Phylum Chordata - Chordates
Internal skeleton with muscle attachments on outer surface
Allows for greater range of movement and development of large
body size
Segmented body plan
All chordates have
(in embryo or adult)
notochord
dorsal nerve tube
post-anal tail
pharyngeal gill slits
Hollow dorsal nerve tube - just below dorsal surface
differentiates into brain and spinal cord in vertebrates
Flexible dorsal notochord - located just below nerve cord
replaced by vertebral column in vertebrate development
persists in some primitive chordates - provides support
Pharyngeal gill slits - a clue to the aquatic ancestry of all chordates
pharynx connects mouth cavity and esophagus to outside
in terrestrial vertebrate embryos slits often form pharyngeal
pouches, become eustachian tubes in some vertebrates
A Post-anal tail
extends beyond the
anus
present in embryo if
not in adult form
Other animals with
tails have a terminal
anus
Phylum Chordata has three Subphyla
Subphylum Urochordata - tunicates or sea squirts
Subphylum Cephalochordata - lancelets
Subphylum Vertebrata - fishes, amphibians, reptiles, birds,
mammals
Subphylum Urochordata - Tunicates or “sea-squirts”
Most are sessile as adults, may be colonial
Possess notochord and nerve cord only in larval stage
Adults secrete cellulose “tunic” around body
Adults are filter feeders
lack a body cavity
and segmentation
Create water currents
with cilia - water drawn
into pharynx
food particles trapped in
mucus produced by
endostyle
Many have symbiotic photosynthetic bacteria
Most chordate characteristics are only seen in tunicates in the
larval stage
Larvae are free-swimming -after finding a suitable
location they settle head-first
and transform into the adult
form
Subphylum Cephalochordata - lancelets
Scaleless, fish-like marine organisms
No obvious head, eyes, nose or ears
Notochord runs entire length of body and persists in adults
Bury in substrate, expose anterior end only
Segmentation of muscles readily visible
Have many more pharyngeal gill slits than fishes
Filter-feed through
the action of cilia in
mouth and pharynx trap food in pharynx
Swim with fish-like
undulations of the
body
The defining chordate characteristics are preserved in the adult
lancelet
Subphylum Vertebrata - Vertebrates
All vertebrates have a distinct head, with skull enclosing a brain
Name derived from bony vertebral segments in spinal column
Vertebral column - protects dorsal nerve cord
notochord surrounded and replaced by vertebral column
Possess a cranium - brain case
Group also called craniate chordates
Most have a bony skeleton, some use only cartilage
All have a closed circulatory system
The vertebral
centra replace
the notochord
Neural spines
develop to
surround and
protect the
neural tube
behind the head
All vertebrates also have a unique set of cells that originate
above the neural tube during development - “neural crest cells”
Neural crest cells migrate throughout vertebrate body and
contribute to the development of many organs
Brief overview of vertebrate evolution
First vertebrates were marine, lacking jaws or paired fins (550 mya)
Jawed fishes then became dominant creatures in the sea (450 mya)
Amphibian ancestors first to invade the land (400 mya)
Largely replaced by reptiles more suited to live on land (300 mya)
Dinosaurs and other reptiles ruled the earth for 150 million years
their extinction (65 mya) allowed mammals to become dominant
Most living
vertebrate groups
have diversified
greatly since their
origin.
