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Chordate
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Not to be confused with Caudata, a group of amphibians.
Not to be confused with Cordate (disambiguation), Cordata (disambiguation), or Caudate (disambiguation).

Chordates
Temporal range: Cambrian Stage 3–Present, 525–0 Ma[1] (Possible Ediacaran record, 555 Ma[2])
LanceletChondrichthyesTunicateTetrapod

Clockwise: lancelet, tunicate, tiger, shark

Scientific classification Edit this classification
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel, 1874[3][4]
Subgroups

And see text

A chordate (/ˈkɔːrdeɪt/ KOR-dayt) is a bilaterian animal belonging to the phylum Chordata (/kɔːrˈdeɪtə/ kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.[8]

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates.[9] These CSIs provide molecular means to reliably distinguish chordates from all other animals.

Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds and mammals), whose notochords are replaced by a cartilaginous/bony axial endoskeleton (spine) and are cladistically and phylogenetically a subgroup of the clade Craniata (i.e. chordates with a skull); Tunicata or Urochordata (sea squirts, salps, and larvaceans), which only retain the synapomorphies during their larval stage; and Cephalochordata (lancelets), which resemble jawless fish but have no gills or a distinct head. The vertebrates and tunicates compose the clade Olfactores, which is sister to Cephalochordata (see diagram under Phylogeny). Extinct taxa such as the conodonts are chordates, but their internal placement is less certain. Hemichordata (which includes the acorn worms) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to the echinoderms, and together they form the clade Ambulacraria, the sister phylum of the chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia, although this called into doubt in a 2021 publication.[10]

Chordata is the third-largest phylum of the animal kingdom (behind only the protostomal phyla Arthropoda and Mollusca) and is also one of the most ancient animal taxa. Chordate fossils have been found from as early as the Cambrian explosion over 539 million years ago.[11] Of the more than 81,000 living species of chordates, about half are ray-finned fishes (class Actinopterygii) and the vast majority of the rest are tetrapods, a terrestrial clade of lobe-finned fishes (Sarcopterygii) who evolved air-breathing using lungs.[12]

Etymology

[edit]

The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation. Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization.[4]

Anatomy

[edit]
The glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.

Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[13]

There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:

1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
9 = space above pharynx
11 = pharynx
12 = vestibule
13 = oral cirri
14 = mouth opening
16 = light sensor
17 = nerves
19 = hepatic caecum (liver-like sack)
Anatomy of the cephalochordate Branchiostoma lanceolatum. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Classification

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The following schema is from the 2015 edition of Vertebrate Palaeontology.[17][18] The invertebrate chordate classes are from Fishes of the World.[19] While it is generally structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

Subphyla

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See also: List of chordate orders
Cephalochordate: lancelet. Pictured species: Branchiostoma lanceolatum

Cephalochordata: Lancelets

[edit]
Main article: Lancelet

Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[25] These burrowing filter-feeders compose the earliest-branching chordate subphylum.[26][27]

Tunicata (Urochordata)

[edit]
Main article: Tunicate (Urochordata)
Tunicates: sea squirts

The tunicates have three distinct adult shapes. Each is a member of one of three monophyletic clades. All tunicate larvae have the standard chordate features, including long, tadpole-like tails. Their larva also have rudimentary brains, light sensors and tilt sensors.[28]

The smallest of the three groups of tunicates is the Appendicularia. They retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of the other two groups.[29]

The other two groups, the sea squirts and the salps, metamorphize into adult forms which lose the notochord, nerve cord, and post anal tail. Both are soft-bodied filter feeders with multiple gill slits. They feed on plankton which they collect in their mucus.

Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus.[28] Most attach firmly to the sea floor, where they remain in one place for life, feeding on plankton.

The salps float in mid-water, feeding on plankton, and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies.[30]

The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[31] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[28]

Comparison of two invertebrate chordates
A. Lancelet, B. Larval tunicate, C. Adult tunicate
--------------------------------------------------------
1. Notochord, 2. Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill slit, 6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic

Craniata (Vertebrata)

[edit]
Main articles: Craniata and Vertebrata
Craniate: hagfish

Craniates all have distinct skulls. They include the hagfish, which have no vertebrae. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails".[32]

Most craniates are vertebrates, in which the notochord is replaced by the vertebral column.[33] It consists of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. Hagfishes have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[34] but they are members of the craniates, the group within which vertebrates are thought to have evolved.[35] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns.[36]

Before molecular phylogenetics, the position of lampreys was ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[37] However, molecular phylogenetics, which uses DNA to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[38] If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a clade, which has been named the Cyclostomata.[39]

Phylogeny

[edit]

Overview

[edit]
Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[40]

There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures.[41]

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic (meaning that the Chordata includes all and only the descendants of a single common ancestor, which is itself a chordate) and that the vertebrates' nearest relatives are tunicates. In 2016, identification of two conserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates also provided strong evidence of the monophyly of Chordata.[9]

