Download Chordates

Chordates (phylum Chordata) are deuterostome
coelomates
-Nearest relatives are echinoderms (the only other
deuterostomes)
Four features characterize chordates
1. Nerve cord
2. Notochord
3. Pharyngeal slits
4. Postanal tail
All chordates have all four of these characteristics at
some time in their lives
Other characteristics also distinguish chordates
1. Chordate muscles are arranged in segmented
blocks called somites
2. Most chordates have an internal skeleton against
which the muscles work
The Chordates
Somites
Phylum chordata can be divided into three subphyla
1. Urochordata
Nonvertebrates
2. Cephalochordata
3. Vertebrata
Tunicates are marine animals
-Larvae are tadpole-like and have notochord and nerve cord
-Are free-swimming but do not feed
-Adults typically lose the tail and notochord
-Are immobile filter-feeders
-Many secrete a tunic (cellulose sac) that surrounds the animal
Lancelets are scaleless chordates
-Notochord persists throughout animal’s life
-Have no distinguishable head
-Feed on plankton
using cilia-generated
currents
-Closest relatives to
vertebrates
Vertebrates: chordates with a spinal column
-Distinguished from nonvertebrates by
1. Vertebral column – Encloses and
protects the dorsal nerve cord
2. Head – Distinct and well-differentiated
possessing sensory organs
Vertebrates also have
-Neural crest – A unique group
of embryonic cells that forms
many vertebrate structures
-Internal organs – Liver,
kidneys, endocrine glands, heart
and closed circulatory system
-Endoskeleton – Made of
cartilage or bone
-Makes possible great size
and extraordinary movement
The first vertebrates appeared in the oceans about
half a billion years ago
-Jawed fishes soon became dominant
-Their descendants, the amphibians, invaded the
land
-Reptiles replaced them as the dominant land
vertebrates. 300 MYA
-Gave rise to birds and mammals 220 MYA
Fishes are the most diverse vertebrate group
-Over half of all vertebrates
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Vary greatly in size,
shape, color and
appearance
500 MYA
Paired fins
Jaws
Fishes have the following characteristics
1. Vertebral column
2. Jaws and paired appendages
3. Internal gills
4. Single-loop blood circulation
5. Nutritional deficiencies
The first fishes had mouths with no
jaws
Skull
-Agnatha (Superclass) extant as
hagfish (class Myxini) and lampreys
(class Cephalaspidomorphi)
Gill slit
Anterior gill arch
-Ostracoderms are now extinct
The development of jaws occurred in
the late Silurian period (438-408
MYA)
-Jaws evolved from the anterior gill
arches that were made of cartilage
The Devonian period (408-360 MYA) saw two types
of jawed fishes
-Spiny fishes (Acanthodii)
-Smaller fishes that dominated early on
-Armored fishes (Placodermi)
-Larger fishes that dominated late
At the end of the period, these early fishes were
replaced by sharks and bony fishes
-These had more improved jaws
In the Carboniferous period (360-280 MYA), sharks
(Class Chondrichthyes) became the dominant sea
predators
-A light, flexible, calcified, cartilaginous skeleton,
and paired fins made them superior swimmers
Sharks were among the first vertebrates to develop
teeth
-Evolved from rough scales on mouth’s skin
-Easily lost but continuously replaced
Sharks (and bony fishes) have a fully developed
lateral line system
-Series of sensory organs under the skin that
detects changes in pressure waves
 Most sharks have internal fertilization. Oviparous,
ovoviviparous, and viviparous.
Bony fishes (Osteichthyes) evolved at the same
time as sharks about 400 MYA
-However, they adopted a heavy internal skeleton
made completely of bone
Bony fishes are the most species rich group of all
vertebrates (> 30,000 living species)
They developed significant adaptations that enabled
them to dominate life in the water
-Swim bladder and gill cover
Swim bladder
-A gas-filled sac that allows
bony fishes to regulate their
buoyant density
-Gas addition = Gas gland
-Gas release = Oval body
Gill cover
-A hard plate, the operculum,
covers gills
-Its flexing permits water
pumping over gills
Ray-finned fishes (class Actinopterygii)
-Parallel bony rays support & stiffen
each fin
-There are no muscles within the fins
Lobe-finned fishes (class
Sarcopterygii)
-Have paired fins that consist of a long
felshy muscular lobe
-Supported by central co-jointed
bones
-Almost certainly the amphibian
ancestors
Amphibians (class Amphibia) are damp-skinned
vertebrates
-First vertebrates to walk on land
Successful invasion of lands by vertebrates required
several adaptations
1. Legs to support body’s weight
2. Lungs to extract oxygen from air
3. Redesigned heart to drive larger muscles
4. Reproduction in H2O to prevent egg drying
5. System to prevent whole body desiccation. The
skin also aids in respiration.
6. Plumonary veins.
Amphibians evolved from lobe-finned fish
Ichthyostega was one of the first amphibians
-Efficient limbs for land crawling
-Improved olfactory and auditory structures
-However, still fishlike
-Spent much of life
in water
In 2006, a transitional fossil was
found between fish and
Ichthyostega
-Tiktaalik lived about 375 MYA
-Shoulder and limb bones like
those of an amphibian
-Fins like those of a lobefinned fish
-Spent most of its life in water
-Capable of moving onto
land to capture food or escape
predators
Modern amphibians are descended from three families of the Tertiary
period (65-2 MYA)
-These accomplished a highly successful invasion of wet habitats all
over the world
Today there are about 5,670 species in 37 different families, comprising
three orders:
-Anura (“without tail”), Caudata (“visible tail”),Apoda (“without legs”)
Over 7000 species of reptiles (class Reptilia) now live
on Earth
All living reptiles exhibit three key features
1. Amniotic eggs, which are watertight
2. Dry skin, which covers body and prevents water
loss
3. Thoracic breathing, which increases lung
capacity
Reptiles, birds and
mammals are
amniotes
The amniotic egg has
four membranes
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
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Chorion – Allows
O2 entry
Amnion – Fluidfilled cavity
Yolk sac –
Provides food
Allantois –
Excretes wastes
Reptiles dominated earth for 250 million years
Reptiles are distinguished by the number of holes on
side of the skull behind eye orbit:
-0 (anapsids), 1 (synapsids), 2 (diapsids)
The synapsids rose to dominance first
-Pelycosaurs were first land
vertebrates to kill beasts their
own size
-Therapsids replaced them
about 250 MYA
-May have been
endotherms
-Replaced by diapsids but
gave rise to mammals.
Archosaurs (Diapsids) were the first land
vertebrates to be bipedal
-Gave rise to dinosaurs about
220 MYA
Dinosaurs dominated for over
150 million years
-Became extinct 65 MYA
-Asteroid’s impact
Modern reptiles developed two important
characteristics:
1. Internal fertilization: Sperm fertilizes egg before
protective membranes are formed
2. Improved circulation: Oxygen is provided to the
body more efficiently
Comparing the circulatory systems of fish, amphibians, and
reptiles.
All living reptiles are ectothermic
-Obtain heat from external sources
There are four surviving orders of reptiles
-Chelonia (turtles and tortoises)
-Rhynchocephalia (tuataras)
-Squamata (lizards and snakes)
-Crocodylia (crocodiles and alligators)
Crocodile
Gharial
In many ways, crocodiles resemble birds far more
than they do other living reptiles
-Build nests and care for their young
-Have a four-chambered heart