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Overview

Brief Summary

Introduction

The Phylum Chordata includes the well-known vertebrates (fishes, amphibians, reptiles, birds, mammals). The vertebrates and hagfishes together comprise the taxon Craniata. The remaining chordates are the tunicates (Urochordata), lancelets (Cephalochordata), and, possibly, some odd extinct groups. With few exceptions, chordates are active animals with bilaterally symmetric bodies that are longitudinally differentiated into head, trunk and tail. The most distinctive morphological features of chordates are the notochord, nerve cord, and visceral clefts and arches.

Chordates are well represented in marine, freshwater and terrestrial habitats from the Equator to the high northern and southern latitudes. The oldest fossil chordates are of Cambrian age. The earliest is Yunnanozoon lividum from the Early Cambrian, 525 Ma (= million years ago), of China. This was just recently described and placed with the cephalochordates (Chen et al., 1995). Another possible cephalochordate is Pikaia (Nelson, 1994) from the Middle Cambrian. These fossils are highly significant because they imply the contemporary existence of the tunicates and craniates in the Early Cambrian during the so-called Cambrian Explosion of animal life. Two other extinct Cambrian taxa, the calcichordates and conodonts, are uncertainly related to other Chordata (Nelson, 1994). In the Tree of Life project, conodonts are placed as a subgroup of vertebrates.

Chordates other than craniates include entirely aquatic forms. The strictly marine Urochordata or Tunicata are commonly known as tunicates, sea squirts, and salps. There are roughly 1,600 species of urochordates; most are small solitary animals but some are colonial, organisms. Nearly all are sessile as adults but they have free-swimming, active larval forms. Urochordates are unknown as fossils. Cephalochordata are also known as amphioxus and lancelets. The group contains only about 20 species of sand-burrowing marine creatures. The Cambrian fossils Yunnanozoon and Pikaia are likely related to modern cephalochordates.

During the Ordovician Period (510 - 439 Ma) jawless or agnathan fishes appeared and diversified. These are the earliest known members of Vertebrata, the chordate subgroup that is most familiar to us. Fossils representing most major lineages of fish-like vertebrates and the earliest tetrapods (Amphibia) were in existence before the end of the Devonian Period (363 Ma). Reptile-like tetrapods originated during the Carboniferous (363 - 290 Ma), mammals differentiated before the end of the Triassic (208 Ma) and birds before the end of the Jurassic (146 Ma).

The smallest chordates (e.g. some of the tunicates and gobioid fishes) are mature at a length of about 1 cm, whereas the largest animals that have ever existed are chordates: some sauropod dinosaurs reached more than 20 m and living blue whales grow to about 30 m.

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Overview

Chordates form a very diverse phylum with species living all over the planet (1). The extant animals in the phylum include the vertebrates—a familiar group that includes fish, amphibians, reptiles, mammals, and birds—plus less well-known creatures including hagfish (which, together, with vertebrates make up the group Craniata), lancelets, and tunicates(1). A crucial defining feature of chordates is a long, cartilage-like(2) structure called the notochord (1,2), which runs along the central axis of the embryo of all chordates (2) and is important in embryonic development (1,2). While in the more modern chordates, the notochord turns into bone before birth(2), in some ancient vertebrates, such as lampreys and sturgeons, the notochord remains in the body for all of the animals’ lives(2); in the even older chordates such as tunicates and lancelets, which do not have backbones, the notochord remains for part or all of the animals’ lives as well, providing the structural support needed for them to swim(1,2). These creatures, particularly lancelets, are probably related to the oldest chordate fossils ever found, which date back to the Early Cambrian period, some 525 million years ago(1).

  • 1. Lundberg, John G. “Chordata.” Tree of Life Web Project. 1995. 1 Sept. 2011. http://tolweb.org/Chordata/2499
  • 2. Stemple, Derek L. “Structure and Function of the Notochord: An Essential Organ for Chordate Development.” Development 132 (2005): 2503-2512.
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Comprehensive Description

Characteristics

The notochord is an elongate, rod-like, skeletal structure dorsal to the gut tube and ventral to the nerve cord. The notochord should not be confused with the backbone or vertebral column of most adult vertebrates. The notochord appears early in embryogeny and plays an important role in promoting or organizing the embryonic development of nearby structures. In most adult chordates the notochord disappears or becomes highly modified. In some non-vertebrate chordates and fishes the notochord persists as a laterally flexible but incompressible skeletal rod that prevents telescopic collapse of the body during swimming.

The nerve cord of chordates develops dorsally in the body as a hollow tube above the notochord. In most species it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect.

