The Evolution of Chordates

Phylum Chordata

belongs to clade

Deuterostomata.

Deutrostomes have

events of development

in common with one

another.

1. Coelom from

archenteron

surrounded by

mesodermal tissue.

2. Formation of the

mouth at the opposite

that of the anus which

developed from

blastopore.

3. Radial cleavage during embryonic development.

*There are some deuterostomes that have been removed

from the clade due to DNA analysis. This includes

ectoprocts and brachiopods. These two phyla have been

moved over to the Clade Lophotorchozoan.

Characteristics of

Chordates (At sometime

during their life)

-Notochord-longitudinal

rod of cartilage between

digestive tube and nerve

cord. Used for support.

Many have only in the

embryonic stage.

-Dorsal hollow nerve cord derived from ectoderm during

neuralation. Other animals have a solid cord on the ventral

side

-Pharyngeal cleft (slits) grooves posterior to the mouth.

Develops into gills, ears, head neck etc.

-Post-anal tails that extend beyond digestive tract (sometime

during their life). Can be reduced in embryonic stage. Used

for movement.

Three subphyla

I. Subphylum

Cephalochordata-

Lancets keeps all 4 char-

acteristics of chordates

through adulthood. Has

series of muscles on

trunk used for

swimming. Does not

have cranium.

II. Subphylum Urochordata -has 4 characteristics of

chordates in larval stage but loses 3 (retains gill slits) as a

sessile adult that undergoes extreme metamorphosis. Does

not have a cranium

III. Subphylum Craniates -has a cranium or head with a brain

with sense organ evolving. Has two sets of Hox genes.

-Evolution of neural crests forming between the neural tube

and ectoderm. These cells give rise to some structures

unique to vertebrate skull and jaw.

-The pharyngeal gill slits become gill slits associated with

breathing. Terrestrial craniates pharyngeal gill slits become

other structures.

Classes in III Subphylum Craniates

1. Class Myxini (hagfishes) skull of cartilage, no vertebrae, no

jaw, retains notochord into adulthood. Swims with

segmented muscles attached to notochord. Produce slime

as protection.

2. Class Petromyzontida (lampreys)are

primitive vertebrate with a skull of

cartilage, it also retains notochord into

adulthood. The notochord is

surrounded by a cartilage like pipe

with primitive like vertebrate

extensions enclosing nerve cord, no

jaw. Mouth is sucker like.

These are the classes

that are found in the

clade vertebrata.

The next important evolutionary development was jaws. This

developed from the rods in the pharyngeal gill slits.

Vertebrates that have jaws are termed gnathostomes

Jaws allowed vertebrates to eat larger and more varied types

of food by shredding and tearing the food apart.

Intermediates fossilized animals between jawless and jawed

vertebrates have been found. Jawed vertebrates also have

larger cranium.

3. Class Condrichthyes- Include sharks and rays. The skele-

ton is composed of cartilage not bone. (Evidence suggests

once it was bone). Evolution of fins increased swimming

efficiency. Dorsal fin stabilizes shark, pectoral and pelvic fins

allows for lift and forward movement. Oil in liver provide

buoyancy, no swim bladder or lungs. Intestine with spiral

valve.

Reproduction-

Oviparous-Eggs laid outside the body

Ovovivparous-Eggs retained in oviduct and develop there

Vivparous-Eggs develop in the uterus with yolk sac placenta

4. Class Actinopterygil- Ray-finned fish have bony

endoskeleton of calcium phosphate. Gills are covered with

an operculum. Buoyancy controlled swim bladder (was once

ancient lung) gas goes in and out by blood. Covered by

scales covered by mucus. Fins are supported by bony rays.

Evolved first in freshwater and moved to marine environment.

Some live in both environments during their life history.

Reproduction mostly

Oviparous-Eggs laid outside the body with external

fertilization

Next evolutionary

advance was a

bony skeleton.

Animals with a

bony skeleton are

in the clade

Osteichthyes

5. Class Actinistia- lobe-finned fish have bony endoske-leton

of calcium phosphate.

Fins are supported by a single rod-shaped bone surrounded

by a thick layer of muscle in their pectoral and pelvic fins.

Allowed them to swim and “walk” underwater. Only two

species survive today and were thought to be extinct until a

fisherman caught one in 1938 in the Indian Ocean.