The vertebrates are composed of eight classes
Four classes are aquatic fishes
Class Myxini - Hagfish
Class Cephalaspidomorphi - Lampreys
Class Chondrichthyes - Sharks, skates and rays
Class Osteichthyes - Bony fishes
Four classes are terrestrial tetrapods
Class Amphibia - Salamanders, frogs and toads
Class Reptilia - Reptiles
Class Aves - Birds
Class Mammalia - Mammals
Characteristics of Fishes
Gills - extract dissolved oxygen from water around them
water passes over filaments rich in blood vessels
blood moves opposite the flow of water - an efficient system
Vertebral column
Internal skeleton with spine surrounding dorsal nerve cord
Skeleton may be made of cartilage or bone
Brain fully encased in protective skull
Single-loop blood circulation
Blood pumped from heart to gills
Oxygenated blood from gills passes to rest of body
Heart is series of four
chambers that
contract in
sequence
Nutritional deficiencies
All vertebrates are unable to synthesize aromatic amino acids
All must consume these amino acids in their diet
Overview of Fish Evolution
First fishes known were jawless with heavy bony armor
Internal skeleton was made of cartilage
Survived by lampreys and hagfish (“Agnathans” - no jaws)
First jawed fishes occurred 410 million years ago
Early jawed fishes
were replaced by
more agile sharks
and bony fishes
Jaws evolved from modified gill arches
the bony area between gill slits
Evolved as a means to close mouth, and then to capture prey
First gill arch behind jaw became supporting strut
Joined rear of lower jaw to rear of skull
Allowed mouth to open very wide, an efficient weapon
Jawless fishes today
Hagfishes are scavengers in cool
waters - especially in deep water
Lampreys have two life-styles
large parasites of other vertebrates
small filter-feeders in freshwater
streams
Sharks became dominant predators about 300 mya
Class Chondrichthyes - sharks, skates, rays
Skeleton is made of cartilage
Have bony teeth and scales with enamel
Have always been large agile predators
enlarged pectoral fins allow greater lift and
maneuvering during swimming
Have internal fertilization
young are well-developed when born
skates and rays have flattened bodies and
are more sedentary
Class Osteichthyes - bony fishes
Evolved at same time as sharks, but in freshwater
Developed heavy skeleton made completely of bone
Ossification replaces cartilage with bone
External plates and scales also made of bone
Added swim bladder for respiration/buoyancy
Primitive fishes use swim
bladder for respiration - a lung
Modern fishes use swim
bladder for buoyancy
Modern bony fishes have highly mobile fins, thin scales and
symmetrical tails
They are an extremely successful group and are dominant in
freshwater and the sea
Two lineages of bony fishes evolved
Ray-finned fishes - most modern fish
Internal skeleton of bony rays supports
and stiffens each fin
No muscles within fins
Swim bladder used for buoyancy
Lobe-finned fishes - 6 living species
5 lungfishes, and the “coelacanth”
have fins with muscular lobes
swim bladder used for respiration
early form gave rise to amphibians
Important characteristics of bony fishes
Lateral line system
Series of small sensory organs that project into a canal beneath
skin surface - sensitive to water pressure changes
allows fish to sense movements in water
Similar structures found in all vertebrate ears - for hearing
Gill cover
Hard plate covering gills called the operculum
Helps to pump water over gills even when stationary
Class Amphibia - Amphibians
includes frogs, salamanders, caecilians
Amphibians were the first “tetrapods”
the first terrestrial vertebrates
There are three orders of Amphibians
Order Anura - frogs and toads
Order Urodela - salamanders
Order Apoda - caecilians
Characteristics of Amphibians
Legs - a key adaptation for terrestrial living
Frog and salamanders have two pairs, lost in caecilians
Lungs - a modified swim bladder used for terrestrial respiration
swallow air by moving floor of mouth
Cutaneous respiration - gas exchange at skin
supplement lung respiration with a highly vascularized skin
Moist skin provides extensive surface area, limits body size
Pulmonary veins
Veins return blood from lungs to heart
Aerated blood leaves heart at greater
pressure than it leaves lungs
Partially divided heart
partially separated pulmonary and systemic pathways
imperfect since there is a single ventricle where oxygenated
and deoxygenated blood can mix
Origin of Amphibians
evolved from lobe-finned
fishes - skeletal structure
of lobe-finned fishes and
early amphibians is very
similar
Earliest amphibian
fossil, Ichthyostega
built for terrestrial life
with lungs and a strong
legs and backbone
