Animal Evolution – The Chordates

Chapter 26 Part 1

Impacts, Issues

Transitions Written in Stone

Fossils such as Archaeopteryx, an ancient

winged dinosaur with feathers, are evolutionary

evidence of transitions between species

26.1 The Chordate Heritage

Chordates

• Most diverse lineage of deuterostomes

• Some are invertebrates; most are vertebrates

• Bilateral and coelomate

• Cephalized and segmented

• Complete digestive system

• Closed circulatory system

• Classified by embryonic characteristics

Modern Chordate Groups

Embryonic Chordate Characteristics

Four characteristics of chordate embryos may

not persist in adults

• Notochord of stiff connective tissue that extends

the length of the body and supports it

• Dorsal, hollow nerve cord parallels the notochord

• Gill slits across the wall of the pharynx

• Tail that extends beyond the anus

Invertebrate Chordates

Lancelets are the only group of chordates that

retains all chordate characteristics as adults

Fig. 26-2b, p. 434

Fig. 26-2b, p. 434

a Dorsal, hollow

nerve cord

b Notochord c Pharynx

with gill slits

d Tail extends

beyond anus

eyespot

tentacle-like

structures

around mouth

segmented

muscles

(myomeres)

midgut hindgut

aorta gonad pore of atrial

cavity anusepidermis

Animation: Lancelet body plan

Invertebrate Chordates

Tunicates have typical chordate larvae, but

adults retain only the pharynx with gill slits

Fig. 26-3a, p. 435

Fig. 26-3a, p. 435

nerve

cord notochord

gut

pharynx with gill slits

Fig. 26-3b, p. 435

Fig. 26-3c, p. 435

Fig. 26-3c, p. 435

pharynx

with gill

slits

Fig. 26-3d, p. 435

1 cm

Craniates

Craniates have a braincase of cartilage or bone

(cranium) that encases the brain, paired eyes,

and other sensory structures on the head

Craniates includes fishes, amphibians, reptiles,

birds, and mammals

Hagfishes are the only modern craniates that

are not vertebrates

Hagfishes

Soft bodied, boneless fishes

Fig. 26-4a, p. 435

Fig. 26-4a, p. 435

tentacles gill slits (twelve pairs) mucous glands

Fig. 26-4b, p. 435

Fig. 26-4c, p. 435

26.1 Key Concepts

Characteristics of Chordates

Four traits characterizes the chordates:

• A supporting rod (notochord)

• A hollow, dorsal nerve cord

• A pharynx with gill slits in the wall

• A tail extending past an anus

Certain invertebrates and all vertebrates belong

to this group

26.2 Vertebrate Traits and Trends

Vertebrates are chordates with an internal

skeleton (endoskeleton) of cartilage or bone

with a supportive backbone (vertebral column)

made up of individual vertebrae

Modern vertebrates (except lampreys) have

jaws derived from gill-supporting structures

Gill-Supporting Structures

Fig. 26-6a, p. 437

supporting

structure

for gill slits

gill slits

A In early jawless fishes, supporting

elements reinforced a series of gill slits

on both sides of the body.

Fig. 26-6b, p. 437

jaw, derived

from support

structure

B In early jawed fishes (e.g., placoderms), the

first elements were modified and served as

jaws. Cartilage reinforced the mouth’s rim.

Fig. 26-6c, p. 437

location of spiracle

(modified gill slit)

jaw support

jaw

C Sharks and other modern jawed

fishes have strong jaw supports.

Animation: Evolution of jaws

Evolution of Internal Skeleton

Fishes evolved appendages (fins) for swimming

Pelvic and pectoral fins gave rise to paired limbs in amphibians, which began the move to land

Evolution of Other Systems

Living in water (fish)

• Blood moves in a single circuit from heart to gills

(respiratory organs that function in water)

Moving to land

• Modification of the respiratory system (lungs)

and circulatory system (two circuits)

• Efficient kidneys to conserve water, and a

system of internal fertilization

Chordate Family Tree

Fig. 26-5, p. 436

lancelets

tunicates

hagfishes

lampreyscartilaginous

fishes

ray-finned

fisheslobe-finned

fishes

lungfishes

amphibians

―reptiles‖

birds mammals

amniotes

tetrapods

swim bladder

or lungs

jawed

vertebratesvertebrates

craniates

ancestral chordates

Origin

of the

first

jawless

fishes.

Jawed

fishes,

including

the placo-

derms

and

sharks,

evolve.

Adaptive

radiation

of fishes,

and the

first

amphibians

move

onto land.

Diversification

of fishes and

amphibians.

Armored

fishes go

extinct.

Reptiles

arise and

start to

diversify.

Early

amphibians

in decline.

Dinosaurs

and marine

reptiles

evolve.

Birds,

mammals,

and modern

amphibians

arise.

Dinosaurs

dominate.

Dinosaur

diversity

peaks, then

extinction

by period’s

end.

Adaptive

radiation

of

mammals.

