Chordata

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)
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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)
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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)
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