Download LECTURE 18 Phylum Chordata and the invasion of land

BIO 120
Field Natural History
Spring
1
LECTURE 18
Phylum Chordata
and the invasion of land
I.
Vertebrates are members of the phylum Chordata.
A. Chordates are distinguished from all other animals by 4 characteristics.
1. A dorsal hollow nerve cord. This differentiates into the spinal cord and the brain.
2. A notochord - a flexible supporting structure below the nerve chord. In most vertebrates, it is replaced by
the backbone, which surrounds the nerve cord.
3. Pharyngeal gill slits. At some stage in their lives, even if only temporarily during the embryonic stages,
chordates possess perforations in the wall of the pharynx ( the anterior portion of the digestive tract).
4. A postanal tail. A tail that extends beyond the end of the digestive tract (anus) – at least during their
embryonic stage.
5. Numbers 2-4 may be lost in the adult stage, but all are present embryonically.
II.
Embryological evidence for membership in the phyla Chordata.
A. Subphylum Urochordata – the tunicates. (1250 species)
1. Most adult tunicates are stationary, filter-feeding organisms that scarcely resemble other chordates.
2. The larvae however, are free-swimming and resemble tadpoles, but only for a few days.
3. They lose their notochord, dorsal nerve cord and the postanal tail. They retain the pharyngeal gill slits,
which they pump seawater through and filter out food particles.
B. Subphylum Cephalochordata – Lancelets. (about 23 species)
1. Lancelets are small (usually under 5 cm) torpedo-shaped, fish-like marine chordates.
2. Many characteristics that link them to the tunicate ‘tadpoles’, but also have some advanced traits that link
them to higher vertebrates.
3. Segmental muscles organized into chevron-shaped units on either side of the notochord. This foreshadows
the similar arrangement found in fishes.
4. Fossil evidence suggest that lancelets are a primitive chordate.
III. Aquatic Chordates – Fish and almost fish
A. Innovation and adaptive radiation of aquatic chordates.
1. The evolution of terrestrial vertebrates was marked by two significant innovations in the body plan.
a. True jaws.
b. Paired fins
B. Jaws? Not yet.
1. The first fish to evolve were called agnathans (Class Agnatha) – this means “without jaws”.
2. They had a mouth, but they could only suck food into their mouths. Many were bottom feeders.
3. Half of existing species are hagfish. Evolutionary relics.
4. The other half of the existing Agnatha are the lampreys. These are eel-like parasites.
a. They may still exist because they have specialized as parasites on other fish.
b. They have a round mouth that functions like a suction cup.
c. They attach to the side of a fish, and rasp through the skin of the fish with their tongue. They then suck
the blood through the hole.
C. The evolution of Jaws.
1. The evolution of fish in the sea has been dominated by two evolutionary questions.
a. What is the best way to grab hold of potential prey?
b. What is the best way to pursue prey through water?
2. The best way to hold onto prey was with strong biting jaws that had jagged bony edges that served as teeth.
a. Jaws allowed predation, rather than filter feeding or scavenging.
b. Jaws also provided a means of defense.
c. The evolution of jaws came about from the conversion of gill structures (arch supports).
d. Natural selection does not create something from nothing, but rather, it modifies exiting structures.
e. Organisms are not necessarily perfect for their environment, but they are the best with the materials at
hand.
BIO 120
Field Natural History
Spring
2
D. Other adaptations of early fish.
1. They had a more streamlined design for swimming – wedge-shaped. Minimizes turbulence.
2. The development of an array of fins that provided more control while swimming.
a. First- a large propulsion fin on the tail that drives the fish forward with its back and forth motion.
b. Second –one or two stabilizing fins on the dorsal (back) side of the fish to keep it from rolling over.
c. Third – There may be a ventral fin (belly) which acts as a keel and keeps the fish from side slipping.
d. Fourth, the presence of paired fins at the shoulders (pectoral) and hips (pelvic).
i. Serve as elevator flaps, so as the tail propels the fish forward, it doesn’t dive into the bottom.
ii. They also help the fish turn left or right or stop suddenly by acting as rudders.
Two major lines of fish developed.
E. The cartilaginous fishes – (Class Chondrichthyes) these are the sharks, the skates and rays.
1. They are called cartilaginous because their skeletons are made from firm, but resilient cartilage.
2. This makes these fish lighter (for a given size) than fish with bones. This makes them faster.
3. Sharks have extremely good eyesight, and a very sensitive chemosensory system (sense of smell).
a. They are among the planet’s most efficient and sophisticated predators.