Most amphibians still must lay their eggs in water to keep them
from drying out
Because of moist skin, adults are also liable to become dehydrated
Modern Amphibians
Order Anura - frogs and toads - tailless amphibians
Frogs have smooth, moist skin; long legs; live in or near water
Toads have bumpy, dry skin; short legs; are adapted to dry
environments
All are carnivores, eat a wide variety of invertebrates
Most return to water to reproduce
Eggs lack water-tight membranes and dry out readily
Eggs fertilized externally
Eggs hatch into algae-eating, swimming larval tadpoles
Tadpoles metamorphose into adults
Order Urodela - salamanders - amphibians with tails
Have elongated bodies, long tails and smooth moist skin
Live in moist places, some live entirely in water
Lay eggs in water or moist areas
Fertilization is external in most species
Juveniles look like adults, do not undergo major metamorphosis
Order Apoda - caecilians - legless amphibians
Highly specialized group of burrowing amphibians
Lack legs, have small eyes, are often blind
Eat worms and soil invertebrates
Male deposits sperm directly into female
young are born alive
Class Reptilia - Reptiles
includes turtles, lizards, snakes, alligators
Reptile improvements on the amphibian plan
Leg design allows better body support
Lungs and heart are more efficient
Dry skin covered with scales
Water-tight eggs
Key Characteristics of Reptiles
Amniotic egg - Water-tight eggs contain food source (yolk) and
four membranes: Chorion, amnion, yolk sac and allantois
Each important an embryonic life-support system
Chorion allows oxygen to enter, but retains water
Amnion encases developing embryo within fluid-filled cavity
Yolk sac holds food for embryo - extracted by blood vessels
Allantois encloses waste products
Dry skin with scales or armor reduces water loss
Scales develop as surface cells fill with protein keratin
Keratin is also used for mammal hair and bird feathers
Thoracic breathing - increase volume of air inspired
Expand and contract rib cage to suck air in and force it out
Capacity limited only by volume of lungs
Improved leg design legs placed more
ventrally under trunk
allows better support of
the body and greater
mobility
allows larger body size
Reptile Evolution - Reptiles were the dominant land animals for
250 my
Four major lineages evolved in series: Pelycosaurs, Therapsids,
Thecodonts, Dinosaurs
Pelycosaurs - improved predators
synapsid skull with temporal openings allowed
better muscle attachment and more powerful bite
Dominant terrestrial vertebrates
for 50 million years
Replaced by Therapsids
Therapsids - Mammal-like reptiles
had higher food intake, and probably had higher metabolic rate
speculated that these were the first endothermic (“warmblooded”) vertebrates - likely evolved in cooler climate
dominant terrestrial vertebrates for 20 million years
Therapsids were the ancestors of mammals
Replaced by Thecodont reptiles - a different lineage
They may have
had hair, like
their mammal
descendants
Thecodonts - greater energy efficiency
Were “cold-blooded” ectotherms like amphibians and early reptiles
environmental change resulted in warmer temperatures
endothermy requires less energy intake for body maintenance
Had an improved skull - diapsid Diapsid reptiles, had two pairs of temporal holes in skull
First land vertebrates to walk on hind limbs only
Dominant for 15 my
Replaced by their direct
descendants, Dinosaurs
Dinosaurs - greater agility
Legs positioned directly underneath body - allowed larger body
Enabled running with speed and agility
Dominated land for 150 million years
Went extinct 65 million years ago - likely due to asteroid impact
Overview of the evolution of “amniotes” - reptiles, birds,
mammals, and their ancestors
Modern Reptiles - Four surviving orders
Turtles - ancient survivors
Have changed little since before time of dinosaurs
Have solid skulls (no temporal openings) like first reptiles
Have changed little since before time of dinosaurs
Lizards and snakes - most modern reptiles are in this group
Also evolved very early - 250 mya
Diversified after disappearance of dinosaurs
Tuataras - a remnant of a group
that was once common
Only two species survive on
islands near New Zealand
Small, sluggish, diapsid reptiles,
Common in Jurassic, declined in
Cretaceous
Unable to compete with lizards,
in most environments
Crocodiles - the most derived “reptiles”
Descended from same line that produced dinosaurs
Little changed in the last 200 million years but with advanced
characteristics they share with birds
parental care, four-chambered heart,
vocalizations for mate attraction
Crocodiles and birds more closely related to dinosaurs and each
other than they are related to lizards and snakes
