First things first:

An Overview of the Chondrichthyan Body and Skeleton

The chondrichthyan body is supported by a long fibrous rod (= notochord), around which lie a series of cartilaginous vertebral elements. Neural arches protect the spinal cord, and hemal arches (present in advanced chondrichthyans) protect blood vessels and arteries that run along the length of the notochord.


From Benton (2005) with permission

Many chondrichthyans possess two dorsal fins, one anterior (just behind the head) and another posterior (in front of the tail). These have internal skeletal supports (basal cartilage with rays extending into the fin), and some forms also have cartilaginous spines lying in front of the dorsal fins. The caudal (= tail) fin in primitive Chondrichthyans was externally symmetrical, though the internal skeleton was asymmetrical. The notochord and its associated cartilaginous elements extend up into the epichordal (= upper) lobe of the fin, while a series of cartilaginous rays extend down into the hypochordal (= lower) lobe. In many extant chondrichtyans, this asymmetry is reflected in the caudal fin's external morphology, with the epichordal lobe significantly longer than the hypochordal lobe (resulting in a "heterocercal" tail). Modern chondrichthyans often possess an anal fin as well.


From Benton (2000) with permission

While many think of the chonrichthyan cartilaginous skeleton as primitive, it is actually a derived (= advanced) state. The agnathan (= jawless) ancestors of chondrichtyans possessed a skeleton that was composed of both cartilage and bone. This means that the entirely cartilaginous skeleton of chondrichtyans is actually the result of an evolutionary modification (i.e. they have lost bone secondarily).

Paired Appendages

The shape and structure of the internal skeletal support of chondrichthyan appendages (i.e. pectoral and pelvic fins) show a great deal of variation. The primitive fin skeleton consisted of a basal cartilage (or cartilages) with radials extending to the outer edge of the fin. It is thought that these fins were not very motile, and were used primarily as hydrofoils, and not for steering. In more derived chondrichthyans, intrinsic fin muscles were present, and further variation in fin shape suggests that they were used to generate lift, and to allow the animal to manipulate its position in the water column. The pectoral and pelvic fins in extant chondricthyans articulate with the pectoral or pelvic girdle via three basal cartilages: the propterygium, the mesopterygium and the metapterygium.


Primitive Paired Fin Skeleton

Advanced Paired Fin Structure


From Benton (2000) with permission

Jaws

The jaws of chondrichthyans are, like the jaws of all other gnathostomes, composed of an upper and lower component. The upper jaw component is a cartilaginous element called the palatoquadrate, and it lies below the neurocranium (the part of the skull that surrounds brain). The lower jaw component is Meckel's cartilage. The palatoquadrate and Meckel's cartilage are thought to be derived from the upper and lower halves, respectively, of the second gill arch of a jawless ancestor. Behind the jaw, the hyomandibular supports the palatoquadrate, and the ceratohyal supports Meckel's cartilage.

From Benton (2005) with permission

Chondrichthyans exhibit a range of jaw attachment styles:

1. Amphistylic attachment The palatoquadrate is attached to the neurocranium in two places (in front and in back). This is seen in many primitive sharks.
2. Hyostylic attachment The palatoquadrate is attached to the neurocranium in the front only, and is supported almost entirely by the hyomandibular. This allows the upper jaw to swing forward, resulting in a wider gape. Many modern sharks exhibit this style of jaw attachment.
3. Autostylic attachment The palatoquadrate is fused completely to the neurocranium, and is not supported by the hyomandibular. Chimaeras exhibit autostylic jaw attachment.

Jaw Schematics provided by Dr. Jon Campbell

Teeth

Chondrichthyans are the first gnathostomes to have true teeth. These teeth are composed of a dentine core (with a central pulp cavity) and an enameloid cap. Dentine is a skeletal tissue that contains a fibrous protein called collagen within a mineralized matrix, and enameloid is a shiny, hypermineralized, brittle tissue that lacks collagen. Chondrichthyan teeth are similar in tissue structure to the teeth of all other vertebrates, though they lack bone at their base. Chondrichthyan teeth are also similar in structure to the dermal denticles that cover the animal's skin (dermal denticles also have a pulp cavity surrounded by dentine, and a layer of enameloid), and this has led to the now widely accepted hypothesis that teeth are derived from dermal denticles that were located on the dental lamina (= the layer of skin that covers the edge) of newly evolved jaws.

The morphology (= shape) of chondrichthyan teeth has change substantially across evolution. As a result, the Chondrichthyans have experimented with a number of disparate tooth types. The primitive morphology was a three-cusped tooth, with a tall central cusp, and shorter cusps on either side. These teeth also had very little root, and this facilitated tooth loss and replacement (a common characteristic of chondrichthyan dentition). Ligament-mediated tooth whorls, the sophisticated tooth replacement mechanism seen in modern sharks, were present in their extinct ancestors, and this ensured that teeth were always sharp and functional.

A Tri-Cusped Elasmobranch Tooth

(Photo by A. Gillis)

  • And now for the specifics...Chondrichthyan Synapomorphies
  • Back to Characters and Anatomy


  • Author: Andrew Gillis
    Last updated: 15 November 2004
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