Chordata is one of three major living deuterostome phyla, the other extant phyla being Echinodermata, Hemichordata, and Xenoturbellida (collectively these other phyla are grouped as Ambulacraria).
In some Deuterostomes such as echinoderms, early development establishes a bilateral bodyplan that later takes on a bi radial symmetry as development precedes.
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It appears that dueterostomes, in general, have undergone genomic reorganizations relative to protostomes, leading one to speculate that these extensive genomic reogranizations may have played an important role in the radiation of deuterostomes after their divergence from protostomes.
Urochordates are known to have jettisoned significant portions of their genomes relative to the other branches of the ancestral chordate population resulting in highly degenerate genomes in some urochordate lines.
It seems that this trend continued to play an important part in the line leading to chordates.
Also of interest is the shift from determinative development to regulative development in embryonic development.
The origin of the craniates and vertebrates may have been preceeded by a period of progressive developmental neoteny in the anscestral chordate population.
Indeed, cephalachordates may represent a branch of this ancestral neotenous chordate population that diverged prior to the advent of the genome duplication event leading to vertebrates.
It would be of great interest to know whether the original genome duplication event occurred at the origin of the craniates or at the origin of the vertebrates.
It has been recently reported that the vertebrate genome has lost at least 11 gene families that are highly conserved in eukaryotes, plants, fungi, and invertebrate metazoans ( ).
The newly erected phylum Vetulicolia is believed to be an extinct phyla of Deuterostomes that may have had close relations to the chordates ( ).
Yunnanozoan lividum is a extinct deuterostome believed to be one of the oldest hemichordates.
The definitive features of Yunnanozoans are:
.jpg "Haikouella fossil reconstruction")
Haikouella lanceolata is an early Cambrian organism with chordate and craniate affinities ( ), although it´s exact status appears to be a matter of debate (, , ).
The ancestral chordates are considered to have given rise to subphyla urochordata, cephlachordata, and craniata/vertebrata.
The Chordate bodyplan possesses the following features at some point during their lifecycle:
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The urochordate Shankouclava is the oldest chordate to appear in the fossil record - dating to the earliest Cambrian approximately 543 million years ago ( ).
These South China Sea fossils demonstrate the antiquity of the urochordate lifestyle as a primarily sessile filter-feeder that strains food substances from water pumped thru the pharyngeal gill apparatus.
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It is generally thought that tunicate urochordates are the modern chordates that are most likely to resemble the primordial chordate ancestor of craniates and vertebrates.
Tunicates possess a benethic or somatic larva that is characterized by the following chordate features:
The larval tunicate does not feed and exists for only a few hours as it swims about in search of a suitable place to settle down and metamorphosize into the adult form.
Since the primary function of the somatic larva is for dispersal, i.e. to find a suitable environmental niche within which it can settle down and metamorphosize to the mature adult bodyplan, swimming behavior along environmentally contingent gradients comprised of gravitational, luminosity, pressure, and/or chemical factors is assumed to be the primitive mode of behavior for the somatic larva ( ).
Thurston Lacalli proposes that this suggests that escape and engagement behaviors in chordates are cephalochordate and vertebrate innovations.
In Ascidians, the neural tube consists of approximately 370 cells of which less than 100 cells are neurons.
The dorsal nerve cord, sensory placodes and the integrating neuromuscular architecture of the somatic larva will be considered as a starting point for understanding the orginal organization of vertebrate central nervous system.
The sensory placodes and their integrating neuroarchitecture will be assumed to be reminiscent of the original vertebrate cerebral vesicle.
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Tunicate adults are sessile or visceral animals characterized by:
The adult tunicate bodyplan will be considered as the prototype of the vertebrate pharyngeal and enteric systems.
Adult Tunicates may be either solitary, colonial, or compound.
In compound ascidians all individuals share a common tunic.
The larval and adult bodyplans are sequentially expressed within the life-cycle of each individual, with the juvenile somatic bodyplan preceding the adult visceral bodyplan.
The transition from the somatic body-plan to the visceral bodyplan is accomplished via the processes of metamorphosis upon settlement upon a suitable environmental substrate.
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I will assume that both anatomical organizations (corresponding to the larval and adult tunicate bodyplan) most closely approximates the anatomical structure(s) relevant to understanding the condition of the ancestral vertebrate bodyplan.
.jpg "The earliest Cephalachordate - Cathaymyrus Diadexus")
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The oldest know member of the Cephalachordates is Cathaymyrus diadexus from Chengjiang, China ( ).
Cathaymyrus lived during the early Cambrian about 530 million years ago and predates Pikaia from the Burgess Shale by about 10 million years.
The emergence of Cephalachordates appears to be a consequence of paedomorphosis in the ancestral chordate population - where the adult organism retains the juvenile bodyplan and proceeds to sexual maturity without undergoing metamorphosis to the adult bodyplan.
It has long been speculated that Cephalachordates are the likely ancestors of vertebrates due to the similarity of their bodyplan to that of vertebrates.
These speculations have recently quieted down due to the emergence of multiple lines of evidence indicating that vertebrates are more directly derived from urochordates and that the Cephalachordates represent an independent line of descent from the same or a close common ancestor of the craniates and vertebrates.
Nonetheless, the cephalochordate bodyplan does offer intriguing clues to the consequences of paedomorphosis in the ancestral chordate population and a reference point for understanding the primitive state of the craniate bodyplan.
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The Cephalochordates possess all chordate features plus:
Although Cephalachordates have a notochord and dorsal nerve cord - they possess no bone or cartilage and therefore have no vertebrae or cranium.
Unlike vertebrates, the notochord of Cephalachordates extends beyond the nerve cord and cerebral vesicle and they have no head.
They do possess anterior sensory structures.
They also do not possess the red corpuscles in the blood that are endemic to vertebrates.
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