Ordovician Carboniferous Jurassic Tertiary

488 443 416 359 299 251 200 146 66

Silurian Devonian Permian Triassic Cretaceous

Animation: Vertebrate evolution

26.2 Key Concepts

Trends Among Vertebrates

In vertebrate lineages, a backbone replaced the

notochord

Jaws and fins evolved in water

Fleshy fins with skeletal supports evolved into

limbs that allowed vertebrates to walk onto land

On land, lungs replaced gills and circulation

changed in concert

26.3 The Jawless Lampreys

Lampreys have no jaws or paired fins; they

undergo metamorphosis, and many are

parasites of other fishes

26.4 The Jawed Fishes

Jawed fishes typically have paired fins and a

body covered with scales

Cartilaginous fishes (Chondrichthyes) have a

cartilage skeleton, gill slits, and teeth that shed

• Sharks and rays

Bony fishes (Osteichthyes) have a bony

skeleton, gill covers, and a swim bladder

• Ray-finned fishes, lungfishes, coelacanth

Ray-Finned Bony Fishes

Fig. 26-9a, p. 439

Fig. 26-9a, p. 439

swim bladder kidney

ovary nerve cord

brain

cloaca intestine stomach liver heart gills

Fig. 26-9b, p. 439

Fig. 26-9c, p. 439

Fig. 26-9d, p. 439

Lungfish

Lungfishes have gills and lunglike sacs for

breathing air

Coelacanth

The only modern lobe-finned fish; closely related

to amphibians

26.5 Amphibians—

First Tetrapods on Land

Tetrapods (four-legged walkers)

• Branched from lobe-finned fishes in Devonian

Amphibians

• Land-dwelling vertebrates that return to water to

breed, undergo metamorphosis, and have a

three-chambered heart

Adapting to Life on Land

Fig. 26-12a, p. 440

Fig. 26-12b, p. 440

Fig. 26-12c, p. 440

Fig. 26-12d, p. 440

Modern Amphibians

Salamanders and newts

• Body form most like early tetrapods, side-to-side

walking motion

Caecilians

• Includes many limbless, blind burrowers

Frogs and toads

• Tailless adults with long, muscular hind legs

Salamander and Caecilian

Frog

26.6 Vanishing Acts

Amphibians depend on standing water to breed

and have a thin skin unprotected by scales

These features make them vulnerable to habitat

loss, disease, and pollution – causing

deformities and threatening species

Frog Deformity

26.3-26.6 Key Concepts

Transition from Water to Land

Vertebrates evolved in the seas, where

cartilaginous and bony fishes still live

Of all vertebrates, modern bony fishes are most

diverse

One group gave rise to aquatic tetrapods (four-

legged walkers), the descendants of which

moved onto dry land

26.7 The Rise of Amniotes

Amniotes are animals with embryos that

develop inside a waterproof egg; their

waterproof skin and highly efficient kidneys

make them well adapted to dry habitats

Amniotes

Four branches of amniotes lead to synapsids

(mammals), anapsids (turtles), lizards and

snakes, and crocodiles and birds

Reptiles are an artificial group referring to

amniotes other than bird or mammals

Dinosaurs are extinct amniotes; birds are their

descendents

The Ruling Reptiles

For 125 million years, dinosaurs dominated the

land and sea (Example: Ichthyosaurs)

Amniote Phylogeny

Fig. 26-16c, p. 442

snakes

lizards

―stem‖

reptiles tuataras

ichthyosaurs

plesiosaurs

birds

therapod

dinosaurs

other

dinosaurs

pterosaursarchosaurs

crocodilians

turtles

therapsidsanapsids

synapsids mammals

PALEOZOIC ERA MESOZOIC ERA

TERTIARY

TO

PRESENT

CARBONIFEROUS PERMIAN TRIASSIC JURASSIC CRETACEOUS

26.8 So Long, Dinosaurs

K-T asteroid impact hypothesis

• Asteroid impacts changed life on Earth, defining

the Cretaceous-Tertiary (K-T) boundary

• Most dinosaurs became extinct

26.9 Diversity of Modern Reptiles

Reptile characteristics

• Scale-covered body

• Cloaca for waste disposal

• Four approximately equal limbs (except snakes)

• Internal fertilization

• Body temperature determined by surroundings

(ectotherms)

Crocodile Body Plan

Fig. 26-18, p. 444

kidney (control

of water, solute

levels in internal

environment)

olfactory

lobe (sense

of smell)

hindbrain,

midbrain,

forebrainspinal

cord

vertebral column

gonad

snout

unmatched

rows of teeth

on upper and

lower jawsesophagus

lung

heart

liver

stomach

intestine cloaca

Animation: Crocodile body plan

Turtles and Tortoises

Turtles and tortoises have a bony, scale-

covered shell attached to the backbone

Fig. 26-19b, p. 445

hard shell vertebral column

Lizards

Lizards, the most diverse reptiles, have many

interesting defenses

Tuataras

The two remaining species of tuataras have a

third eye under the skin of the forehead

Snakes

Snakes are legless, but some have bony

remnants of hindlimbs

Fig. 26-19f (1), p. 445

venom

gland

hollow

fang

Crocodilians

Crocodilians, close relatives of birds, are the

only reptiles with a four-chambered heart

Animation: Bony fish body plan

Animation: Cartilaginous fishes

Animation: Evolution of limb bones

Animation: Jawless fishes

Animation: Salamander locomotion

Animation: Tortoise shell and skeleton

Animation: Tunicate body plan