Crocodiles, Thecodonts, Dinosaurs, and Birds form a
monophyletic group - the Archosaurs
Other Important Characteristics of Reptiles
Internal fertilization - a requirement with a shelled egg
Improved circulatory system - Circulatory system provides more
oxygen to body
Septum in heart extended from
atrium partway into ventricle
Decreases mixing of
oxygenated and deoxygenated
Septum totally divides ventricle
in crocodiles and birds (a fourchambered heart)
All living reptiles are ectothermic, obtain heat from external
sources - regulate body temperature through behaviors - basking
or seeking shade
Order Chelonia - Turtles and tortoises
Turtles generally live in water, tortoises live on land
Bodies are encased in a protective shell
Carapace covers dorsal surface
Plastron covers ventral portion
Vertebrae and ribs fused with carapace
Lack teeth, but have sharp beak
Order Rhynchocephalia - Tuataras
Two species makes up entire order
found only on islands of New Zealand
Has spiny crest running down back
Has inconspicuous parietal or “third eye” on top of head
has lens, retina and is connected to brain
May function as a thermostat, protect it from overheating
Order Squamata - Lizards and snakes
Males have paired copulatory organs
lower jaw not joined directly to skull
allows flexibility in jaw movements
most are predatory carnivores
Lizards have limbs, snakes do not
Snakes lack movable eyelids and
external ears - can’t hear
Lizards are more ancient group
Lizards rely on agility and speed
to catch prey and avoid
predators
Most snakes depend on stealth
Many lizards can loose tail to
escape predator and
regenerate new one
Order Crocodilia - Crocodiles and Alligators
Primitive-looking reptiles also includes caimans and gavials
Similar to 200 million year old fossil forms
Live in or near water in tropical or subtropical regions
Are aggressive carnivores, bodies adapted for hunting by stealth
Eyes and nostrils on top of head, lie submerged in water
Enormous mouths with sharp teeth and strong neck
Can feed underwater, valve prevents water from entering air
passage
The only living reptiles that care for their young
Class Aves - Birds
Most successful of all terrestrial vertebrates
great diversity, 8800 species
Key Characteristics of Birds
Produce amniotic eggs - like reptiles
Have keratin scales on feet and legs - like reptiles
Lack teeth - developmental mutants have reptile-like teeth
Have very small tail - unlike reptiles
Feathers
Modified reptilian scales
Provide lift for flight and insulate
Develop from follicles
Has shaft, with barbs, with
barbules, with interlocking hooks
Flexible, light-weight, and strong
Can be replaced, like scales
Flight skeleton
Bird bones are thin, hollow,
and light-weight
Many have internal struts to
strengthen the bone with
little added weight
Many bones are fused to
each other - reduces
number
Have fused collarbones
(wishbone) for recoil of
wings during flight
Have keeled breastbone - for
attachment of flight
muscles
The Evolution of Birds
Archaeopteryx, the first bird - fossils date to 150 mya
Shares features with small therapod dinosaurs
Skull has teeth
Very few bones are fused to each other
Bones are solid (bird bones are hollow)
Has long reptilian tail
Has no breastbone
Considered a bird because of feathers on wings and tail
Modern Birds
Beak and feet characterize many bird habits
Have unique physiology to cope with high energy demands of flight
Efficient respiration - “flow-through lungs”
Gas flows through
lungs in only one
direction and opposite
direction of blood flow
- an efficient system
No “dead-air” in bird
lungs
Efficient circulation
Has four-chambered heart like mammals but
independently evolved
Wall dividing ventricle is
complete, two circulations
do not mix
Flight muscles get fully
oxygenated blood
Endothermy
Birds maintain higher body temperatures than most mammals
Metabolism in flight muscles proceeds at faster rate
Feathers provide insulation to conserve heat
Class Mammalia - Mammals
The least diverse group of vertebrates 4100 species
Almost all large land vertebrates are mammals
Most mammals are not large
3200 species are rodents, bats, shrews, moles
Greatest diversity was reached about 15 mya
numbers are declining due to relatively
cool climate
Key Mammalian Characteristics
Hair
Helps regulate body temperature, allows life in colder climates
Not derived from reptilian scales or feathers
Each hair extends like stiff thread from bulb-like hair follicle
Composed of dead cells filled with fibrous keratin protein
Insulates against heat loss
Provides camouflage
Whiskers function as sensory structures
Serve as defensive weapons as in porcupines and hedgehogs