The visceral (also called pharyngeal or gill) clefts and arches are located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the esophagus. The visceral clefts appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts. Thus the clefts are continuous, slit-like passages connecting the pharynx to the exterior. The soft and skeletal tissues between adjacent clefts are the visceral arches. The embryonic fate of the clefts and slits varies greatly depending on the taxonomic subgroup. In many of the non-vertebrate chordates, such as tunicates and cephalochordates, the clefts and arches are elaborated as straining devices concerned with capture of small food particles from water. In typical fish-like vertebrates and juvenile amphibians the walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood. In adult amphibians and the amniote tetrapods (= reptiles, birds and mammals) the anteriormost cleft transforms into the auditory (Eustachian) tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands and lymphatic tissues in the throat region. The skeleton and muscles of the visceral arches are the source of a great diversity of adult structures in the vertebrates. For example, in humans (and other mammals) visceral arch derivatives include the jaw and facial muscles, the embryonic cartilaginous skeleton of the lower jaw, the alisphenoid bone in the side wall of the braincase, the three middle ear ossicles (malleus, incus and stapes), the skeleton and some musculature of the tongue, the skeleton and muscles of the larynx, and the cartilaginous tracheal rings.

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Ecology

Associations

Animal / parasite
larva of Calliphora parasitises Chordata
Other: minor host/prey

Animal / carrion / dead animal feeder
larva of Cynomya mortuorum feeds on dead Chordata
Other: major host/prey

Animal / carrion / dead animal feeder
gymnothecium of Illosporium curreyi feeds on dead bone of Chordata

Animal / parasite / ectoparasite
larva of Lucilia sericata ectoparasitises wound of Chordata

Plant / resting place / within
imago of Oxyomus sylvestris may be found in corpse of Chordata

Animal / carrion / dead animal feeder
larva of Phormia regina feeds on dead carrion of Chordata

Animal / parasite / ectoparasite
larva of Phormia terraenovae ectoparasitises Chordata
Other: minor host/prey

In Great Britain and/or Ireland:
Animal / carrion / dead animal feeder
Podops inuncta feeds on dead Chordata
Remarks: Other: uncertain

Animal / parasite / endoparasite
larva of Ravinia pernix endoparasitises gut of Chordata
Other: minor host/prey

Animal / parasite / endoparasite
larva of Sarcophaga carnaria endoparasitises gut of Chordata

Animal / carrion / dead animal feeder
larva of Sarcophaga jacobsoni feeds on dead Chordata

Animal / carrion / dead animal feeder
larva of Trox perlatus feeds on dead dry detritus, esp bone of corpse of Chordata

Animal / carrion / dead animal feeder
Trox sabulosus feeds on dead dried carcass of Chordata

Animal / carrion / dead animal feeder
Trox scaber feeds on dead debris of carcass of Chordata

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Evolution and Systematics

Evolution

Discussion of Phylogenetic Relationships

View Chordata Tree

Tree based on summary in Nelson (1994) plus Yunnanozoon added as uncertain basal sister group to cephalochordates. Euconodonts are to be found within vertebrates, a subgroup of craniates.

As noted below, the relationships of some of the presumed fossil chordates is based on scant evidence and there is debate about the position of especially the calcichordates and conodonts (see references cited below and Chen et al. 1995). There is strong morphological, especially embryological, evidence for monophyly of the Urochordata, Cephalochordata and Craniata, with the latter two being sister taxa. Schaeffer (1987) details several embyological synapomorphies, in addition to those noted here, that support these same relations among and monophyly of the three living chordate groups.

  1. Calcichordata. Jeffries (1986) provides descriptions and comparisons and argues for the placement of calcichordates near the Chordata. Other workers believe that calcichordates are closer to echinoderms. Reconstructions of these fossil organisms include visceral (pharyngeal or gill) slits that would suggest chordate affinities, but the mineralized skeleton was composed of calcite, like echinoderms, not bone as in many chordates.
  2. Urochordata. Evidence that tunicates are chordates comes clearly from the larval "tadpole" stage which shows pharyngeal slits and arches, dorsal hollow nerve cord, notochord and post-anal muscular (unsegmented) tail. Adults of most members are sessile filter feeders with an expanded pharynx and, like cephalochordates and larval lampreys, with an endostyle, a mucous food trap in the pharyngeal floor that is homologous with the thyroid gland of vertebrates.
  3. Cephalochordata. Among the living chordates there is little doubt that lancelets are most closely related to the Craniates based on synapomorphies such as segmented axial muscles and metameric organization of the visceral (pharyngeal) arches. Uniquely, the notochord of cephalochordates extends to the tip of the snout, the gonads are segmentally organized, adults have a high number (50+) gill arches, and there is a hood-like atrium covering the pharyngeal region. The Early Cambrian fossil Yunnanozoon possesses the extended notochord and segmental gonads, but lack the atrium and increased number of gill arches.
  4. Craniata. Because hagfishes (Myxini) lack all traces of vertebrae, i.e. a backbone, Janvier (1981) groups the Myxini with all other vertebrates in the higher taxon Craniata (referring to the presence of a head skeleton). The taxon Vertebrata is, therefore, in a strict sense, applied to those animals known or believed to possess at least a simple backbone of neural arches. Synapomorphies of the Craniata include: presence of a cartilaginous (and often bony) head skeleton; relatively large brain plus a unique set of sensory and motor cranial nerves; nephrons as the functional excretory unit; neural crest embryonic tissue.

