Phylum Chordata
A Tribute to the Diversity We Know
Introduction
•
Members of the Phylum Chordata include animals
with which we are probably most likely familiar
(including fish, birds, reptiles, amphibians and
beasts like ourselves)
•
As unlikely as it seems, based on embryological
evidence, the echinoderms appear to be the mostlikely ancestors to the early chordates
•
Primitive stemmed echinoderms are thought to
have shifted from arm-feeding to filter-feeding
acquiring a body plan similar to urochordates
•
Unfortunately, the fossil record is poor and
intermediates are lacking. The rest of the
evolutionary picture is better documented.
Introduction
• Four structural characteristics set
chordates apart from all other phyla:
–
–
–
–
a notochord
a pharyngeal gill slits
postanal tail
a hollow dorsal nerve cord
• These attributes are always found in the
larval forms or early embryo (although
they may be absent in the adult).
1) The Notochord
• The notochord is mainly
composed of fibrous
connective tissue
• For those animals in
which it persists into the
adult form, the notochord
provides support (it acts
like our backbone) and
increases swimming
efficiency
• In animals like ourselves,
bony structures called
vertebrae develop near
the notochord and
eventually replace it
during embryogenesis
2) Pharyngeal Gill Slits
• Pharyngeal gill slits are cuts in
the pharynx that connect to a
cavity surrounding the
pharynx
• For organisms in which they
remain in the adult, they are
often elaborated into
respiratory structures (and are
sometimes involved in filtering
food during feeding).
• The morphological equivalent
of gill silts are seen briefly
during our own development
(weeks 4-5), but they usually
close or develop into other
structures
• Occasionally, the slits do not
close, resulting in the newborn
having an opening in the neck
area (a cervical fistula).
3) The Postanal Tail
• In all the phyla we have
studied up to this point,
the anus was terminal (at
the tip of the tail)
• Chordates, on the other
hand, follow the anus with
a tail of variable length
(again, an adaptation for
locomotion).
• In us, the tail is short and
fused (the coccyx at the
base of your spine).
4) Hollow Dorsal Nerve Cord
• Our nerve cord, like that of other chordates
is hollow (even in the adult).
• Well, what else is there to say?!?!?
Let’s start at the very beginning . . .
• Phylum Chordata is divided into three
subphyla:
– Subphylum Urochordata
– Subphylum Cephalochordata
– Subphylum Vertebrata
Subphylum Urochordata
• At first glance, you might
mistake this creature for a
sponge.
• Adult tunicates look like small
sacs (about 3 cm tall) and are
stationary, lacking a nerve cord,
a notochord, and a post-anal
tail.
• Lacking three of the four
distinguishing hallmarks of the
chordates, it would seem
impossible for these animals to
be placed in phylum Chordata.
Subphylum Urochordata
• However, tunicates
begin life in a larval
state, which have a
post-anal tail, a nerve
cord, and a notochord.
• Therefore, these
immobile animals with
tadpole-like larvae are
considered chordates.
Subphylum Urochordata
• Urochordates have a
notochord that extends
from just behind the tail to
the head (rather than from
head to tail; Urochordata
means "tail-cord")
• Many urochordates are
more commonly referred
to as “sea squirts”
• Contain cancer-fighting
chemicals
Subphylum Urochordata
• The body of an adult tunicate is quite simple, being essentially a sack
with large gill structures that form two siphons through which water
enters and exits. Water is filtered inside the sack-shaped body.
Subphylum Cephalochordata
• Lancelets are common bottomdwelling forms that possess all
four chordate characteristics
• They dig into the sand and lie
with their anterior end
protruding from the burrow.
• Unlike the urochordates, the
notochord extends along the
entire length of their body.
• This structure imparts rigidity to
their body and permits more
coordinated swimming
movements.