All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include three species that are regarded as fish,[citation needed] and hence vertebrates. Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has proven difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" shows that many of the traditional classes are paraphyletic.[citation needed]

Deuterostomes
Diagram of the evolutionary relationships of chordates[14]

While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux.[42]

The majority of animals more complex than jellyfish and other cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates.[43] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.[44][45] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years ago, and hence well before the start of the Cambrian 538.8 million years ago.[43] Three enigmatic species that are possible very early tunicates, and therefore deuterostomes, were also found from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia, the sac-like Yarnemia ascidiformis, and one from a second new Ausia-like genus from the Onega Peninsula of northern Russia, Burykhia hunti. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.[46][47] Ausia and Burykhia lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.[47] The Russian Precambrian fossil Yarnemia is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of Ausia and Burykhia, so this identification has been questioned.[citation needed]

A skeleton of the blue whale, the largest animal, extant or extinct, ever discovered. Mounted outside the Long Marine Laboratory at the University of California, Santa Cruz. The largest blue whale ever reliably recorded measured 98ft (30m) long.
A peregrine falcon, the world's fastest animal. Peregrines use gravity and aerodynamics to achieve their top speed of around 242mph (390km/h), as opposed to locomotion.

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, 542 million years ago.[48] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[49] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[50][51] Another fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth.[51] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[40][52] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[53] On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[54] The best known and earliest unequivocally identified Tunicate is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming (South China).[55]

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected.[14] Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved.[56] Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives.[14][57]

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago.[57] However, molecular estimates of dates often disagree with each other and with the fossil record,[57] and their assumption that the molecular clock runs at a known constant rate has been challenged.[58][59]

Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.[citation needed]

A specific relationship between vertebrates and tunicates is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not cephalochordates, indicating vertebrates are more closely related to tunicates than cephalochordates.[9]

Cladogram

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Below is a phylogenetic tree of the phylum. Lines of the cladogram show probable evolutionary relationships between both extinct taxa, which are denoted with a dagger (†), and extant taxa.[60][61][62][63][64]

Chordata

Closest non-chordate relatives

[edit]
Acorn worms or Enteropneusts are example of hemichordates.

The closest relatives of the chordates are believed to be the hemichordates and Echinodermata, which together form the Ambulacraria. The Chordata and Ambulacraria together form the superphylum Deuterostomia.

Hemichordates

[edit]
Main article: Hemichordate

Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord.

There are two living groups of hemichordates. The solitary enteropneusts, commonly known as "acorn worms", have long proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though seafloor sediments. Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct graptolites, colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.[65]

Echinoderms

[edit]
A red knob sea star, Protoreaster linckii is an example of an asterozoan echinoderm.
Main article: Echinoderm

Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry only as larvae – in adulthood they have radial symmetry, meaning that their body pattern is shaped like a wheel; they have tube feet; and their bodies are supported by dermal skeletons made of calcite, a material not used by chordates. Their hard, calcified shells keep their bodies well protected from the environment, and these skeletons enclose their bodies, but are also covered by thin skins. The feet are powered by another unique feature of echinoderms, a water vascular system of canals that also functions as a "lung" and surrounded by muscles that act as pumps. Crinoids are typically sessile and look rather like flowers (hence the common name "sea lilies"), and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few species can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example starfish and brittle stars, sea urchins and sea cucumbers.[66]

See also

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Notes

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Chordate

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from Grokipedia
Chordates (phylum Chordata) are a diverse group of animals distinguished by four key anatomical features present at some point in their life cycle: a flexible, supportive ; a dorsal, hollow cord; pharyngeal slits or pouches; and a muscular post-anal . These synapomorphies unite the , which derives its name from the Greek word "chordē," meaning string, referring to the . The provides structural support along the dorsal side and is typically replaced by a vertebral column in vertebrates, while the cord develops into the , often with an anterior ; pharyngeal slits function in filter feeding, respiration, or embryonic development; and the post-anal aids in locomotion. The Chordata encompasses approximately 72,000 described (as of 2024), predominantly vertebrates such as fishes, amphibians, reptiles, birds, and mammals, including humans, alongside two smaller invertebrate subphyla: Urochordata ( or sea squirts, about 3,000 ) and Cephalochordata (lancelets, around 30 ). Urochordates are marine, sessile or planktonic with a only in their larval stage, while cephalochordates are small, burrowing marine animals retaining the throughout adulthood. Vertebrates, the most speciose , feature a backbone enclosing the nerve cord and dominate terrestrial, aquatic, and aerial environments, with adaptations like jaws, paired appendages, and advanced sensory systems driving their evolutionary success. Chordates exhibit a closed with a heart in most , segmented body muscles (myomeres), and development where the anus forms before the mouth. This phylum's evolutionary radiation began in the Cambrian period, with fossil evidence from over 500 million years ago, leading to ecological roles ranging from primary producers in aquatic food webs to apex predators and decomposers on land.