F. The bony fishes – (Class Osteichthyes)
1. Bony fish evolved at the same time as sharks.
2. Instead of gaining speed through lightness, as sharks did, bony fishes adopted a heavy internal skeleton
made completely of bone.
3. Such an internal skeleton is strong, and provides a basis against which very strong muscles could pull.
4. Although bones are heavier than cartilage, bony fish are still buoyant because they possess a swim bladder.
a. This is a gas-filled sac that allows them to regulate their buoyant density and allows them to remain
suspended at any depth in the water effortlessly.
b. In contrast, sharks must constantly move through the water or sink.
c. The swim bladder is the precursor to the lungs of terrestrial animals.
5. 18,000 species bony fish with swim bladders. That’s more species than all other vertebrate species combined.
G. Two types of bony fish.
1. Ray-finned fish. These are the most dominant fish in existence today.
2. Lobe-finned fish. Only a few species survive today. Surprisingly, they provide a clue to the evolution of
land animals.
a. Lung-fish - a pouch connected to the upper digestive tract evolved into a primitive lung that enabled it
to breathe air. (6 species exist)
b. Coelacanth- the sole survivor of the primitive group of fleshy-finned fish that are thought to have
crawled around in swamps on their four, stubby, fleshy fins.
i. Thought to be extinct for hundreds of millions of years – only known from fossils. Discovered in
the western Indian Ocean in 1938.
ii. Each fin consists of a long, fleshy muscular lobe supported by a central core of bone.
iii. Unlike ray-finned fish, which have no muscles in their fins, the coelacanth has muscles within
each lobe that can move the fins independently of each other. Ray-finned fish could not do this!
IV.
The invasion of land.
A. Class Amphibia (from Greek “both lives”)
1. The anatomical similarities of the limb bones of the lobe-finned fish with those of amphibians suggests that
the lobe-finned fish might be the ancestors to the tetrapods (4 foots).
a. The bones in the fins and legs are homologous structures.
2. The evolution of tetrapods from lobe-finned fishes may have been from one of the following selection
pressures.
a. For dispersal between ponds when one pond dries-up.
b. Escape competition or predation in the water.
c. Exploit a new resource – they may have eaten early land arthropods – the insects.
3. Frogs and salamanders are considered the direct descendants of fish. They are the sole survivors of the very
first group of vertebrates to walk on land.
BIO 120
Field Natural History
Spring
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4. Three innovations that permitted amphibians to invade land.
a. They have 4 legs –Legs were necessary to support the body’s weight as well as allow movement from
place to place.
b. Lungs. Lungs are necessary, even though there is far more oxygen in the air than in water. The
delicate structure of fish gills requires the buoyancy of water to support them, or else they collapse.
c. The heart had to be redesigned to deliver the greater amount of oxygen required by the exertion of
walking muscles.
5. But 2 limitations still exist for most amphibians.
a. Reproduction is still carried out in the water or else the eggs would dry out.
b. Like early plants, they lacked a waterproof outer layer. Most amphibians are not very drought
resistant and need to return to water. Many others, toads and salamander, live in damp or moist places.
c. They are the most numerous terrestrial arthropod on earth today – there are more species amphibians
than of mammals.
B. Class Reptilia
1. If you think of amphibians as the first draft of a manuscript about survival on land, then reptiles were the
finished book.
2. For each of the five key challenges of living on land, reptiles improved on the innovations first seen in
amphibians.
3. The key reptilian adaptation is the amniotic egg.
a. Most reptiles lay watertight eggs that contain a food source (a yolk) with a waterproof shell and a
series of 4 membranes. Each membrane plays a role in making the egg an independent life-support
system, some by allowing oxygen to pass in, but prevent water from escaping.
b. The reptile egg is analogous to the seed of angiosperms.
4. Dry skin – reptiles have skin covered with scales made of keratin, the same protein as in our hair.
a. This is a waterproof layer on the skin, protecting them from dehydration. This is analogous to the waxy
cuticle of plants that helped them invade land.
5. Leg arrangement – The legs of reptiles are arranged to be more effective at supporting body weight, thus
allowing reptiles to be bigger than amphibians, and to run.
6.& 7. The heart and lungs were altered to make them more efficient for a faster moving animal.
8. Reptiles have been a very successful group – more common than mammals.
a. There are 3 reptile species for every mammal species.