6. Class Dipnoi- lung fish

have bony endoskeleton

of calcium phosphate

and lobbed-fins. These

fish are freshwater fish

living in stagnant water

in the southern

hemisphere. They gulp

air that goes to the

lungs. This oxygen is to

supplement the oxygen

that gills are extracting

from the water. Lobe-fin

fish are the closest living

relative to amphibians or

tetrapods.

The next development is the evolution of four limbs (Clade

Tetrapod). Tetrapods have four limbs. The neck has two

veterbrae instead of one giving rise to side to side motion as

well as up and down motion. Pelvic girdle fused to the

backbone to reduce to force put on the legs. Pharyngeal

clefts (no longer slits) give rise to the parts of the ears,

glands and other structures.

The next develop-

ment was the

evolution of four

limbs (Clade

Tetrapod). The

closest living tetra-

pod to the lungfishes

are the amphibians.

The fossil record

indicates that many

other clades evolved

to give rise to the

class Amphibia. The

evolution of legs

allowed the verte-

brates to move on to

land. Amphibia

means dual life.

7. Class Amphibia- Most amphibians reproduce in water. 3

chambered heart, lungs sac-like. Uses skin to help

breath. 3 orders of amphibians.

A. Oder Urodela-salamanders retain tail as adults-legs out

to. Walks side to side.

B. Order Anuara-tadpoles juvenile form “fish-like” with

gills, coiled intestine, long tail. Later absorbs tails. Legs

emerge. Digestive tract specializes.

C. Order Apoda- legless nearly blind. Look like earthworms.

Lost legs in the course of evolution.

Most amphibians reproduce in water, then live on land but

there are some amphibians entirely aquatic or entirely

terrestrial. Most salamanders and caecilians do not

undergo metamorphosis instead the juvenile form looks

like the adult form.

Clade Amniotes- are tetrapods that have an amniotic egg

which allowed tetrapods to be entirely terrestrial and not

go back to the water to reproduce. Amniotic egg has

extraembryonic membranes or membranes derived the

embryo but no longer a part of the embryo.

Membranes within Eggs

a. Amnion-Contains fluid that surround the embryo.

Cushions against mechanical shock.

b. Allantois-Garbage bag contains waste materials. Also

aids in gas exchange with the chorion

c. Yolk sac- contains yolk or stockpile of nutrients for the

embryo. The egg white stores protein and is a source of

amino acids for the embryo

d. Chorion combine with the allantois and aids in gas

exchange.Surrounds the embryo and all the membranes.

e. Albumen found outside the chorion but inside the shell

as a source of amino acids.

Clade Amniotes- gave rise the two clades- reptiles and

synapsids. The Class Reptiles gave rise to diapsids and

anapsida. These clades are based on the structure of

the skull. The Class Mammalia in the clade synapsid.

The holes in the skull are thought to provide better

attachment for jaw muscles which allowed the animals

to capture and digest a greater variety of prey.

The reptiles have scales, and internal fertilization. Most are

ectothermic except for birds which are endothermic and

have an extreme high metabolic rate.

Turtles are thought to have been anapsids but other

molecular evidence have them more closely related to

crocodiles.

Snakes, lizards, and tuataras are diapsids belonging to the

linage lipidosaurs. This group is greatly varied. Snakes

evolved from tetrapods. Their scales are overlapping.

Crocodiles and birds are diapsids belonging to the linage

archosaurs. This linage also includes dinosaurs. Birds

are adapted for flight

-Hollow bones

-4 chambered heart (crododiles also have 4 chambers)

-Endotherms for high metabolic rate

-Feathers which are modified scales and legs are

covered with scales.

-Beaks (adapted for food source) Once had teeth.

Class Mammals belong the clade synapsids which have two

holes in the skull.

-4 chambered heart

-endotherms

-mammary glands that produce milk

Three clades

-Monotremes-Egg layers (only 5 species) no nipples but

produce milk

The other two clades produce eggs and all amniotic

membranes are present but offspring develop in the

uterus forming a placenta

-Marsupials- Higher metabolic rate, nipples, young

complete development in pouch (marsupium).

-Eutherians- Higher metabolic rate, nipples, no pouch,

development completed in the uterus. Embryo retained

longer.

Marsupials were isolated from Eutherians in Australia.

Adaptive radiation and convergent evolution occurred.

Many of the marsu-

pials in Australia fill

the same niche as

a placental

mammal in North

America. Case of

convergent

evolution due to

the same selection

pressures.

This shows the relationship between the various orders

mammals