Introduction and Etymology

Definition and Diagnostic Traits

The phylum Chordata constitutes a major within the superphylum Deuterostomia, distinguished by four key synapomorphies that appear at some stage during the life cycle of its members: a , a dorsal hollow cord, pharyngeal slits, and a post-anal tail. These shared derived traits define the , which includes over 65,000 species ranging from forms to complex vertebrates, and reflect an evolutionary innovation in organization among deuterostomes. The is a flexible, rod-like structure composed of vacuolated cells embedded in an elastic , situated along the dorsal midline to provide axial support and enable bending during locomotion. It functions as a in early developmental stages across all chordates and induces formation, but in vertebrates, it typically regresses post-embryonically, being replaced by the vertebral column derived from surrounding . In contrast, it persists throughout adulthood in basal chordates such as lancelets ( spp.), extending nearly the full body length to maintain structural integrity. The dorsal hollow nerve cord forms as a fluid-filled tube along the dorsal surface, arising from ectodermal during , and serves as the precursor to the , with anterior expansions developing into the in more derived forms. This arrangement contrasts with the ventral, solid nerve cords of protostomes and is a hallmark of neural organization. Pharyngeal slits consist of perforations in the lateral walls of the , originally aiding suspension feeding by allowing water to enter and exit while trapping food particles; in embryos, they derive from endodermal outpocketings and are universal among chordates, though their adult persistence and function vary, such as in respiration for aquatic species. The post-anal tail projects beyond the anus and incorporates segmental muscle blocks (myomeres) innervated by the nerve cord, facilitating undulatory swimming or burrowing movements. While transient in the larval stages of many chordates—such as the tadpole-like larvae of urochordates—it remains prominent in adults of basal groups like lancelets, where it powers sinusoidal body motions for filter-feeding in . Across the , these traits often manifest most completely during embryonic or larval phases, underscoring the chordate body's conserved developmental blueprint despite diverse adult morphologies.

Etymology and Historical Context

The term Chordata derives from the Greek word khordē (χορδή), meaning "cord," "string," or "gut," in reference to the , a flexible, rod-like structure that serves as a defining embryonic feature of the . The name was first proposed by in 1866 to denote a group encompassing and vertebrates (including lancelets) based on shared anatomical traits, though it gained formal taxonomic recognition through in 1885. Early recognition of chordates centered on vertebrates, with Jean-Baptiste Lamarck introducing the concept of "animaux à vertébrés" (vertebrate animals) in 1801 to describe mammals, birds, reptiles, and fish as a unified group distinguished by their spinal columns. Georges Cuvier advanced this in 1812 by establishing Vertebrata as an embranchement (major division) within the animal kingdom, emphasizing functional anatomy and separating them from invertebrates. Throughout the 19th century, debates raged over the boundaries between vertebrates and invertebrates, particularly concerning tunicates (ascidians) and lancelets; Lamarck initially classified tunicates as a subclass of mollusks in 1816 due to their sessile adult forms and test-like coverings, while Cuvier treated them as a distinct class (Tunicata) but without linking them to vertebrates. A pivotal shift occurred in 1866 when Alexander Kowalevsky demonstrated through embryological studies that ascidian larvae possess a , dorsal hollow nerve cord, and pharyngeal slits—traits homologous to those in embryos—prompting their reclassification as proto-chordates rather than mollusks. Similar investigations by Francis M. Balfour on lancelets () in the 1880s revealed comparable embryonic features, solidifying their inclusion alongside and in the Chordata. These embryology-based insights challenged earlier adult-morphology-focused views and expanded the phylum beyond traditional vertebrates. Classification evolved from Linnaean hierarchies, which relied on observable adult similarities to organize vertebrates into classes within broader animal divisions, to 20th-century cladistic methods pioneered by Willi Hennig, which prioritize synapomorphies—shared derived traits like the present across life stages—to define monophyletic groups. This cladistic framework, widely adopted since the 1970s, underscores the evolutionary unity of Chordata by reconstructing phylogenies based on common ancestry rather than superficial resemblances.

