Calcichordate Theory
Encyclopedia
The Calcichordate Theory holds that each separate lineage of chordate
(Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate
, and thus the echinoderm
s and the chordates are sister groups, with the hemichordates as an out-group.
It was formulated by British Museum
paleontologist Richard Jefferies
.
are grouped together in a clade
called Calcichordata. Cornutes and mitrates are viewed as sister groups, and mitrates represent stem group chordate
s. The mitrates (and thus the chordates) are all Dexiothetes
, dexiothetism being a synapomorphy
for the clade.
. All chordates share a common ancestor which lost its echinoderm
stereom
calcite
skeleton
. 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 group.
The central part of the Calcichordate Theory lies in the interpretation of the phylogeny of the two groups of stylophorans, 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. Mitrates are thought to have formed their tail from the proximal 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 left-hand side, with the right hand side a novel feature. This would explain the bizarre embryology
of Amphioxus, a basal cephalochordate widely held to be the prime example of a chordate bauplan.
. The large orifice seen is most likely the mouth, with many of the slits along the side assumed to be gill slit
s. While the Cornuta were interpreted as lying with the flat side ventrally, Jefferies suggested that in Mitrata the flat side was dorsal and the convex side ventral, while the tail was curved underneath to provide forward thrust
; many mitrates are preserved with the tail underneath.
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.
with Pikaia
, which is around the same time that carpoids are found, although the mitrates may not be stem group.
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
(Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate
Mitrate
Mitrates are a group of stem group Echinoderms, which may be closely related to the hemichordates.-Morphology:The organisms were a few millimetres long. Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite...
, and thus the echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s and the chordates are sister groups, with the hemichordates as an out-group.
It was formulated by British Museum
British Museum
The British Museum is a museum of human history and culture in London. Its collections, which number more than seven million objects, are amongst the largest and most comprehensive in the world and originate from all continents, illustrating and documenting the story of human culture from its...
paleontologist Richard Jefferies
Dick Jefferies
Richard P.S. Jefferies was a paleontologist famous for developing the Calcichordate Theory of the origin of chordates, now widely discredited. Jefferies joined the British Museum in 1960, and was largely based there for the remainder of his career....
.
Details
The carpoids Cornuta and MitrataMitrate
Mitrates are a group of stem group Echinoderms, which may be closely related to the hemichordates.-Morphology:The organisms were a few millimetres long. Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite...
are grouped together in a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
called Calcichordata. Cornutes and mitrates are viewed as sister groups, and mitrates represent stem group chordate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s. The mitrates (and thus the chordates) are all Dexiothetes
Dexiothetism
Dexiothetism refers to a reorganisation of a clade's bauplan, with right becoming ventral and left becoming dorsal. The organism would then recruit a new left hand side.-Details:...
, dexiothetism being a synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
for the clade.
Hypothetical Phylogeny
In the evolution of the chordates, the ancestors of the chordates underwent a profound remodeling of their bauplan, becoming dexiotheteticDexiothetism
Dexiothetism refers to a reorganisation of a clade's bauplan, with right becoming ventral and left becoming dorsal. The organism would then recruit a new left hand side.-Details:...
. All chordates share a common ancestor which lost its echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
stereom
Stereom
Stereom is a calcium carbonate material that makes up the internal skeletons found in sea urchins, and all other 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 being a matrix of calcite...
calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
. 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 group.
The central part of the Calcichordate Theory lies in the interpretation of the phylogeny of the two groups of stylophorans, 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. Mitrates are thought to have formed their tail from the proximal 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 left-hand side, with the right hand side a novel feature. This would explain the bizarre embryology
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
of Amphioxus, a basal cephalochordate widely held to be the prime example of a chordate bauplan.
Anatomy according to the Calcichordate view
The appendage of the carpoids is regarded as a tail, with the central canal probably containing a notochordNotochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
. The large orifice seen is most likely the mouth, with many of the slits along the side assumed to be gill slit
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, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s. While the Cornuta were interpreted as lying with the flat side ventrally, Jefferies suggested that in Mitrata the flat side was dorsal and the convex side ventral, while the tail was curved underneath to provide forward thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
; 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 synapomorphySynapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
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 CambrianCambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
with Pikaia
Pikaia
Pikaia gracilens is an extinct animal known from the Middle Cambrian Burgess Shale of British Columbia. -Discovery:It was discovered by Charles Walcott and was first described by him in 1911. It was named after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation...
, 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.See also
- Three-taxon problem
- Carpoids
- Cornutes
- MitrateMitrateMitrates are a group of stem group Echinoderms, which may be closely related to the hemichordates.-Morphology:The organisms were a few millimetres long. Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite...
s - StylophoraStylophoraThe stylophorans are an extinct, possibly polyphyletic group allied to the echinoderms, comprising the cornutes and mitrates. It is synonymous with the subphylum Calcichordata....
ns - EchinodermEchinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s