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From Wikipedia, the free encyclopedia

Temporal range:
Cambrian Stage 3Holocene, 521–0 Ma
The glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.
Scientific classification edit
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel, 1874[1][2]

And see text

A chordate (/kɔːrdt/) is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail: these five anatomical features define this phylum. Chordates are also bilaterally symmetric; and have a coelom, metameric segmentation, and a circulatory system.

The Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds, and mammal)s; Tunicata (salps and sea squirts); and Cephalochordata (which includes lancelets). There are also extinct taxa such as the Vetulicolia. Hemichordata (which includes the acorn worms) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes.

Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically (phylogenetically), vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull. The Craniata and Tunicata compose the clade Olfactores. (See diagram under Phylogeny.)

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  • ✪ Chordates - CrashCourse Biology #24
  • ✪ Chordate salient features, BSc, MSc Lecture by Ms. Priyanka khan.
  • ✪ Chordate Evolution (1/2)
  • ✪ Phylum chordata: general characters and basic overview


Next time someone asks you who you think you are, just give them the facts. You're a mammalian amniotic tetrapodal sarcopterygiian osteichthyan gnathostomal vertebrate cranial chordate. Yeah, it's a mouthful. And in order to understand what it means, you're going to have to understand the most complex group of animals on earth, and what it takes to get from this to this. The phylum Chordata accounts for all 52,000 species of vertebrates on earth and several thousand species of invertebrates. Together they range from tiny, brainless filter feeders all the way up to Scarlett Johansson. Now, you know by now that when we talk about classifying animals, we're really talking about their shared ancestry, each new branch on this tree marking an important new evolutionary milestone. And just like with tissue layers and segmentation in simpler animals, there are traits we can look for to track the evolution of chordates. By the time all of those traits appear in one organism, we'll have arrived at the most complex class within the most complex phlyum: the mammals. But first let's start with the fundamentals. We've talked before about synapomorphic traits: traits that set a group of animals apart from its ancestors and from other groups that came from the same ancestors. Chordates share four synapomorphies that make us who we are. Each of them is present at some point in every chordate's life cycle. How about a volunteer to demonstrate these traits? Ah, I see that the lancelets are raising their...mouthparts. The lancelets, also known as cephalochordata, literally "head-cords," are one of the three sub-phyla of chordates. And unlike almost all other chordates, these tiny, brainless, invertebrate filter feeders retain all four of these characteristics for their entire lives. You probably already know where most of these traits are going to appear, since the phylum is named after it: the spinal cord, or at least something that resembles a spinal chord. First there's the notochord, a structure made of cartilage that runs between an animal's digestive tube and its nerve cord. In most vertebrates, a skeleton develops around the notochord and allows the muscles to attach. In humans, the notochord is reduced to the disks of cartilage that we have between our vertebrae. Second, we have the nerve cord itself, called the dorsal hollow nerve cord a tube made of nerve fibers that develops into the central nervous system. This is what makes chordates different from other animal phyla, which have solid, ventral nerve cords, meaning they run along the front or stomach side. Third, all chordates have pharyngeal slits. In invertebrates like our lancelet here, they function as filters for feeding. In fish and other aquatic animals, they're gill slits, and in land-dwelling vertebrates like us they disappear before we're born, but that tissue develops into areas around our jaws, ears, and other structures in the head and neck. And finally, we can't forget our fourth synapomorphy, the post-anal tail, which is exactly what it sounds like. It helps propel aquatic animals through the water, it makes our dog look happy when she wags it, and in humans it shrinks during embryonic development into what is known as the coccyx, or tailbone. It's right here. And trust me, when it comes to tail placement, post-anal is the way to go. These four traits all began to appear during the Cambrian explosion more than 500 million years ago, and today they're shared by members of all three chordate subphyla even if the animals in those