Anatomy and Physiology

General Anatomical Features

Chordates exhibit a bilaterian body plan characterized by bilateral symmetry and triploblastic organization, with a distinct anteroposterior axis and cephalization in more derived forms. This plan often includes metameric segmentation, particularly evident in the arrangement of muscle blocks known as myomeres, which facilitate coordinated body movements. The notochord, a flexible rod-like structure composed of vacuolated cells surrounded by a fibrous sheath, runs along the dorsal midline and serves as a primary skeletal element, providing hydrostatic support and a foundation for the development of an endoskeleton in vertebrates through the formation of vertebral precursors. In basal chordates, the notochord persists as the main axial support, enabling elongation and flexibility essential for locomotion. The in chordates varies but shares a basic pattern of fluid along the body axis. In , it is an open lacking true capillaries, where a simple tubular heart exhibits reversing peristaltic contractions to circulate through lacunae and sinuses. In cephalochordates, it is a without a but with a contractile subintestinal vessel that propels colorless blood forward in ventral vessels and backward in the dorsal . Vertebrates, in contrast, possess a with a multi-chambered heart that evolved from this ancestral pulsatile tube, ensuring efficient oxygen and nutrient delivery under higher pressures. Respiratory adaptations center on pharyngeal slits, perforations in the pharyngeal wall that originally facilitated filter-feeding by creating a current for particle capture, while also enabling through diffusion across thin epithelia in aquatic environments. The features segmental myomeres—W-shaped blocks of striated muscle separated by myosepta—that contract sequentially to produce undulating waves for , a mechanism conserved across chordate lineages for efficient . Sensory and support structures derive primarily from the dorsal hollow nerve cord, a tubular assemblage of neural tissue that forms the and contrasts with the ventral solid nerve cords of other bilaterians. In basal chordates, this cord lacks extensive anterior enlargement but includes sensory receptors for light, pressure, and chemical cues, with precursors to regions emerging at the anterior end to integrate environmental signals. The additionally supports formation by providing midline signaling cues during development. These anatomical features integrate to support key physiological functions across diverse habitats. The notochord and myomeres together enable powerful , allowing chordates to navigate marine and freshwater environments with energy-efficient generation. Pharyngeal slits not only aid in suspension feeding by trapping but also contribute to through selective ion transport, maintaining internal balance in varying salinities. The coordinates these processes, linking sensory input to motor output for adaptive behaviors such as predator avoidance and resource acquisition.

Developmental Biology

Chordate development is characterized by early embryonic processes that establish the defining traits of the , beginning with where the blastula invaginates to form the three germ layers: , , and . During , the forms as the primary gut cavity, with mesodermal cells ingressing to position precursors for the and somites. In model chordates like the ascidian , this stage involves rapid cell divisions transitioning to morphogenetic movements, setting the stage for axial elongation. follows, where the dorsal thickens into a that invaginates to form the , the precursor to the ; this process is conserved across chordates and relies on precise coordination of progression, including a prolonged in epidermal cells to facilitate tube closure without disrupting morphogenesis. The arises from axial induced during , serving as a signaling center that induces formation through secreted factors. Pharyngeal development in chordates involves the evagination of endodermal pouches from the anterior , which perforate to form pharyngeal slits, a process regulated by conserved genes such as Pax1/9, Eya, Six, and Tbx1. These slits emerge as simple perforations in basal chordates like amphioxus, providing via endodermal tissues, and represent an ancestral feature adapted for filter-feeding and respiration. Concurrently, development proceeds through somitogenesis, where paraxial segments into somites along the posterior axis, contributing to the post-anal 's musculature and skeletal elements; in vertebrates, this involves a presomitic mesoderm clock driven by Notch and Wnt signaling, while cephalochordates exhibit similar segmentation from tailbud mesoderm. The post-anal extends beyond the , formed by heterogeneous cell populations in the tailbud, including , , and myotomes, enabling propulsion in larval stages. Central to these processes are key genetic pathways, including clusters that direct anterior-posterior (A-P) patterning through collinear expression along the body axis, specifying regional identities in the and from onward; this combinatorial code is conserved across chordates and modulated by gradients. The T-box Brachyury (Bra) plays a pivotal role in notochord specification, acting downstream of vegetal FGF signaling to activate notochord genes in synergy with factors like Foxa.a, forming a feed-forward regulatory network essential for mesodermal differentiation into notochord tissue. Developmental strategies vary among chordate subphyla, with urochordates exhibiting indirect development featuring a free-swimming larva that undergoes into a sessile , involving hormone-mediated remodeling where chordate traits like the and tail are resorbed. In contrast, cephalochordates display direct development without a pronounced larval phase or , progressing smoothly from to juvenile while retaining chordate features throughout. These differences highlight evolutionary adaptations in life history, with likely ancestral to chordates but modified or lost in lineages like vertebrates.

Classification and Taxonomy

Taxonomic Framework

The phylum Chordata is classified within the superphylum Deuterostomia, alongside the phyla Echinodermata and , based on shared developmental features such as radial cleavage and enterocoely. This phylum encompasses approximately 80,000 described species as of 2025, predominantly marine but with significant terrestrial and freshwater diversity. Chordata is traditionally divided into three subphyla—Cephalochordata, Urochordata, and Vertebrata—differentiated primarily by the persistence and extent of the , as well as the presence or absence of a developed head structure. In Cephalochordata and Urochordata, classified as non-craniates, the is either limited to the tail region or transient during larval stages, and a distinct head is lacking; in contrast, Vertebrata, the craniates, feature a that is largely replaced by a vertebral column and includes a well-developed cranium enclosing the . Within the subphylum Vertebrata, taxonomic ranks include the paraphyletic superclass , comprising jawless fishes such as lampreys and hagfishes, and the monophyletic superclass , which includes all jawed vertebrates from cartilaginous fishes to tetrapods. Contemporary chordate classification has shifted toward cladistic methods, emphasizing monophyletic groups defined by shared derived (synapomorphic) traits and phylogenetic analyses, rather than solely traditional morphological hierarchies that sometimes grouped unrelated forms. Molecular data from genomic sequencing has prompted recent nomenclature revisions, notably the establishment of the clade, which groups Urochordata and Vertebrata as sister taxa to the outgroup Cephalochordata, supported by evidence from patterns and conserved developmental genes.

