Chordata  

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.

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.

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.

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.