subphyla look pretty much nothing like each other For instance, our new friends here in cephalochordata are the oldest living subphylum, but you can't forget the other invertebrate group of chordates, urochordata, literally "tail-cords." There are over 2,000 species here, including sea squirts. If you're confused about why this ended up in a phylum with us, it's because they have tadpole-like larvae with all four chordate characteristics. The adults, which actually have a highly developed internal structure, with a heart and other organs, retain the pharyngeal slits but all other chordate features disappear or reform into other structures. The third and last and most complex subphylum, is the Vertebrata, and has the most species in it because its members have a hard backbone, which has allowed for an explosion in diversity, from tiny minnows to the great blue whale. You can see how fantastic this diversity really is when you break down Vertebrata into its many, many classes from slimy, sea-snakey things to us warm and fuzzy mammals. And as these classes become more complex, you can identify the traits they each developed that gave them an evolutionary edge over the ones that came before. For example, how's this for an awesome trait: a brain! Vertebrates with a head that contains sensory organs and a brain are called craniates. They also always have a heart with at least two chambers. Since this is science, you're going to have to know that there's going to be an exception for every rule that you're going to have to remember. and the exception in this case is the Myxini, or hagfish the only vertebrate class that has no vertebrae but is classified with us because it has a skull. This snake-like creature swims by using segmented muscles to exert force against its notochord. Whatever, hagfish. Closely-related to it is the class petromyzontida, otherwise known as lampreys, the oldest living lineage of vertebrates. Now these have a backbone, made of cartilage, and maybe even more important, a more complex nervous system. With the advent of the backbone we see vertebrates getting larger, developing more complex skeletons, and becoming more effective at catching food and avoiding predators. But do you notice anything missing? Lampreys and other early vertebrates are agnathans, literally "no-jaws." And if you want to be able to chew food, it really helps to have a jaw and teeth. Most scientists think that the jaw evolved from structures that supported the first two pharyngeal slits near the mouth. And teeth? Well, the current theory is that they evolved from sharp scales on the face! Gnathostomes, or "jaw-mouths", arrived on the scene 470 million years ago, and one of the oldest and most successful groups of gnathostomes that have survived to the present day are the class chondrichthyes, the "cartilage fish". You know them as sharks, skates and rays, and as their name says, their skeleton is made up mostly of cartilage, but they show the beginnings of a calcified skeleton. Chondrichthyans haven't changed much over the past 300 million years or so, and their success stems from the paired fins that allow for efficient swimming, and those jaws for biting off delicious hunks of flesh. If we're going to eventually get to mammals, we need bones, and we find those with the evolution of fish. Meet Osteichthyes, which technically means "the bony fish" unlike cartilaginous fish, members of this group have a mineralized endoskeleton. Now, Osteichthyes is sometimes considered a superclass, because it includes a whole slew of diverse classes that descended from it. There's actually some controversy among taxonomists about what to call it, but the main thing to know is that the majority of all vertebrates fall under Osteichthyes, and that includes you. It's broken up into two main groups, which themselves include a bunch of classes. The first is Actinopterygii, or ray-finned fishes, and with 27,000 species, pretty much every fish you've ever heard of is in this group. Ray-finned fishes evolved in fresh water, spread out into the oceans, and some eventually came back to fresh water. In the second group, things start to get really strange and interesting. These are the lobe-finned fishes, or sarcopterygii, a name derived from bones surrounded by muscle found in their pectoral and pelvic fins. And that sounds like something that could be used for walking! Lobe-fins include the coelacanths, which consist of one living species lungfish, which gulp air into their lungs; and tetrapods, which have adapted to land with four limbs. So this is weird, right? Even though land animals clearly are not fish, since tetrapods evolved from bony fish, they are filed under this group. These taxonomists, man. I want to party with them sometime. But first: Imagine that you're a fisherman off the coast of South Africa in the western Indian Ocean about 75 years