Major Subphyla Overview

The chordate phylum is divided into three major subphyla: Cephalochordata, Urochordata, and Vertebrata, each exhibiting distinct defining features, levels of diversity, and ecological contributions while sharing core chordate traits like a , dorsal hollow nerve cord, pharyngeal slits, and post-anal tail at some life stage. These subphyla represent a spectrum from simple, forms to complex vertebrates, with basal groups primarily confined to marine environments where they serve as key filter-feeders in benthic and pelagic ecosystems, facilitating nutrient cycling and supporting food webs. In contrast, Vertebrata dominates diverse habitats, including terrestrial ones, through adaptive radiations that underpin global biodiversity and ecological stability. Cephalochordata, comprising approximately 30 species as of 2025, includes small, fish-like filter-feeders such as the lancelet Branchiostoma, which inhabit shallow marine sediments worldwide. These organisms retain a persistent notochord throughout life without developing vertebrae, emphasizing their primitive chordate morphology. Ecologically, cephalochordates play a vital role as benthic filter-feeders, siphoning plankton and organic particles to contribute to sediment aeration and nutrient remineralization in coastal ecosystems. Urochordata encompasses around 3,000 of as of 2025, featuring sessile ascidians that attach to substrates and planktonic that drift in open waters. Characteristic of this subphylum are tadpole-like larvae with a temporary and the secretion of a tunic for protection, which is lost or modified in adults. Urochordates function as efficient marine filter-feeders, clearing water of microorganisms and serving as prey for higher trophic levels, thus maintaining water quality and supporting pelagic food chains. Vertebrata, the most diverse with approximately 70,000-75,000 as of 2025, is defined by the presence of a cranium enclosing the and cells that give rise to diverse cell types, spanning jawless fishes like lampreys to mammals. Unlike the other , the is largely replaced by a vertebral column in adults, enabling structural support for active lifestyles. dominate terrestrial and aquatic habitats, fulfilling roles from primary consumers in oceans to apex predators on land, driving ecosystem dynamics through predation, , and . Comparatively, Urochordata and Vertebrata lose the in adulthood, contrasting with the persistent structure in Cephalochordata, which underscores evolutionary shifts toward specialization. Basal subphyla like Cephalochordata and Urochordata are overwhelmingly marine, reinforcing oceanic , while Vertebrata's expansion into terrestrial realms highlights their adaptive versatility across environments.

Diversity of Subphyla

Cephalochordata

Cephalochordates, commonly known as lancelets or amphioxus, are small, benthic marine animals that exemplify the primitive chordate body plan, retaining key diagnostic traits such as a , , pharyngeal slits, and post-anal throughout their lives. Their morphology features a slender, translucent, fish-like body, typically 5-8 cm long, with a pointed rostrum at the anterior end and a tapered , lacking paired fins, jaws, or a distinct head. The body is laterally compressed, segmented by V-shaped myomeres, and includes an atrium enclosing the with over 100 gill slits for filter-feeding. Feeding occurs through an oral hood equipped with cirri that act as an incurrent siphon to draw in containing , while traps particles on the gill slits, and exits via an excurrent atriopore near the . Lancelets inhabit shallow, coastal marine environments worldwide, from tropical to temperate regions, where they burrow tail-first into sandy or muddy sediments, leaving only their anterior end exposed for feeding. Densities can reach up to 5,000 individuals per square meter in suitable substrates, such as those in the or Mediterranean. Ecologically, they serve as detritivores and filter-feeders, recycling nutrients in benthic communities by processing and , and they form a food source for larger marine organisms and, in some Asian regions, for consumption. Their global distribution spans three genera—Branchiostoma, Asymmetron, and Epigonichthys—with about 30-35 described species adapted to soft-bottom habitats. Reproduction in cephalochordates is gonochoristic, with separate sexes predominant, though rare hermaphroditism occurs in some populations; spawning is seasonal, typically in spring or summer, triggered by lunar cycles and occurring at . Gametes are released into the water column for , yielding free-swimming, planktonic larvae that resemble miniature adults and drift for weeks to months before metamorphosing and settling into the sediment as juveniles. Lifespans vary by species, ranging from 2-3 years in Branchiostoma floridae to 5-8 years in B. lanceolatum. As the sister group to the (urochordates and vertebrates) within Chordata, cephalochordates like Branchiostoma floridae serve as key model organisms for , particularly through genome sequencing that illuminates the ancestral chordate and the origins of innovations. The sequencing of the amphioxus revealed a lack of whole-genome duplications seen in , highlighting conserved families and regulatory elements that underpin chordate evolution. Subsequent studies, including CRISPR-based editing, have leveraged this to explore primitive neural and mesodermal patterning, providing insights into how vertebrate-specific traits arose from a basal chordate .

