Natural History of Vertebrates
Lecture Notes
Chapter 5 - Radiation of the Chondrichthyes
These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).
List of Terms
Chondrichthyes first appear in the Late Silurian and are represented by two modern groups (the Holocephali and the Neoselachii). There are three fairly distinct grades in shark evolution.
early elasmobranchs (represented by Cladoselache, figure 5-1)
- appeared in the Late Silurian and were extinct by the Middle Triassic
- large fins
- a keel present at the base of the tail indicates that it may have been a fast, open-water swimmer.
- The paired fins (pectoral and pelvic) were triangular with the base of the fin broadly attached to
the body. This arrangement prevents the fin from being turned to change the angle of attack with
the water, thus the fin served as a stationary hydrofoil.
- mouth was terminal
- jaw suspension was amphistylic, which means that the upper jaw, the palatoquadrate, was firmly attached to the cranium by ligaments in three places; the symphysis, behind the eye, and the ear region. This jaw suspension prevented the jaw from being protruded when feeding.
- teeth had three cusps, with the middle cusp generally larger
Teeth were replaced on a regular basis as new teeth were constantly being developed from a tooth whorl (figure 5-2)
- Caudal fin had the notochord in the upper lobe and the rest of the fin was supported by fin rays.
- Fertilization was internal as males had claspers, which indicates a complex courtship behavior
and also limited numbers of young produced by each female.
second elasmobranch radiation (represented by Hybodus, figure 5-3)
- appear in the Carboniferous and extinct by the Late Cretaceous
- The paired fins, instead of being broadly attached to the body, were attached via a stalk around
which the fin could rotate. Ceratotrichia extended to the margin of the fins and intrinsic muscle
of the fin allowed the fin to change shape. Presumably, Hybodus had much more control over its
movements than did Cladoselache (figure 5-3)
- In the caudal fin, the upper lobe was larger than the lower lobe and the fin was stiffer and thus
produced more forward thrust
- Dentition was heterodont with slashing and piercing teeth forward in the jaw and crushing teeth to the
rear. This allowed a more diverse diet (figure 5-3)
- mouth is still terminal, retained an amphistylic jaw suspension, unrestricted notochord, and
multicusped teeth
- claspers are common among the different species
modern radiation (Neoselachii)
- appear in Early Triassic, modern genera by Cretaceous
- probably arose from a Cladoselache-like ancestor
- They developed a snout that overhangs a ventral mouth with
hyostylic jaw suspension. An enlarged hyomandibula attaches to the otic region of the braincase
and braces the posterior part of the palatoquadrate. In addition, flexible ligaments attach the
palatoquadrate to the front of the cranium (figure 5-5). This type of jaw suspension allows a
wider gape and also allows the jaws to be protruded forward for plucking items or to get deeper
penetration of the teeth into a prey item. During feeding a shark often bites deeply and then
shakes its head from side to side to tear off large pieces of flesh from its prey.
- Notochord has been replaced by cartilaginous centra that calcify; neural and haemal arches
protect the spinal cord and blood vessels that run next to the notochord.
- the scapulocoracoid cartilage fuses at the midline and provides for additional rigidity in the pectroral girdle.
- They have placoid scales, single cusped with a single pulp cavity, that make the skin of a shark feel very
rough. The projection of these scales reduces turbulence in the flow of water near a shark and
thus increase swimming efficiency.
- Sharks lack a swim bladder but control buoyancy by having a liver that stores a large quantity of
oil such that a 460 kg shark may weigh only 3.5 kg saltwater. The oil compensates for the
weight of scales, teeth, and calcified cartilages and allows a shark to maintain near neutral
buoyancy.
- The sensory system of sharks is very well developed. In fact they receive such an abundance of
information that their brain size is equal to that of some tetrapods.
Mechanoreception is through receptors called neuromast cells that are distributed throughout
their head and body as part of the lateralis system (lateral line system) (study pages 80-81 and figures 4-4 and 4-5). They also detect vibration through the sensory areas of the inner ear.
Electroreception is through receptors called ampullae of Lorenzini (study pages 82-86 and figures 4-7, 4-8, and 4-9). These are also distributed over the surface of the head region and can detect voltages as low as 0.01 microvolt/cm. Electroreception allows a shark to detect prey by the electrical signals given off
by the prey's muscular contractions.
Chemoreception is also well developed and allows a shark to detect and follow a blood trail in the
water. Can detect some chemicals as dilute as 1 part in 10,000,000,000 (1 drop in about 79,000 gallons of water).
Vision is highly developed and sharks have a tapetum lucidum that increases a sharks visual
sensitivity in dim light. Sharks can also cover the tapetum lucidum by expanding cells that
contain melanin to reduce light sensitivity in bright light.
When approaching a prey item, sharks switch from one sense to another as they approach a
potential prey item. chemoreception --> mechanorecpetion --> visual --> electroreception.
- Fertilization is internal via claspers (males) that are used as an intromittent organ (figure 5-6).
Elasmobranchs in general have eggs with large yolks and thus produce fewer eggs. In oviparous
forms, the fertilized eggs are deposited in a sac that gets entangled in algae. Development to
hatching usually takes several months. Some forms retain the eggs in the female's body until
hatching.
In some species all of the nutrients for development comes from the yolk and the mother only supplies
gases and inorganic ions (lecithotrophy).
In some species, the mother supplies nutrients to the developing embryos (matrotrophy). In
some cases, the mother continues to ovulate and the embryos eat the unfertilized eggs. Others
develop long projections of the oviduct wall that enter the mouth and gill openings and secrete a
milky fluid. Finally, some develop a yolk sac placenta such that nutrients pass directly from the
bloodstream of the mother to the bloodstream of the embryo (placentotrophic viviparity).
However, once born there is no more parental care and the young are born as small versions of
the adults.
- Pleurotremate elasmobranchs (sharks) number about 360 extant species.
Hypotremate elasmobranchs (skates and rays) number about 460 extant species. These have a
benthic life style. The pectoral fins are greatly enlarged and swim by undulating these enlarged
fins such that they seem to fly through the water (figure 5-8). Primarily eat benthic
invertebrates many of which have shells, thus skates and rays are durophagus. Generally
sexually dimorphic with females larger than males. Males of at least one species, change the
type of dentition during the breeding season to sharp cusped teeth for grasping the female. Some (skates)
have tissues that can generate an electric field in some cases up to 200 volts of potential.
Skates have two dorsal fins on a rather stout tail and are oviparous. Rays do not have dorsal fins, but do have a venomous dorsal barb from a tail that is generally slender, and rays are viviparous.
Holocephali (ratfishes, figure 5-9)
- appear in the fossil record during the Jurassic
- about 34 extant forms
- generally confined to deep water (80 m or more)
- ancestors are not well known
- oviparous with eggs laid in horny shell
- fertilization is internal via claspers.
- benthic invertebrates make up most of the diet
Last updated on 5 February 2008
Provide comments to Dwight Moore at mooredwi@emporia.edu
Return to the Natural History of Vertebrates Home Page at Emporia State University.