# Chordate
A chordate is an animal of the phylum Chordata, defined by possessing a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some stage of its life cycle, whether or not the structures persist into adulthood[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata). The phylum contains roughly 65,000 species in three living groups: the vertebrates (fishes, amphibians, reptiles, birds, and mammals), the tunicates (sea squirts, salps, and appendicularians), and the cephalochordates (lancelets or amphioxus)[2](https://www.nature.com/articles/nature04336). Chordates are deuterostomes, animals in which the anus forms before the mouth during embryonic development, and they share this larger group with the echinoderms and hemichordates[3](https://assets-global.website-files.com/67239b384a0382b73e416180/691a61bf91e0c0494127856c_fovowevezimesani.pdf) • [4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12125).
| Key fact | Detail |
|---|---|
| Defining features | A notochord, pharyngeal slits, a hollow dorsal nerve tube, and a postanal tail, present at some point in the life cycle[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata) |
| Fifth synapomorphy | An endostyle or its derivative, the thyroid gland, is often listed as a fifth chordate characteristic[3](https://assets-global.website-files.com/67239b384a0382b73e416180/691a61bf91e0c0494127856c_fovowevezimesani.pdf) |
| Living subgroups | Cephalochordates (lancelets), tunicates, and vertebrates, including lamprey and hagfish[2](https://www.nature.com/articles/nature04336) |
| Higher classification | Deuterostomes, alongside echinoderms and hemichordates (Ambulacraria)[4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12125) |
| Closest relatives of vertebrates | Tunicates, established by a phylogenomic dataset of 146 nuclear genes (33,800 aligned amino acids)[2](https://www.nature.com/articles/nature04336) |
| Divergence time | The three chordate groups diverged from each other about 550 million years ago[5](https://www.nature.com/articles/nrg2419) |
| Fossil record | Lancelets are modern survivors of an ancient lineage with fossils dating back to the Cambrian period[6](https://www.nature.com/articles/nature06967) |
## Defining features
**The four diagnostic characters** are shared by members of all the major chordate groups[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata). A firm yet pliant rod called the notochord extends along the body below the central nervous system and above the coelom, providing stiffness and structural support when the animal moves[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata) • [7](https://animaldiversity.org/accounts/Chordata/). Pharyngeal slits are lateral openings from the pharynx, the organ behind the mouth that forms part of the digestive tract[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata). The dorsal hollow nerve cord is a tube formed from ectoderm that rolls up, lying dorsal to the notochord, and usually bears an enlarged anterior end, the brain[7](https://animaldiversity.org/accounts/Chordata/) • [8](https://sist.sathyabama.ac.in/sist_coursematerial/uploads/SBC1201.pdf). The post-anal tail is an extension of the body posterior to the anus[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata).
Every chordate possesses these characteristics at some stage of life, although not all of them keep the structures as fully working features for their entire existence.[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata) In many vertebrates the notochord is almost completely replaced by blocks of bone forming the vertebrae[1](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata), and in tunicates the notochord is confined to the tail region of the larva[9](https://www.wbnsou.ac.in/UGC_Disclosure/SLM_Plagarism/UG_Zoology.pdf). Some treatments add a fifth characteristic, an endostyle or thyroid gland, giving five synapomorphies in total[3](https://assets-global.website-files.com/67239b384a0382b73e416180/691a61bf91e0c0494127856c_fovowevezimesani.pdf).
**Beyond the formal definition**, chordates share a broader body plan. According to the Animal Diversity Web account, chordates show bilateral symmetry, a segmented body including segmented muscles, three germ layers with a well-developed coelom, a ventral heart with dorsal and ventral blood vessels in a closed circulatory system, a complete digestive system, and usually a bony or cartilaginous endoskeleton[7](https://animaldiversity.org/accounts/Chordata/).
## The three living groups
The phylum is conventionally divided into three subphyla: Vertebrata (or Craniata), Urochordata (Tunicata), and Cephalochordata[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC4211455/) • [11](https://sciencereviews.info/files/SR_3.2.pdf).
**Cephalochordates**, the lancelets or amphioxus, retain the notochord from rostrum to tail throughout life, a character described as primitive[9](https://www.wbnsou.ac.in/UGC_Disclosure/SLM_Plagarism/UG_Zoology.pdf). Their body plan and genome resemble those of vertebrates more closely than do those of tunicates[12](https://pubmed.ncbi.nlm.nih.gov/33602487/), and amphioxus species evolve more slowly than the slowest-evolving vertebrate known, the elephant shark[13](https://www.sciencedirect.com/science/chapter/bookseries/pii/S0070215320301046).
**Tunicates** start as tadpole-like larvae and then lose the notochord, living a sessile life as adults in groups such as the sea squirts; the group also includes the pelagic salps and appendicularians[2](https://www.nature.com/articles/nature04336). Their genomes are highly rearranged and evolving especially rapidly compared with vertebrates[6](https://www.nature.com/articles/nature06967) • [13](https://www.sciencedirect.com/science/chapter/bookseries/pii/S0070215320301046).
**Vertebrates** include the jawless lamprey and hagfish as well as the jawed fishes and all tetrapods[2](https://www.nature.com/articles/nature04336). Their origin is associated with a fundamental embryological revolution and a doubling or quadrupling of the genome[4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12125).