The traditional taxa Agnatha (jawless fishes), Ostracodermi (fossil jawless fishes) and Cyclostomata (living lampreys and hagfishes) are non-monophyletic assemblages that are no longer recommended. Details of jawless fish relationships are introduced on the Craniata and Vertebrata pages.

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Functional Adaptations

Functional adaptation

Slime reduces drag: fish
 

Skins of fish reduce drag by being covered by a slime layer of complex proteins, polysaccharides, and bacteria.

   
  "Specific organisms can be targeted for specialized purposes. Slick 'no drag' hulls have been sought for centuries. The slickest surface is not a waxed or polished or Teflon surface, contrary to common belief. Using biomimicry, most fish have a clearly articulated slime layer of complex proteins, polysaccharides and bacteria. They are very fast and achieve this speed from the physical/chemical interface of water molecules with the surfaces of the bacteria and components which harbor either thinly affixed water molecules or compounds which have the appropriate hydrophilicity/hydrophobicity balance." (Guritza 2002:1)

  Learn more about this functional adaptation.
  • Guritza, Dennis A. (inventor, assignee). 2002. Stenoprophiluric matrices, and methods of making and using the same. Patent: World Intellectual Property Organization WO/2002/036112.
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Functional adaptation

Crystals and fibers provide strength, flexibility: bones
 

The composition of bones grants them strength, light weight, and some flexibility via small inorganic crystals and thin collagen fibers.

             
  "Nature has no reason for making a bone round or square. The outlines of bones, therefore, follow the stress lines or are vertical to them so that they give an indication of the pressures the bone has to withstand. But this ideal distribution of bone material along the stress lines would have been to little avail were the material itself not so well adapted to extraordinary pressure. Just like fiberglass made of synthetics threaded with glass fiber, bone tissue is made up of two constituents which greatly differ in their mechanical properties. About half the bone volume is made up of inorganic crystalline material. It consists of phosphate, calcium, and hydroxyl ions and comes very close to hydroxylapatite in structure. It appears in the bone in the form of tiny crystals, only about 200 atomic diameters in size. They are inserted between thin fiber hairs of the elastic material collagen and seem to be linked with them. Many of these parallel inorganic and organic building blocks form fascicles, which may be interwoven in various ways. The end product is a material that is considerably stiffer than collagen, though low in weight, but by far not as brittle and inelastic as pure hydroxylapatite. Besides, because of the continuous alternation between brittle and elastic material, there is little chance for a fracture to spread unchecked." (Tributsch 1984:32-33)

"Mineralized collagen fibrils are highly conserved nanostructural building blocks of bone. By a combination of molecular dynamics simulation and theoretical analysis it is shown that the characteristic nanostructure of mineralized collagen fibrils is vital for its high strength and its ability to sustain large deformation, as is relevant to the physiological role of bone, creating a strong and tough material. An analysis of the molecular mechanisms of protein and mineral phases under large deformation of mineralized collagen fibrils reveals a fibrillar toughening mechanism that leads to a manifold increase of energy dissipation compared to fibrils without mineral phase. This fibrillar toughening mechanism increases the resistance to fracture by forming large local yield regions around crack-like defects, a mechanism that protects the integrity of the entire structure by allowing for localized failure. As a consequence, mineralized collagen fibrils are able to tolerate microcracks of the order of several hundred micrometres in size without causing any macroscopic failure of the tissue, which may be essential to enable bone remodelling. The analysis proves that adding nanoscopic small platelets to collagen fibrils increases their Young's modulus and yield strength as well as their fracture strength. We find that mineralized collagen fibrils have a Young's modulus of 6.23 GPa (versus 4.59 GPa for the collagen fibril), yield at a tensile strain of 6.7% (versus 5% for the collagen fibril) and feature a fracture stress of 0.6 GPa (versus 0.3 GPa for the collagen fibril)." (Buehler 2007:1)
  Learn more about this functional adaptation.
  • Tributsch, H. 1984. How life learned to live. Cambridge, MA: The MIT Press. 218 p.
  • Buehler, Markus J. 2007. Molecular nanomechanics of nascent bone: fibrillar toughening by mineralization. Nanotechnology. 18(29): 295102.
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Functional adaptation

Fluid lubricates joints: vertebrates
 

Joints of vertebrates are protected by use of a lubricant, synovial fluid.