Subphylum Vertebrata
• Members of the Subphylum
Vertebrata differ from the
urochordates and
cephalochordates in having the
notochord replaced by a vertebral
column composed of bone and/or
cartilage. The vertebral column,
along with the cranium, limb girdles,
and limbs, make up the
endoskeleton. This internal
skeleton is an adaptation for
efficient locomotion, as was the
notochord
Subphylum Vertebrata
• Subphylum Vertebrata has several divisions you need to
be familiar with:
– Superclass Agnatha – Jawless Fish; Lampry Eel;
Ostracoderm (fossil)
– Superclass Gnathostomata
• Class Placodermi – First Jawed Fish (Fossils)
• Class Chondricthyes – Cartilagenous Fish; Sharks; Rays
• Class Osteicthyes
– Subclass Actinopterygii – Ray-finned Fish; Goldfish; Sea Horse
– Subclass Sarcopterygii – Lobe-finned Fish; Coelocanth
Subphylum Vertebrata
• Subphylum Vertebrata has several divisions you need to
be familiar with:
– Superclass Tetrapoda
• Class Amphibia
» Order Caudata – Tailed Amphibians; Salamanders; Newts
» Order Anura – Tailless Amphibians; Frogs; Toads
» Order Gymnophiona – Caecillians
• Class Reptilia
– Subclass Anapsida – Turtles; Tortoises
– Subclass Diapsida – Crocodiles; Snakes; Lizards
• Class Aves
– Subclass Archaeornithes – Archaeopteryx (Extinct)
– Subclass Neornithes – All Other Birds
• Class Mammailia
– Subclass Prototheria – Monotremes – Platypus; Echinda
– Subclass Metatheria – Marsupials – Kangaroo; Koala
– Subclass Eutheria – Placentals
Superclass Agnatha
• Agnatha (Jawless fish) are
primitive fishes with a fibrous
skeleton and an eel-like body
• They lack a jaw as well as the
scales and paired fins we
usually associate with fish
• Agnathans are mostly
represented as heavily armored
fossil forms, but today’s living
species are smooth-skinned,
completely unprotected by
armor
Superclass Agnatha
• Many species are parasitic
(they attach to the outer surface
of a fish with their sucker-like
mouth)
• Rasping teeth (arranged in a
whorl) cut into the host. The
lamprey then sucks blood from
the wound (a fish hickey?).
• When it is finished its
bloodmeal, the fish is released.
The injured fish usually dies
from blood loss or infection.
Superclass Agnatha
•
The agnaths are considered
to be an evolutionary dead
end, a side branch in the
phyletic tree that did not
lead to the more advanced
fish
•
The first agnaths were
ostracoderms
•
Though extinct, they give us
clues into how vertebrates
evolved from the
cephalochordates
Superclass Agnatha
• Notice the similarities of
the agnath larve to the
lancelet
• There are two main
classes under Agnatha:
– Class Petromyzontes
(lampreys)
– Class Myxini (hagfish)
Superclass Gnathostomata
•
Gnathastomata (Jawed fish)
contain a verticle biting appendage
•
Jaws are what makes it possible
for some gnathostomes to crack
nuts, rip of sections of meat, and
even crop grass.
•
Teeth have modified into a wide
range of forms, from teeth
designed to bite off and chew, to
teeth designed to strip bark, to
teeth designed to inject poison, to
teeth grown out into defensive
tusks.
Superclass Gnathostomata
Superclass Gnathostomata can be broken up
into the following classes:
• Class Placodermi – First Jawed Fish (Fossils)
• Class Chondricthyes – Cartilagenous Fish; Sharks;
Rays
• Class Osteicthyes
– Subclass Actinopterygii – Ray-finned Fish; Goldfish; Sea
Horse
– Subclass Sarcopterygii – Lobe-finned Fish; Coelocanth
Class Placodermi
•
The Placodermi are characterized by a
dermal armor consisting of a head
armor and a thoracic armor.
•
In the thoracic armor, the foremost
dermal plates form a complete "ring"
around the body and always include at
least one median dorsal plate
•
Jaws probably developed from the
anterior visceral arch that first served to
support the gills. The presence of jaws
and development of paired appendages
resulted in efficient eating and
locomotion
Class Chondricthyes
• Sharks, skates, rays, and
chimaeras are all members of
the Class Chondrichthyes.
• Their endoskeleton is entirely
cartilaginous (Chondros =
"cartilage"; "icthys" = "fish") and
all are carnivorous (as
exemplified by the great white
shark.