## Phylogeny and the position of the groups
The relationships among the three groups were long debated. Until the mid-1990s, comparative adult morphology placed cephalochordates closer to the craniates than tunicates were[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC2615822/). Molecular phylogenetics overturned this: a 2006 phylogenomic analysis of 146 nuclear genes concluded that tunicates, not cephalochordates, are the closest living relatives of vertebrates[2](https://www.nature.com/articles/nature04336). Subsequent work with standard maximum-likelihood and Bayesian site-heterogeneous models gave unequivocal support for this sister-group relationship, named Olfactores[15](https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/dvg.20450), and the vertebrate-plus-tunicate clade with cephalochordates as sister lineage is now the framework used in early vertebrate evolution studies[4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12125).
Chordate monophyly itself was once questioned: one early molecular analysis failed to support a monophyletic Chordata, grouping urochordates with hemichordates[16](https://academic.oup.com/mbe/article-pdf/11/4/648/11167082/9mccl.pdf), and a 2006 study noted that cephalochordates grouped with echinoderms with non-significant statistical support, a hypothesis needing additional data[2](https://www.nature.com/articles/nature04336). Later phylogenomic work, including a study that revealed monophyletic chordates within the deuterostomes, resolved the phylum as a natural group[17](https://pubmed.ncbi.nlm.nih.gov/29653534/).
## Genomic insights into chordate origins
**The amphioxus genome**, sequenced by shotgun sequencing from a single male, is 520 Mb and contains 21,900 protein-coding genes, with the highest level of polymorphism yet found in a sequenced organism, implying a large effective population size[5](https://www.nature.com/articles/nrg2419). Comparative analysis identified 17 ancestral chordate linkage groups conserved in both modern amphioxus and vertebrate genomes despite over half a billion years of independent evolution[6](https://www.nature.com/articles/nature06967).
The vertebrate lineage underwent two rounds of whole genome duplication at its base; many duplicate genes were subsequently lost, but those coding for transcription factors and signaling proteins were preferentially retained[18](https://www.nature.com/articles/nrn2703). This genomic expansion underpinned vertebrate novelties such as the neural crest and cranial placodes, structures amphioxus lacks: amphioxus has only thousands of neurons, no neural crest, few placodes, and no midbrain-hindbrain organizer, yet comparisons of gene expression show that vertebrates built these features on a foundation already present in the ancestral chordate[18](https://www.nature.com/articles/nrn2703).
The recognition of chordates as a natural group has its own history. The Russian embryologist Alexander Kowalevski discovered in 1866 that the central rod in ascidian larvae is derived from a cellular structure similar to the vertebrate notochord, the finding that led to tunicates being grouped with vertebrates in the phylum Chordata[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC4632506/).
## References
1. [Chordata - an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/immunology-and-microbiology/chordata)
2. [Delsuc, F. et al. (2006). Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature 439, 965-968](https://www.nature.com/articles/nature04336)
3. [Anatomy of Chordates (zoology text, five chordate characteristics)](https://assets-global.website-files.com/67239b384a0382b73e416180/691a61bf91e0c0494127856c_fovowevezimesani.pdf)
4. [Donoghue, P. C. J. & Keating, J. N. (2014). Early vertebrate evolution. Palaeontology 57(5), 879-893](https://onlinelibrary.wiley.com/doi/10.1111/pala.12125)
5. [Goymer, P. (2008). Chordate origins. Nature Reviews Genetics 9, 571](https://www.nature.com/articles/nrg2419)
6. [Putnam, N. H. et al. (2008). The amphioxus genome and the evolution of the chordate karyotype. Nature 453, 1064-1071](https://www.nature.com/articles/nature06967)
7. [Myers, P. (2001). Chordata, Animal Diversity Web, University of Michigan](https://animaldiversity.org/accounts/Chordata/)
8. [Phylum Chordata, Sathyabama Institute course material](https://sist.sathyabama.ac.in/sist_coursematerial/uploads/SBC1201.pdf)
9. [NSOU Zoology study material, Protochordata summary](https://www.wbnsou.ac.in/UGC_Disclosure/SLM_Plagarism/UG_Zoology.pdf)
10. [Satoh, N. et al. (2014). Chordate evolution and the three-phylum system. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4211455/)
11. [Caputi, L. (2024). Tunicate genomics review, Science Reviews - Biology 3(2), 22-32](https://sciencereviews.info/files/SR_3.2.pdf)
12. [Transitional chordates and vertebrate origins: Tunicates (PubMed, 2021)](https://pubmed.ncbi.nlm.nih.gov/33602487/)
13. [Cephalochordates: A window into vertebrate origins, ScienceDirect](https://www.sciencedirect.com/science/chapter/bookseries/pii/S0070215320301046)
14. [Deciphering deuterostome phylogeny: molecular, morphological and palaeontological perspectives. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC2615822/)
15. [Delsuc, F. et al. (2008). Additional molecular support for the new chordate phylogeny. Genesis 46, 592-604](https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/dvg.20450)
16. [Deuterostome Phylogeny and the Sister Group of the Chordates, Molecular Biology and Evolution 11(4), 648](https://academic.oup.com/mbe/article-pdf/11/4/648/11167082/9mccl.pdf)
17. [Delsuc, F. et al. (2018). A phylogenomic framework and timescale for comparative studies of tunicates. BMC Biology 16, 39](https://pubmed.ncbi.nlm.nih.gov/29653534/)
18. [Holland, L. Z. (2009). Chordate roots of the vertebrate nervous system. Nature Reviews Neuroscience 10, 736-746](https://www.nature.com/articles/nrn2703)
19. [Tunicates: exploring the sea shores and roaming the open ocean. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4632506/)
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*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Other invertebrate lineages › Echinoderms and nonvertebrate chordates*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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