the evolution of chordates of the phyla...aids in gas exchange with the chorion c. yolk sac-...
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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
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