Milk-producing glands
All mammals have mammary glands that produce milk
Mammary glands are modified sweat glands
Milk is rich in fat, sugar, protein with 95% water
High calorie food needed to support rapid growth of newborn
Evolution of Mammals
Arose from Therapsids in Triassic, 220 mya
First mammals were small insectivores
Had large eyes - they may have been nocturnal
Mammal jaw reduced to massive bone with a single joint
Allows strong bite
Two jaw bones moved to middle ear to make three bone chain
that improved hearing - malleus, incus, stapes
Were a minor group until the Cretaceous (65 mya)
Two groups - Prototheria and Theria
Prototheria - egg-laying mammals
close relatives of therapsids
Small, resembled modern shrews
Theria - live-bearing mammals
Subdivided into marsupials (pouched mammals) and
placental mammals
Characteristics of Modern Mammals
Endothermy
Allows activity in cooler weather and times of day
Allows colonization of harsh environments, deserts to arctic
Hair provides insulation to support endothermy
Higher metabolic rate required as well
Efficient circulation
provided by four chambered heart
Improved respiration
Diaphragm breathing muscle allows
greater volume of air to be inspired
Placenta
Most mammals are placental and viviparous (live bearing)
Blood stream of mother and fetus in close contact at placenta
Food, water, oxygen pass from mother to child
Wastes pass from child to mother, carried away
Teeth
Reptiles have homodont dentition - all teeth are the same
Mammal dentition is heterodont - variation in teeth in mouth
Highly specialized to match food eaten
Mammals as a group eat a wide variety of foods but individual
species often have highly specialized feeding
Digestive systems for eating plants
Most mammals are herbivores
with help of symbiotic bacteria cellulose is major food
Some mammals have four-chambered stomachs
first chamber is large, holds most cellulose-digesting bacteria
Partially digested plants are regurgitated (cud) and chewed
Swallowed again and digested by succeeding stomachs
then passes to intestine
Other mammals digest plant material in the large intestine
Have relatively small stomachs, do not chew a cud
Bacteria live in pouch called the caecum, off the large intestine
Herbivores must eat great mass to gain sufficient nutrition
Hooves, Horns, and Antlers
claws, fingernails, hair, and hooves are made of keratin
Hooves cushion and protect toes in running mammals
Horns have a core of bone surrounded by keratin sheath
not shed, bony core is attached to skull
Outer layer is compacted hair-like layers
Antlers are made only of bone
Male deer grow and shed a set of antlers each year
Covered by thin skin layer of velvet while growing
Velvet dies and is scraped off when antlers are fully grown
Antlers used to combat rival males in fall and winter
Shed in spring after breeding season
Rhinoceros horn composed of keratinized hair
Flight
Bats are only mammals capable of powered flight
Wings are modified forelimbs
leathery membrane of skin stretched over bones of four fingers
edges attach to side of body and to hind leg
Bats at rest hang upside down from legs
Have sonar system to
navigate in dark and
find insects
High frequency pulses
emitted through mouth
or nose
Sound waves reflect off
objects, captured by
large ears
Modern Mammals
nineteen orders
Seventeen are placental mammals
Two are non-placental mammals :
monotremes and marsupials
Monotremes - Egg-laying mammals
Includes duck-billed platypus and two species of echidna
Monotremes retain some reptilian characteristics
Lay shelled eggs which they incubate in a nest
Shoulder and pelvic bones similar to early reptiles
Have a cloaca - a single opening for feces, urine, and eggs
More closely related to early extinct mammals
Have functioning mammary glands
Marsupials - Pouched-mammals
Marsupial fertilized egg is amniotic but lacks shell
Marsupial egg has much yolk to nourish embryo
Embryonic marsupial is born early in development
Crawls to marsupial pouch, attaches to nipple, continues to
develop for months
Nearly all modern marsupials live in
Australia and New Guinea
Only 20 of 280 total species live
elsewhere
Marsupials in Australia and New
Guinea fill niches that placental
mammals fill elsewhere
Virginia opossum is the only marsupial
in North America
Placental mammals
Have well-developed placenta
that nourishes embryos
for entire development
Eggs have little yolk
Placenta forms early in course
of development
Embryos develop in uterus (womb)
of mother
Fetal placenta formed from
chorion and allantois
Maternal placenta formed from
wall of uterus
Young undergo considerable
development before birth
Many have extensive parental care