Urochordata

Urochordata, commonly known as , represent a diverse group of within the phylum Chordata, characterized by their sac-like bodies enclosed in a protective outer covering. This encompasses approximately 3,000 , predominantly found in oceanic environments from shallow coastal waters to the . Tunicates exhibit a range of lifestyles, from sessile to planktonic, and play significant roles in marine ecosystems as . Their classification into three primary classes—Ascidiacea, , and Larvacea—reflects distinct adaptations to these habitats. The class , often referred to as sea squirts, comprises the majority of species and features sessile adults that attach to substrates such as rocks, docks, or seafloor debris. These organisms typically form solitary individuals or colonial aggregates, with adults reaching sizes from a few millimeters to over 10 centimeters. In contrast, includes pelagic forms like salps, doliolids, and , which are gelatinous, barrel-shaped swimmers that propel themselves through jet-like contractions of their bodies. Larvacea, also known as Appendicularia, consists of small, ic species that retain a larval-like morphology throughout life, secreting a mucous "house" for feeding and protection. These classes highlight the subphylum's ecological versatility, with dominating benthic communities and and Larvacea contributing to open-ocean . A defining feature of urochordates is the , an outer secretion composed primarily of —a rare among animals—embedded with proteins, sulfated , and other compounds, providing structural support and defense against predators and . This encases the body, leaving openings for two : the oral siphon for ingesting water and the atrial siphon for expelling filtered water and waste. Water enters via the oral siphon, passes through the lined with slits for particle capture, and exits the atrial siphon, facilitating suspension feeding on and . Notably, the characteristic chordate traits—, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail—are prominent only in the free-swimming tadpole-like , which undergoes to a sessile or pelagic adult, often losing the and tail. Reproduction in Urochordata combines asexual and sexual strategies, enhancing dispersal and population resilience. via is prevalent in colonial , where new individuals develop from body tissues, forming interconnected zooids that share a common ; this process allows rapid expansion in favorable environments. involves hermaphroditic adults releasing gametes into the water, with fertilization yielding larvae that swim for hours to days before settling and metamorphosing. These larvae, equipped with sensory organs for substrate detection, ensure wide dispersal across marine currents. often alternate between sexual and asexual generations, with chains of budded individuals in asexual phases, while Larvacea reproduce sexually but maintain a perpetual larval form. Ecologically, tunicates serve as key grazers in marine food webs, filtering vast quantities of and —up to thousands of liters of water per individual daily—transferring to higher trophic levels as prey for , seabirds, and . However, many species, particularly invasive , act as fouling organisms, adhering to ship hulls, gear, and artificial structures, which increases drag, damages equipment, and facilitates non-native species spread via . Such biofouling can disrupt local ecosystems by outcompeting native and altering benthic community structure. Urochordata, together with Vertebrata, form the Olfactores, sister to Cephalochordata in chordate phylogeny, underscoring their evolutionary significance.

Vertebrata

Vertebrata, also known as Craniata, represent the most diverse and advanced within Chordata, encompassing animals that possess a cranium or enclosing the , a defining feature that distinguishes them from other chordates. This , composed of or bone, provides protection for the and supports sensory organs, marking a key evolutionary innovation in craniate development. Additionally, vertebrates are characterized by the presence of cells, a unique embryonic cell population that arises from the dorsal and migrates to form critical craniofacial structures such as bones, , ganglia, and pigment cells. Unlike the persistent notochord in other chordates, the vertebrate is largely replaced by a vertebral column, a segmented series of bony or cartilaginous vertebrae that encases and protects the while providing structural support for the body. These features collectively enable greater , or concentration of sensory and neural tissues in the head, facilitating complex behaviors and adaptations. The major groups within Vertebrata are broadly divided into jawless and jawed forms, reflecting significant evolutionary transitions. Jawless vertebrates, classified as cyclostomes, include lampreys and , which retain primitive traits such as a cartilaginous and lack paired fins, but possess a cranium and rudimentary vertebrae. Jawed vertebrates, or gnathostomes, comprise the vast majority and include cartilaginous fishes (e.g., and rays), bony fishes (osteichthyans), and tetrapods, with jaws evolving from arches to enable efficient feeding and predation. Among gnathostomes, a pivotal transition occurred in sarcopterygians (lobe-finned fishes), where robust fins with internal bones gradually evolved into weight-bearing limbs, allowing early tetrapods like amphibians to venture onto land during the period. Further diversification led to amniotes—reptiles, birds, and mammals—which developed amniotic eggs and other adaptations for fully terrestrial life, including waterproof skin and efficient lungs. Vertebrate diversity spans aquatic, terrestrial, and aerial environments, with approximately 66,000 described demonstrating remarkable adaptability. Aquatic vertebrates, primarily fishes, dominate in number and inhabit diverse marine and freshwater ecosystems, while terrestrial forms include amphibians that bridge aquatic and habitats, reptiles adapted to arid conditions, and endothermic birds and mammals that achieve aerial flight or high-energy lifestyles. Sensory advancements underpin this radiation, including paired eyes with image-forming capabilities derived from lateral eye fields, enabling stereoscopic vision and color perception in many , and sophisticated inner ears that provide balance, hearing, and orientation through and otoliths. These sensory innovations, honed over millions of years, support navigation, foraging, and predator avoidance across habitats. As the dominant chordate subphylum, vertebrates hold profound economic and medical importance to humans, serving as sources of through fisheries and that support global and . Medically, non-human vertebrates are essential model organisms in biomedical research, contributing to breakthroughs in understanding diseases, developing vaccines, and advancing treatments for conditions like cancer and via studies on and .