   
  "Whereas technology tries to polish the hard metal of bearings to as fine a finish as possible, nature covers the touching surfaces with a spongelike substance which is comparatively stiff yet quite elastic: cartilaginous tissue, which differs from hard bone tissue essentially in that it lacks deposits of calcium crystals. One could compare this tissue to fiberglass from which the fibers have been removed. The fine pores of the cartilaginous sliding layer are soaked through with lubricating synovial fluid. When the joint is subjected to pressure, the layer compresses and the fluid is pushed out of the thin ducts. The gliding principle is the same as the one used for air-cushion vehicles, with the difference that in bone bearings the cushion (of fluid) is produced on the spot…Synovial fluid might also compete in the market with modern lubricants. It contains slightly less protein than blood serum, but on the other hand it carries an organic acid with very long molecules which are probably linked to proteins. The more the gliding speeds vary in the lubrication layer, the lower the viscosity of the fluid." (Tributsch 1984:41-42)
  Learn more about this functional adaptation.
  • Tributsch, H. 1984. How life learned to live. Cambridge, MA: The MIT Press. 218 p.
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Functional adaptation

Gills exchange oxygen efficiently: fish
 

The gills of fish remove oxygen from water with extreme efficiency because water flows countercurrent to capillary blood flow.

   
  "Water flow over the secondary lamellae is countercurrent to capillary blood flow, resulting in extremely efficient oxygen extraction. Gills also function in monovalent ion regulation (via specialized chloride cells) and nitrogenous waste excretion (ammonia)." (Fowler and Miller 2003:4)
  Learn more about this functional adaptation.
  • Fowler, ME; Miller, RE. 2003. Zoo and Wild Animal Medicine. Philadelphia: W.B. Saunders Co.
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Functional adaptation

Cells manage monovalent ions: fish
 

The gills of fish manage monovalent ion concentrations via specialized chloride cells.

   
  "Water flow over the secondary lamellae is countercurrent to capillary blood flow, resulting in extremely efficient oxygen extraction. Gills also function in monovalent ion regulation (via specialized chloride cells) and nitrogenous waste excretion (ammonia)." (Fowler and Miller 2003:4)
  Learn more about this functional adaptation.
  • Fowler, ME; Miller, RE. 2003. Zoo and Wild Animal Medicine. Philadelphia: W.B. Saunders Co.
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Functional adaptation

Tendons and bones form seamless attachment: Chordates
 

The attachment of tendons and bones is a strong and smooth transition due to the same fibre material spanning a gradual transition of mineralization.

     
  "In addition, architects can learn from connections and transitions between systems and subsystems of biological entities. In the building sector, connections between parts and elements are almost always discontinuous and articulated as dividing seams, instead of a smoother transition in materiality and thus functionality (such as can be seen in the way tendon and bone connect, deploying the same fibre material yet across a smooth transition of mineralisation). The understanding and deployment of gradient thresholds in materiality and environmental conditions can yield the potential for complex performance capacities of material systems. This will require a detailed understanding of the relation between material makeup and resultant behavioural characteristics." (Courtesy of the Biomimicry Guild)
  Learn more about this functional adaptation.
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Functional adaptation

Highly effective swimming: fish
 

Fish have effective maneuverability, braking, stability and thrust thanks to multiple fins.

         
  "By flexing the pectoral and caudal fins the fish can turn up, down, or sideways. The pectoral fins are also used as brakes, being pushed forwards like the flaps on an aircraft wing. The positions of the paired fins, especially the pelvic fins, are constantly adjusted to keep the fish from pitching or rolling: the pectorals tend to produce lift, which is counteracted by the downward thrust of the pelvics." (Foy and Oxford Scientific Films 1982:186)
  Learn more about this functional adaptation.
  • Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
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Functional adaptation

Eggs are buoyant: oviparous fish
 

The eggs of many fish are buoyant due to the presence of discrete oil droplets within each egg.

   
  "These fish eggs, equally supported by water on all sides, have retained an almost pure spherical form, and so have the tiny oil droplets inside them, used to make the eggs buoyant." (Foy and Oxford Scientific Films 1982:20)
  Learn more about this functional adaptation.
  • Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:415,586Public Records:226,603
Specimens with Sequences:327,665Public Species:17,344
Specimens with Barcodes:309,466Public BINs:25,761
Species:30,578         
Species With Barcodes:26,794         
          
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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Chordata

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Barcode data

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