknown to Jeffries; they had not been discovered when he formulated the hypothesis in the early 1960s based on the examination of a group of

fossils, and were still unknown when he wrote subsequent papers in 1981 and 1997 defending the hypothesis. Stylophorans are classed as echinoderms based on their possession of at least two shared and unique features (

apomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

) of the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

;

stereom Stereom is a calcium carbonate material that makes up the internal skeletons found in all echinoderms, both living and fossilized forms. It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest b ...

plates and 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 mov ...

. Because they show no sign of

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, take the face of a human being which has a pla ...

, the position of stylophorans within Echinodermata remains unresolved as of 2019.

Details

The

carpoid Homalozoa is an obsolete extinct subphylum of Paleozoic era echinoderms, prehistoric marine invertebrates. They are also referred to as carpoids. Description The Homalozoa lacked the typical pentamer body form of other echinoderms, but all were ...

Cornuta Cornuta is an extinct order of echinoderms. Along with the mitrates, they form one half of the Stylophora The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornu ...

and Mitrata are grouped together in a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

called

Calcichordata The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a v ...

. Cornutes and mitrates are viewed as sister groups, and mitrates represent

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. ...

s. The mitrates (and thus the chordates) are all Dexiothetes, dexiothetism being a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

for the clade.

Hypothetical phylogeny

In the evolution of the chordates, the ancestors of the chordates underwent a profound remodeling of their

bauplan A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually applied to animals, envisages a "bluepr ...

, becoming dexiothetetic. All chordates share a common ancestor which lost its

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

skeleton A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

. However, later revisions of the theory had each separate lineage losing its calcitic skeleton independently, as it evolved from its own mitrate ancestor, making the chordates a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group.

The calcichordate family tree in its original incarnation, with all chordates originating from a soft bodied ancestor. Later versions of the theory had each chordate group having a mitrate ancestor each, requiring three episodes of loss of stereom. Mitrates are assumed to have evolved from the cornutes.

The central part of the Calcichordate Theory lies in the interpretation of the

phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

of the two groups of

stylophoran The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a va ...

s, which are termed calcichordates in the theory. Mitrates (and the rest of the calcichordates that evolved from them) are dexiothetic as a synapomorphy, having evolved from a

cornute Cornuta is an extinct order of echinoderms. Along with the mitrate Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates. Along with the cornuta, cornutes, they form one half of the Stylop ...

. Mitrates are thought to have formed their tail from the

proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

part of the cornute tail, with the distal part atomised, and evolving new appendages. The left hand side in this scheme would be cognate with the

Pterobranch Pterobranchia is a Class (biology), class of small worm-shaped animals. They belong to the Hemichordata, and live in secretion, secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia a ...

left-hand side, with the right hand side a novel feature. This would explain the bizarre

embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, '' -logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and ...

Amphioxus The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochorda ...

, a basal

cephalochordate A cephalochordate (from Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or prima ...

widely held to be the prime example of a chordate

Anatomy according to the Calcichordate view

The appendage of the carpoids is regarded as a tail, with the central canal probably containing a

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...

. The large orifice seen is most likely the mouth, with many of the slits along the side assumed to be

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 con ...

s. While the Cornuta were interpreted as lying with the flat side

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

ly, Jefferies suggested that in Mitrata the flat side was

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

and the convex side ventral, while the tail was curved underneath to provide forward

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...

; many mitrates are preserved with the tail underneath.

Criticism

The calcichordate theory is not widely accepted as a viable theory on the origins of the chordates. Many cite its overall unparsimonius nature as unnecessary, however there are many specific points that can be raised. All in all, the carpoids are much closer to echinoderms than to chordates.

Loss of Stereom

Stereom calcite is considered to be a

of the echinoderms, and there is no evidence for it ever having evolved in any other lineage, and there is no crown group echinoderm that seems to have lost it secondarily. The theory implies either that it must have been lost once or three times, which is considered to be very unlikely.

Timing of appearance

Chordates are known to exist from the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

with ''

Pikaia ''Pikaia gracilens'' is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway ...

'', which is around the same time that carpoids are found, although the mitrates may not be stem group.

Genetic evidence

Genetic evidence has shown that Hemichordata is the sister group to Echinodermata.

References

{{reflist

External links

Abstract of review at Springerlink.com
Paleontological concepts and hypotheses Chordate zoology