• The notocord persists into the
adult, and they have both
median and paired fins
Class Chondricthyes
•
•
•
•
They have two-chambered hearts
(one auricle, one ventricle). Only
deoxygenated venous blood flows
through the heart (which is then
pumped through gills before going
to the rest of the body).
Red blood cells are present, but
they're nucleated and oval.
Respiration is by five to seven
pairs of gills, each located in a cleft
and supported by cartilaginous
visceral arches.
Sexes are separate (dioecious),
and the gonads are usually paired.
Class Chondricthyes
•
The caudal fin of sharks differs from that of bony
fishes because it is asymmetrical. This heterocercal
fin provides both lift and forward thrust for the animal
when swimming (an important consideration since
sharks are heavier than water and have no swim
bladder to keep them buoyant).
•
Although there are many rows of teeth, only the outer
row or two is functional. The inner rows are
replacement teeth, which move forward when outer
teeth break off. Unlike ours, the upper jaw is not
rigidly connected to the braincase and can be moved
to open the jaws.
•
Although most sharks are predatory or scavengers,
the largest sharks (whale sharks and basking sharks)
feed on planktonic organisms they filter from the sea.
•
The outer skin layer is the epidermis, beneath which
a thick, fibrous dermis can be seen. The outer, hard
portion of the scale is enameloid, one of the toughest
materials produced in the animal kingdom.
Class Osteicthyes
•
The bony fish (Osteon = "bone";
"icthys" = "fish") are the most
diverse and numerous of all
vertebrates
•
They differ from most of the
cartilaginous fishes in having a
terminal mouth and a flap
(operculum) covering the gills.
•
In addition, most have a swim
bladder, which is ordinarily used to
adjust their buoyancy, although
among the air-breathing fishes it is
attached to the pharynx and serves
as a simple lung.
Class Osteicthyes
•
The skin has many mucus glands
and is usually adorned with dermal
scales.
•
Their jaws are well developed,
articulated with the skull, and
armed with teeth.
•
They have a two-chambered heart
built on the same plan as the
Chondrichthyes
•
The sexes are separate, most are
oviparous, and fertilization is
usually external.
Class Osteicthyes
• There are two subclasses:
Subclass Actinopterygii
(ray-finned fishes)
Subclass Sarcopterygii
(lobe-finned fishes)
Subclass Actinopterygii
• The Actinopterygii is the
larger of the two subclasses
• These animals have slender fin
rays suporting their fins and
lack the odd appendages of the
lobe-finned fish.
• Most have a symmetrical
caudal fin (homocercal tail) and
a swim bladder.
• This group includes most of the
fishes with which you are
familiar (bass, goldfish,
guppies, sea horses,
sturgeons, and tuna).
Subclass Sarcopterygii
•
•
•
•
•
Sarcopterygians have a fleshy lobe at
the base of their fins that is leg-like in
appearance.
They include fossil forms that are
ancestors of the amphibians, the true
lungfishes, and the coelacanth.
Today's coelacanths are "living fossils,"
represented by a single species
(Latimeria) found near the coast of
Madagascar.
Until 1938, when a coelacanth was first
captured by chance, they were known
only from the fossil record. Since then
several dozen have been captured and
some of their behavior has been filmed
using robotic cameras.
Latimeria is important because it
provides an opportunity to compare
observations from the fossil record with
a living animal.
Superclass Tetrapoda
• Tetrapoda means "four feet",
and the group was so-named
as its members primitively had
four limbs, as opposed to fins.
• This taxon includes about 3000
extant species of amphibians
(frogs, salamanders, and
caecilians) and approximately
18100 extant species of
amniotes (mammals, reptiles,
and birds).
Superclass Tetrapoda
• Superclass Tetrapoda has four major class
divisions:
•
•
•
•
Class Amphibia
Class Reptilia
Class Aves
Class Mammailia
Class Amphibia
•
The ancestors of today's
amphibians were the first
chordates to venture onto land
•
Although many changes in the
anatomy, physiology, and behavior
were required for terrestrial
colonization, several preadaptations among the early
amphibians eased their
colonization of terrestrial habitats.
