Phylum Chordata belongs to clade
Deuterostomata. Deuterostomes 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. 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 neurulation. 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
characteristics 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.
Phylum Chordata belongs to clade Deuterostomata. Deuterostomes 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.
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. Produces slime as protection.
2. Class Petromyzontidae (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 Chondrichthyes- Include sharks and
rays. The skeleton 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 provides
buoyancy, no swim bladder. Intestine with
spiral valve.
ReproductionOviparous-Eggs laid outside the body
Ovoviviparous-Eggs retained in oviduct and develop there
Viviparous-Eggs develop in the uterus with yolk sac placenta
Next evolutionary advance was a bony skeleton. Animals with a bony skeleton are in the Clade
Osteichthyes4. Class Actinopterygii- 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. Bony rays
support fins. Evolved first in freshwater and moved to marine environment. Some live in both
environments during their life history.
Reproduction mostly oviparous5. Class Actinistia- lobe-finned fish have bony endoskeleton 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
6. Class Dipnoi- lung fish have a bony endoskeleton of calcium phosphate and lobbed-fins. These
fish are freshwater fish living in stagnant water in the southern hemisphere. They gulp air to
supplement the oxygen that gills are extracting from the water. Only six species survive today.
The next development is the evolution of four limbs (Clade Tetrapod). Tetrapods have four limbs.
The neck has two vertebrae 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 closest living tetrapod 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 vertebrates 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. Order Urodela-salamanders retain tail as adults-legs out to. Walks side to side.
B.Order Anura-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
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 combines 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 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 lepidosaurs. 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 (crocodiles 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
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.
Marsupials were isolated from Eutherians in Australia. Adaptive radiation and convergent
evolution occurred.
This shows the relationship between the various orders mammals