Chordate

Lancelets (Cephalochordata)

The cephalochordates are small, "vaguely fish-shaped" animals that lack brains and clearly defined heads, as well as lacking specialized sensory organs.^[28] These burrowing filter-feeders compose the earliest-branching chordate subphylum.^[29][30]

Tunicates (Urochordata)

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.^[31]

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.^[32]

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

Sea squirts are sessile organisms and consist mainly of water pumps and filter-feeding apparatus.^[31] Most attach firmly to the sea floor, where they remain in one place for life.

The salps float in mid-water and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies.^[33]

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.^[34] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.^[31]

Comparison of two invertebrate chordates

A. Lancelet, B. Larval tunicate, C. Adult tunicate
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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. Oral/Inhalant siphon, 12. Exhalant siphon, 13. Heart, 14. Stomach, 15. Esophagus, 16. Intestines, 17. Atrium, 21. Tunic.

Vertebrates (Craniata)

All craniate species have a distinct skull. However, not all of them possess vertebrae. The notochord is accompanied or replaced by the vertebral column in most craniates.^[35] Which consists of a series of cylindrical, and bony or cartilaginous vertebrae and projections that link the vertebrae. And these generally have neural arches that protect the spinal cord.

Hagfish have incomplete braincases and lack vertebrae. Thus, they were regarded as non-vertebrate craniates.^[36][37] However, the cladistic exclusion of hagfish from the vertebrates is controversial. As they may have secondarily lost their vertebral columns, making them vertebrates.^[38]

Molecular phylogenetics, which uses DNA sequences to classify organisms, has generally supported the grouping of hagfish and lampreys (which have vertebrae) in a clade named Cyclostomata,^[39][40] suggesting that the hagfish are indeed vertebrates.^[38] Examination of fossil hagfish also supports this conclusion.^[41] Indicating that Vertebrata is the same group as Craniata, instead of the former being a subset of the latter.^[38]

Overview

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 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 identified by DNA analysis as there is no physical trace of any chordate-like structures.^[43]

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic (meaning, all chordates are descend from a single, common ancestor, which can be considered 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]}

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.^[44]

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.^[45] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.^[46][47] 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.^[45] 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.^[48][49] 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.^[49] 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]}

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.^[50] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.^[51] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.^[52][53] 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.^[53] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.^[42][54] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Mya), is also regarded as a primitive chordate.^[55] 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.^[56] 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).^[57]

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.^[16] 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.^[58] 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.^[16][59]

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.^[59] However, molecular estimates of dates often disagree with each other and with the fossil record,^[59] and their assumption that the molecular clock runs at a known constant rate has been challenged.^[60][61]

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

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.^{[62][63][64][65][66]}

Hemichordates

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.^[67]

Echinoderms

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.^[68]