•
Two barriers, however, keep most
amphibians from a totally terrestrial
lifestyle: respiration and
reproduction
Class Amphibia
•
•
•
•
Today's amphibians have a moist,
glandular skin with no scales (with a
few exceptions).
Most have two pair of limbs adapted for
walking and/or swimming. Their hearts
have three chambers (two auricles and
one ventricle)
Respiration is by the skin, lining of the
mouth, gills, and/or lungs, depending
on the species or stage in their life
history. Most cannot survive away from
water for very long because they lose
too much moisture through their thin
respiratory surfaces.
In addition, most amphibians require
water for reproduction and have an
aquatic larval stage. Fertilization may
be internal or external and most are
oviparous.
Class Amphibia
• There are three surviving orders:
– Order Caudata (newts and salamanders)
– Order Anura (frogs and toads)
– Order Gymnophiona (caecilians)
(Extinct Euryops)
Order Caudata
• Caudatans (salamanders and
newts) have poorly developed
limbs and retain a tail as adults
(the name Caudata refers to
the presence of a tail).
• They prey on worms, insects,
and small mollusks. Some
species have no lungs and
depend entirely on cutaneous
respiration. Others, such as the
mudpuppy (Necturus) and the
axolotl (Ambystoma), retain the
larval gills as adults.
Order Anura
• Frogs and toads belong to the
order Anura ("Anura" refers to
the lack of a tail in adults).
• They differ from the caudates
by having a more complex
skeletal system with stronger
limbs and developmental
metamorphosis (from a tailed
and limbless polliwog to a
tailless limbed adult).
• Of all the amphibians, anurans
have been most successful in
their occupation of terrestrial
habitats (including trees).
Order Gymnophiona
•
•
•
Caecilians, as they are more
commonly called, are wormlike
creatures found in tropical forests.
They make their livings by
burrowing through the soft soil
searching for worms and other tasty
invertebrates.
Several adaptations are related to
their subterranean habits: They
have no legs, are almost totally
blind (atrophied eyes are hidden
under the skin), and have sensory
tentacles.
Their skin is annulated, and some
have minute dermal scales. This
combination of characteristics
makes them look very much like
overgrown earthworms.
Class Reptilia
• Reptiles have acquired several
advances over amphibians that have
allowed them to move successfully into
terrestrial habitats.
• Their skin, for example, is more heavily
thickened and is protected with surface
scales that are impervious to water.
• A reptile's scales are very different in
structure from that of fish. The outer
layer of skin is a thick layer of dead,
keratin-filled cornified cells. These cells
are organized into horny scales
covering the entire outer surface.
• Since reptiles have internal fertilization,
water isn't even needed for mating.
Class Reptilia
•
There are two major subclasses:
• Subclass Anapsida – Turtles; Tortoises; some dinosaurs
• Subclass Diapsida – Crocodiles; Snakes; Lizards; most dinosaurs
•
This is based on the presence or
absence of certain temporal openings:
Subclass Anapsida
• The term Anapsida ("no
arch") refers to all those
reptile groups that lack
skull openings behind the
eyes.
• Subclass Anapsida
contains most of the
extremely primitive
ancestral reptiles as well
as turtles and tortoises
Subclass Diapsida
• Most all other reptiles are
diapsids (including lizards,
crocodiles, snakes, and most
dinosaurs)
• The main diagnostic physical
character for a diapsid is the
presence of two openings on
each side of the skull; the
upper and lower temporal
openings.
• Birds even exhibit this
temporal arrangement
Class Aves
• Birds are endothermic (warm-blooded)
vertebrates with feathers. Their anterior
limbs are modified as wings for flight,
while the posterior pair is adapted for
walking, swimming, or perching.
• Other adaptations related to flight include
changes in the skeletal, respiratory,
circulatory, reproductive, digestive, and
excretory systems.
• Today's birds still retain many reptilian
characteristics such as similarities in
behavior, skull structure, and scales on
their beak, legs, and feet.
Class Aves
•
The long hollow and porous bones of birds are
thin and slender to aid in flight. Many bones
overtime have fused together to give support.
•
Teeth have been lost and replaced by a light
beak.
