1. Bone is a tissue unique to vertebrates.

2. It is a strong, hard material that is a composite of inorganic calcium phosphate mineral crystals plus organic collagen fibers.

John Ruber, of Oregon State University has made a rather interesting hypothesis:

If metabolism is slow and steady, oxygen can be used by the muscles in aerobic to produce energy. But if there is a rapid and powerful burst of energy, anaerobic glycolysis is used to break down sugars in the muscles. The result of anaerobic glycolysis is lactic acid buildup in the blood.

As the lactic acid is removed from the muscles and oxygen debt is payed back to the muscles, blood acidity fluctuates and calcium in bones is dissolved. If the skeleton was of calcium carbonate, much of it would dissolve during this process, leaving the body structurally weak.

- Forebrain - large olfactory bulbs (smell, taste and other chemosensation), small cerebral hemispheres (memory), and a pineal structure (produces melatonin and is responsible for biological clocks).

- Midbrain - large optic lobes (vision).

- Hindbrain - small cerebellum (higher muscular coordination) but medula oblongata is well developed (communicate with the spinal nerve cord and thus controls reflexes, heartbeat, & respiration).

A. Agnatha - jawless vertebrates
B. Gnathostoma - vertebrates with jaws

The animals you know as FISH are actually several different animals - some agnathans and some gnathostomes. In other words, fish is a common name, like worm, and not a taxonomic name.

The fish include

1. All of the Superclass Agnatha - Jawless Fish

2. Some of the Superclass Gnathostoma - Jawed Vertebrates

a. Class Acanthodia

b. Class Placodermi

c. Class Osteichthyes - Bony Fish

d. Class Chondrichtyes - Cartilagenous fish

B. These most primitive jawless fish are little different from cephalochordates with bone.

Lamprey

Hagfish

a. Found in the early Paleozoic (Ordovician) 505 mya

b. Small (few inches to a foot long initially).

c. Freshwater bottom-feeding animals. -- The circular mouth lacks jaws. Use their gills as both straining devices and as respiratory structures.

d. There is no internal bony skeleton instead thick bony plates and scales that cover the body.

These scales provide three advantages

i. phosphate reserves
ii. to stiffen and allow the body two maintain its shape as size increases.
iii. protection against predation and abrasion as moved along the substrate.

e. There are no fins. Only a tail sticks out behind the box of bony scales, and it is able to flex from side to side thus propelling the animal forward.

f. Notocord retained at maturity and partially enclosed in vertebra.

g. Single median nostril, 2 lateral eyes, median pineal eye (photoreceptor to adjust activity day/night)

h. Closed circulatory system with arteries and veins and a 2-chambered heart that consists of an atrium (which receives blood) and a ventricle (which pumps blood into the arteries).

i. At the anterior end of the nerve cord is a brain in a thin case of bone.

During the Silurian, the continents had collided to form three large land masses:

Euramerica - North America and Europe

Angaraland - Siberia and Asia

Gondwanaland - Southern continents (South America, Australia, Africa, Antartica)

(1) more streamlined with dorsoventrally flattened heads, and could probably move relatively rapidly (compared to earlier fish) just under the surface of the water.

(2) To aid in balance while moving rapidly in this way, these fish had a dorsal bony spine and the tail was longer on its ventral surface than on its dorsal surface

(3) Headshields became elborate and sometimes hydrodynamically wing shaped.

(4) In the Devonian fish with paired fins appeared. In other respects they were similar to the earlier agnathans in that they had a plated headshield and a flexible tail that provided the propulsion.

In one group, the eyes are small and set close together on the top of the headsheld. In addition there are large sensory areas on each side of the headshield covered with very tiny bony plates. The organs probably served as pressure sensors in murkey water, although it has also been suggested that they sensed electrical fields, just as sharks do today.

IV. Evolution of Jaws

A. Connected with Respiration - Functional shift.

1. The evolution of jaws and a resulting extension of potential food supply was the key to a tremendous ecological expansion and evolutionary success of jawed vertebrates. It also allowed gills to specialize in gas exchange (rather than serve the dual function of gas exchange and filter feeding).

2. No series of fossils shows the stages of jaw development, but embryological evidence suggests that they evolved by modifying slender bones in the throat region that support the gill arches of jawless fish.

a. The gill slit is supported by an upper and lower bone that are hinged, so becoming a hinged jaw is not to far fetched.

b. Began as a pump to move water through the gills.

2. Gill arches behind the jaw with spikes called gill rakers to strain out food particles from the respiratory current.

3. Have paired fins - several on dorsal surface but not really used for locomotion (that came from tail) probably used for manuverability.

4. Bone around the brain and cartilage in the fins.

The evolution of fish after the acanthodian fish (since the appearance of jaws) has largely been a story of modifying the jaw and increased manuverability.

Following the evolution of the Acanthodians, there is an apparent three-way split in the evolutionary tree of vertebrates:

1. The Placoderms (all extinct)

2. The Chondrichthyese (sharks, skates, rays, etc)

3. The Osteichthyese (and their relatives the land vertebrates) (bony fish)

1. Paired pelvic fins

2. Jaws evolved as the mouth was displaced posteriorly and became associated with the bony supports of the gill slits. This allows the adaptation to feed on many different types of food and transformed the vertebrates from filter feeders into predators.

3. Had a spectacular head shield and a long powerful tail.

1. Members of the Chondrichthyes all lack true bone and have a skeleton made of cartilage. Only their teeth, and sometimes their vertebrae, are calcified.

2. Gill slits exposed.

3. Body covered with placoid scales in mouth and used as teeth.

4. Excellent sense of smell and electrical sense that allows them to be good hunters.

5. Circulatory system as in Agnathans - 2-chambered heart with an atrium and ventrical.

6. Brain more highly developed and all parts larger but especially the Forebrain where there are 2 very large olfactory bulbs (in other words smell and taste are the most important senses).

7. Very successful in the Devonian (408-360 mya) - much of the diversity disappeared at the end of the Permian, but this group is still around today. They should not be considered primitive but instead they are animals that discovered a sucessful way of life 350 million years ago and have not needed to change it.

Class Osteichtyes

1. The first bony fish appeared about 410 mya along with the placoderms and probably related to them in the Devonian.

2. These fish have a bony skeleton and an operculum or flap covering the gill opening.

3. For the first time we see organisms with an axial (body) and appendicular (arms and legs) skeleton. They have two pairs of fins attached to bony girdles - the pelvic and pectoral fins. These fins, together wih the tail fin give greater mobility. The dorsal and anal fins give stability.

4. The jaws become more complex and capable of extensible movement.

Many features are retained from ancestors:

5. Circulatory system - 2-chambered heart with atrium and ventrical.

6. Brain - larger than in agnathans but still consists of forebrain, midbrain and, hindbrain

7. Dermal scales - apparently remnanets of bony sheilds of ancestors just not ossified.