Phylogeny and Evolution

Phylogenetic Relationships

The phylogenetic relationships within Chordata are characterized by a basal position for Cephalochordata, with —comprising Urochordata () and —as its sister clade. This structure positions cephalochordates, such as amphioxus, as the outgroup to the more derived , reflecting an early divergence that shapes the evolutionary tree of living chordates. Recent genomic sequencing of the early-diverging cephalochordate Asymmetron amphioxus (2025) further supports this basal position, offering insights into ancestral chordate genome organization. Within , cyclostomes (lampreys and ) form a monophyletic group at the base, sister to the gnathostomes (jawed ), supported by genomic analyses revealing shared ancestral features like the absence of certain gnathostome-specific innovations. This underscores the progression from simple lancelet-like forms to the complex vertebrate lineage. Molecular phylogenomics provides robust evidence for these relationships, drawing from large-scale datasets including 18S rRNA sequences and multigene analyses that consistently recover Cephalochordata as sister to with high bootstrap support. clusters further corroborate this topology, as cephalochordates retain a single, intact cluster resembling the ancestral chordate condition, while and vertebrates exhibit duplications and rearrangements indicative of their closer affinity. Morphological synapomorphies, such as pharyngeal pouches and slits, unite all chordates but show derived modifications in , like the elaboration of these structures into gills or jaws in vertebrates, reinforcing the molecular tree. Prior to widespread molecular data in the late and , alternative hypotheses prevailed based on morphology, often placing Urochordata as the basal chordate group or even aligning them more closely with non-chordate due to their sessile adult forms and apparent simplicity. These views, rooted in 19th- and early 20th-century embryological studies, suggested Cephalochordata as sister to Vertebrata, with as a primitive offshoot; however, phylogenomic studies using nuclear genes and mitochondrial genomes resolved as vertebrate sisters, highlighting secondary simplifications in urochordate body plans. This emphasized the derived nature of morphology, aligning with evo-devo insights. Divergence time estimates, calibrated via molecular clocks and fossil constraints, place the origin of crown-group chordates in the early , approximately 550 million years ago (Ma), coinciding with the split between Cephalochordata and . The subsequent radiation within occurred between 550 and 520 Ma, setting the stage for diversification amid the of animal life. These timelines integrate relaxed clock models with paleontological data, providing a framework for understanding chordate evolutionary dynamics.