•
Feathers are cornified epidermal appendages
that are probably related to scales. They are
used for thermoregulation, communication, and
as a flight surface.
•
There are two subclasses of Aves:
– Subclass Archaeornithes (Extinct Archaeopteryx)
– Subclass Neornithes (All other birds)
Subclass Archaeornithes
•
•
•
The Archaeornithes, are
represented by a single extinct
species (Archaeopteryx).
The first Archaeopteryx specimen
was discovered during the
nineteenth century. It was about
the size of a crow, had a long
reptilian tail and a reptilian skull
with no beak. It had three fingers
on its wings, each bearing a claw.
Because of its small sternum and
flexible trunk, it's unlikely that
Archaeopteryx was a strong flier.
Its characteristics are so reptilian
that, were it not for the feathers
fossilized with the specimen, it
would not be recognizable as a
potential ancestor of birds.
Subclass Neornithes
• All birds other than
Archaeopteryx belong to the
subclass Neornithes.
• While most neornithes fly, ratites
can not. Examples of living
ratites include emus, rheas,
ostriches, and penguins.
• Neornithes that fly are carinates
(they have a large carina).
Class Mammalia
• Members of the class Mammalia
possess both hair and mammary glands.
• Their integument is complex and has
many glands used for a variety of
purposes: thermoregulation and
excretion (sweat glands), communication
(scent glands), care of the hair and skin
(sebaceous oil glands), and for feeding of
the young (mammary glands).
Class Mammalia
•
They are thermic and have relatively
high rates of metabolism. In keeping
with their higher metabolic rates,
adaptations for efficient feeding include
heterodont teeth in most species and a
secondary palate to separate the
respiratory and food passages (so they
can breathe and chew at the same
time).
•
The circulatory systems are efficient,
and they have a four-chambered heart
with separate pulmonary and systemic
circulations.
•
Their brains are highly developed,
fertilization is internal, and most have
placental attachment of the young.
Class Mammalia
• There are three subclasses
based on birthing systems:
– Subclass Prototheria
(egg-laying monotremes)
– Subclass Metatheria
(marsupials)
– Subclass Eutheria
(placental)
Subclass Prototheria
•
Members of the subclass Prototheria are so
different from placentals that they may have
developed from a different theriapsid species
•
Today's monotremes are found in Australia,
Tasmania, and New Guinea. The only living
monotremes are the duckbill platypus and
echidna (spiny anteater).
•
Monotremes have several primitive
characteristics: They lack teeth as adults, the
braincase and other skeletal elements are
reptilian in structure, they have a single
ventral orifice (connected to a cloaca), and
they are oviparous.
•
In these ways they are reptilian in their
structure, reproduction, and physiology.
Nonetheless, monotremes do possess hair
and feed their young milk, so they do qualify
as mammals
Subclass Metatheria
•
Metatherians are the marsupial
mammals (kangaroos, koalas,
opossums, Tasmanian wolves, and
wombats).
•
Like the placental mammals,
marsupials start their lives attached
by a placenta to the maternal
circulation. Marsupial placental
development, however is shortlived; and following their birth the
embryos attach to a nipple within a
skin pouch (marsupium) where
they continue their development.
•
They are nocturnal in their habits
and are often found in association
with humans
Subclass Eutheria
• Eutherian mammals
are placental beasts.
• The number and
arrangement of the
teeth are important in
the classification of
these mammals
• All of these animals
give live birth and
place heavy
importance on raising
their young
Subclass Eutheria
You need to know the Eutherian orders:
Artiodactyla (deer, goat, pig)
Macroscelidea (elephant shrews)
Carnivora (bears, wolves)
Perissodactyla (horses, rhinos)
Cetacea (whales, dolphins)
Pholidota (the pangolin)
Chiroptera (bats)
Primates (monkeys, apes)
Edentata (armadillo, sloth)
Proboscidea (elephants)
Hyracoidea (hyraxes)
Rodentia (rats, mice)
Insectivora (shrew, hedgehog)
Sirenia (manatees)
Lagomorpha (rabbits)
Tubulidentata (aardvarks)
Pinnipedia (seal, walrus)