ago. Just put that in your brain and hold on to it. And you've just pulled up a fish that no one has ever seen. Not only that, you've caught a fish that was thought to have become extinct 75 million years ago. This is exactly what happened in 1938, when Captain Hendrick Goosen hauled up a coelacanth, and it has mystified scientists ever since. A second population has since been identified near Indonesia in 1999, but the deep-sea creatures remain extremely rare. The coelacanth fascinates scientists because of its paired lobe fins. They extend from the body like legs and move in an alternating pattern. In other words, they move more like a horse than like a fish. And in fact those paired fins are supported by the very same bones that we have in our arms and legs. The coelacanth also has a hinged joint in the skull so it can widen its mouth to eat large prey, as well as thick scales that don't exist on any living fish. It's not good eating, but why would you want to eat what's essentially a living fossil? Alright, now we're talking about tetrapods, which of course means "four feet," and getting those four feet onto land was really awesome for those early creatures because that meant that they could escape the increasingly brutal and predatory world of the ocean. Tetrapods gradually replaced their fins with limbs, and developed entirely new body parts that were never seen before, like necks, with the help of additional vertebra, that separated the body from the head. The first tetrapods are today found in the class Amphibia, which were the first creatures to develop a three-chambered heart. There are more than 6,000 known species of amphibians like frogs and salamanders, most of which begin life as tadpoles in water, and then later develop legs, lungs and a digestive system, and often migrate to land for adulthood. But amphibians lay eggs that don't have shells, so they dehydrate quickly, so they have to be laid in water. So this leads us to our next evolutionary milestone for chordates: the amniotic egg. Amniotes are tetrapods that have eggs adapted for life on land a group that includes reptiles, birds and mammals. The amniotic egg was crucial for the success of land-dwellers, allowing embryos to develop in their own "private pond" of the amniotic sac, often surrounded by a hard shell in the case of reptiles and birds. The class Reptilia represents the earliest amniotes. Like amphibians, they have a 3-chambered heart, but they're totally terrestrial. And here's where we find our dinosaurs, snakes, turtles and lizards. You often hear reptiles described as "cold-blooded." This does not mean that their blood is cold. They're actually ectothermic, which means that they absorb external heat as their main source of body heat. Hence the lizard that likes to lay in the sun all day. The oldest group of reptiles, the archosaurs, mostly disappeared when most of the dinosaurs died out 65 million years ago. But two lineages of archosaurs survived. One includes the modern reptiles crocodiles and alligators, and the other is a type of dinosaur that we now call birds, the class Aves. There are big, obvious differences between these two surviving archosaurs: one is designed for eating and fighting big animals, while the other is designed for flying around and being graceful and stuff. The not-so-obvious but equally important difference is that birds are endotherms, which means that they can crank up their metabolism to regulate their body temperature. Making all that heat requires a big furnace, which is provided thanks to the evolution of a four-chambered heart. There's only one other group of animals that developed this trait, independently of birds, by the way, and it allowed them to spread through the planet. I'm talking, of course, about the class Mammalia, otherwise known as amniotes that have hair, three special ear bones and mammary glands. And most mammals have evolved to dispense with the hard egg shell altogether the embryo avoiding predation and other environmental dangers by developing inside the mother's body. In this class of chordates you'll find me, dame Judi Dench, your dog, your cat, Shamu the orca, African elephants, the South American pudu, and 5,300 other known species of mammals. It all began with a simple ancestor more than 500 million years in this crazy chordate phylum, but we've finally made it! And now you know exactly who you are. Thank you for watching this episode of Crash Course Biology, We hope that it was helpful and that you feel like a smarter person There's review stuff next to me that you can click on to go to those parts of the video that you maybe want to watch a little bit more of; to reinforce it in your brain. Thank to everybody who helped put this together. And if you have any questions for us you can get in touch with us on Facebook or Twitter. Or, of course, down in the comments below. Goodbye.



Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[3]

  • A notochord, a fairly stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, and in wholly aquatic species this helps the animal to swim by flexing its tail.
  • A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system.
  • Pharyngeal slits. The pharynx is the part of the throat immediately behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live.
  • Post-anal tail. A muscular tail that extends backwards behind the anus.
  • An endostyle. This is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.[4] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.[3]

There are soft constraints that separate chordates from certain 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
18 = metapleural fold
19 = hepatic caecum (liver-like sack)
Anatomy of the cephalochordate Amphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.


The following schema is from the third edition of Vertebrate Palaeontology.[7] The invertebrate chordate classes are from Fishes of the World.[8] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.


Craniata (Vertebrata)

Craniate: Hagfish
Craniate: Hagfish

Craniates, one of the three subdivisions of chordates, 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".[12]

Most craniates are vertebrates, in which the notochord is replaced by the vertebral column.[13] These consist 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. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[14] but as members of the craniates, the group from which vertebrates are thought to have evolved.[15] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns.[16]

The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[17] However, molecular phylogenetics, which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[18] 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.[19]


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

Tunicates: sea squirts
Tunicates: sea squirts

Most tunicates appear as adults in two major forms, known as "sea squirts" and salps, both of which are soft-bodied filter-feeders that lack the standard features of chordates. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus;[20] 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.[21] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[20] The third main group of tunicates, Appendicularia (also known as Larvacea), retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps.[22] The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[23] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[20]

Cephalochordata: Lancelets

Cephalochordate: Lancelet
Cephalochordate: Lancelet

Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[24] These burrowing filter-feeders compose the earliest-branching chordate sub-phylum.[25][26][27]



Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[28]
Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[28]

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.[29]

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 include all and only the descendants of a single common ancestor, which is itself a chordate, and that craniates' nearest relatives are tunicates.

All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that they are 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 also proved 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.








Craniates (vertebrates)

Diagram of the family tree of chordates[4]

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.[30]

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.[31] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.[32][33] 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 541 million years ago.[31] The Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.[34]

A skeleton of the blue whale, the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz
A skeleton of the blue whale, the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz
A peregrine falcon, the world's fastest animal
A peregrine falcon, the world's fastest animal

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.[35] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[36] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[37][38] 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.[38] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[28][39] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[40] 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.[41]

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.[4] 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.[42] 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.[4][43]

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.[43] However, molecular estimates of dates often disagree with each other and with the fossil record,[43] and their assumption that the molecular clock runs at a known constant rate has been challenged.[44][45]

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.


Phylogenetic tree of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported[46][47][48][49]






Appendicularia (formerly Larvacea)




Myxini (hagfish)

Hyperoartia (Petromyzontida) (Lampreys)


Myllokunmingia fengjiaoa

Zhongjianichthys rostratus











Acanthodii, incl Chondrichthyes


Actinopterygii (ray-finned fish)


Actinistia (coelacanths)

Dipnoi (lungfishes)






 Lepidosauromorpha (lizards, snakes, tuatara, and their extinct relatives)

 Archosauromorpha (crocodiles, birds, and their extinct relatives)


Closest nonchordate relatives

Acorn worms or Enteropneusts are example of hemichordates.
Acorn worms or Enteropneusts are example of 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.[50]


A red knob sea star, Protoreaster linckii is an example of Asterozoan Echinoderm.
A red knob sea star, Protoreaster linckii is an example of Asterozoan 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 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 look rather like flowers, and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example starfish, sea urchins and sea cucumbers.[51]

History of name

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.[2]

See also


  1. ^ Haeckel, E. (1874). Anthropogenie oder Entwicklungsgeschichte des Menschen. Leipzig: Engelmann.
  2. ^ a b Nielsen, C. (July 2012). "The authorship of higher chordate taxa". Zoologica Scripta. 41 (4): 435–436. doi:10.1111/j.1463-6409.2012.00536.x.
  3. ^ a b Rychel, A.L.; Smith, S.E.; Shimamoto, H.T. & Swalla, B.J. (March 2006). "Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage". Molecular Biology and Evolution. 23 (3): 541–549. doi:10.1093/molbev/msj055. PMID 16280542.
  4. ^ a b c d Ruppert, E. (January 2005). "Key characters uniting hemichordates and chordates: homologies or homoplasies?". Canadian Journal of Zoology. 83: 8–23. doi:10.1139/Z04-158. Retrieved 2008-09-22.
  5. ^ Valentine, J.W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. p. 7 The phylum chordata includes three subphylums: Cephalochoradta, Urochordata, Vertebras. It is determined that urochordates are closer to vertebrates that cephalochordates are due to their chordate–like features as a larval form. ISBN 978-0-226-84548-7."Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually, organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy".
  6. ^ R.C.Brusca, G.J.Brusca. Invertebrates. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p. 47, ISBN 0-87893-097-3.
  7. ^ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing. The classification scheme is available online
  8. ^ Nelson, J. S. (2006). Fishes of the World (4th ed.). New York: John Wiley and Sons, Inc. ISBN 978-0-471-25031-9.
  9. ^ Benton, M.J. (2004). Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd.
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