Animal Evolution – The Chordates

Chapter 26 Part 1

Impacts, IssuesTransitions 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 fishes

lobe-finned fishes

lungfishes

amphibians

“reptiles”

birds mammals

amniotes

tetrapods

swim bladder or lungs

jawed vertebrates

vertebrates

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, forebrain

spinal cord

vertebral column

gonad

snout

unmatched rows of teeth on upper and lower jaws

esophagus

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