Evolutionary History and Fossil Record

The earliest known fossils suggestive of chordates date to the Early Cambrian Chengjiang biota in Province, , approximately 520 million years ago (Mya), where soft-bodied forms like Yunnanozoon exhibit a notochord-like structure and pharyngeal slits, positioning them as potential stem-chordates or primitive vertebrates. Similarly, Haikouichthys from the same biota displays vertebrate-like features, including a cranium, segmental muscle blocks, and a post-anal tail, marking it as one of the oldest putative vertebrates. A 2024 reinterpretation of Pikaia from the Middle Burgess (~508 Mya) confirms key chordate features, including a and gut canal, strengthening evidence for early chordate body plan . Additionally, in 2024, the soft-bodied stem-vertebrate Nuucichthys rhynchocephalus was described from the Drumian Marjum Formation in (~505 Mya), expanding the known diversity of Cambrian chordates in the . These lagerstätten provide exceptional preservation of soft tissues, offering critical insights into basal chordate anatomy otherwise absent from the record. Major evolutionary events in chordate history include the Ordovician radiation of jawless fishes (agnathans), around 485–443 Mya, when ostracoderms and other armored forms diversified in nearshore marine environments, adapting to filter-feeding and detritivory amid rising levels and availability. The period (419–359 Mya) witnessed the emergence of tetrapods from sarcopterygian ancestors, with fossils like and early amphibians such as demonstrating transitional limb and skeletal adaptations for shallow-water locomotion and eventual terrestrial incursion. diversification accelerated in the (252–66 Mya), following their Late origins, as synapsids and sauropsids radiated into diverse terrestrial niches, including the rise of dinosaurs and early mammals, driven by ecological opportunities post-Permian extinction. Significant gaps persist in the chordate record, particularly for soft-bodied basal forms, due to taphonomic biases favoring hard-part preservation, which obscures the full diversity of early chordates before the . analyses, calibrated against data, indicate that chordate lineages may have diverged in the pre-Cambrian period (>541 Mya), potentially over 100 million years earlier than the oldest macrofossils suggest, highlighting discrepancies between times and paleontological evidence. Key extinctions and adaptations shaped chordate trajectories, notably the Late Devonian decline of placoderms around 359 Mya, coinciding with the Kellwasser and Hangenberg events that eliminated many armored jawed fishes, possibly due to anoxic episodes and competition from more agile chondrichthyans and osteichthyans. Rising atmospheric oxygen levels during the Paleozoic, from ~10% to 30% by the Devonian, facilitated increases in body size and metabolic demands, enabling the evolution of larger, more active chordates like early tetrapods and contributing to their ecological dominance.

Relationships to Other Animal Groups

Closest Non-Chordate Relatives

The closest non-chordate relatives of chordates are the hemichordates and echinoderms, which together form the Ambulacraria, the to Chordata within the deuterostomes. This relationship is supported by phylogenomic analyses that resolve Ambulacraria as monophyletic and closely allied to chordates based on shared developmental and genetic features. Hemichordates, comprising the classes Enteropneusta (acorn worms) and Pterobranchia (pterobranchs), exhibit several traits reminiscent of chordates, including pharyngeal gill slits used for filter feeding and respiration. Their tornaria larvae, which are free-swimming and possess a ciliated band for locomotion and feeding, closely resemble the auricularia larvae of echinoderms and share developmental similarities with early chordate stages, such as those of amphioxus. Echinoderms, including familiar forms like (Asteroidea) and sea urchins (Echinoidea), display bilateral in their planktonic larvae, which undergo to develop the characteristic pentaradial of adults. This radial supports a unique —a network of fluid-filled canals and —that facilitates locomotion, feeding, and , marking a key distinct from chordate body plans. Ambulacrarians and chordates share deuterostome characteristics, such as enterocoelic formation, where the arises from mesodermal pouches evaginating from the during embryogenesis. Genetic homologies further unite them, including conserved BMP signaling pathways that pattern the dorsoventral axis; in hemichordates, BMP activity establishes ventral identity and represses neural development, mirroring its role in chordates but with an inverted orientation relative to the . Despite these similarities, ambulacrarians lack defining chordate features like a and a dorsal hollow cord; instead, hemichordates possess a ventral nerve cord, and echinoderms have a decentralized without a centralized cord. These differences highlight the divergence within deuterostomes while underscoring the common ancestry of Ambulacraria and Chordata.

Broader Deuterostome Context

Deuterostomes are defined by key embryological features, including the formation of the from the blastopore while the develops secondarily from a separate , radial cleavage of the , and indeterminate cell fate during early development, where isolated blastomeres can give rise to complete larvae. These traits distinguish deuterostomes from protostomes, in which the blastopore typically becomes the and cleavage is spiral and determinate. formation occurs via enterocoely, with mesodermal pouches budding from the . The clade encompasses four extant phyla—Chordata, Echinodermata, , and Xenoturbellida—along with extinct lineages such as . Xenoturbellida comprises simple, sac-like marine worms lacking a , gut, or gonads, yet sharing molecular affinities with other deuterostomes. , known from fossils, represents a possible stem-group deuterostome, with specimens showing pharyngeal structures resembling gill slits that parallel early chordate anatomy. Deuterostomes diverged from protostomes around 600 million years ago in the late , marking a pivotal event in bilaterian . This common likely possessed a diffuse , as seen in modern echinoderms with their decentralized nerve nets and radial in adults, in contrast to the centralized, tubular that evolved within the chordate lineage. Such innovations highlight how deuterostome diversity arose from a shared developmental framework, influencing bilaterian variations through modifications in regulatory networks. Whole-genome analyses have historically supported monophyly by identifying conserved syntenic regions and orthologous genes, such as those involved in dorsoventral patterning, that unite the and illuminate bilaterian origins from a common ancestor with modular genetic toolkits. However, recent phylogenomic studies using large-scale datasets have revealed weak branch support for this grouping, attributing traditional monophyly signals to systematic biases like long-branch attraction or incomplete lineage sorting, prompting reevaluation of deuterostome relationships. As of 2025, further analyses suggest that the may be an artifact of these errors, with support diminishing when biases are mitigated.

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