Chapter 25 Animal Evolution – the Chordates

Chapter 25 Animal Evolution – The Chordates

• Archaeopteryx – First early bird fossil discovered – Lived about 150 million years ago • Sinosauropteryx prima – First fossil of this species discovered in 1994 in China – Feathered dinosaur – Supports theory that birds descended from dinosaurs

© Cengage Learning 2016 25.2 Chordate Traits and Evolutionary Trends • Chordates – Group of bilaterally symmetrical, coelomate animals – Have a complete digestive system and a closed circulatory system • Chordate traits – Stiff, flexible connective tissue (notochord) – Hollow nerve cord runs parallel to notochord – Gill slits – Muscular tail

© Cengage Learning 2016 Chordate Traits and Evolutionary Trends • Invertebrate chordates – Lancelets • Fish-shaped body • Adults are about five centimeters long • Live in sediment • Filter food particles out of the water – Tunicates • Only larvae have typical chordate traits • Undergo metamorphosis into barrel-shaped adult • Suck in water through a tube to get nutrients

pharynx with gill slits dorsal nerve cord

notochord

anus

tail that extends beyond the anus

pharynx with gill slits

A Free-swimming tunicate larva with all the deﬁning chordate traits.

water ﬂows in

water flows out pharynx with gill slits

secreted “tunic”

2 cm

B Adult tunicate. The only deﬁning chordate trait it retains is the pharynx with gill slits. The species in the photo is sessile as an adult.

(A, B left) From Russell/Wolfe/Hertz/Starr. Biology, 1e. © 2008 Cengage Learning, Inc.; (B right) Ethan Daniels/Shutterstock. © Cengage Learning 2016 Chordate Traits and Evolutionary Trends • Most chordates are vertebrates • Endoskeleton – Internal skeleton • First vertebrates: fishes that sucked up food – Later, jaws evolved • New feeding strategies emerged

Vertebrates

Tetrapods

Amniotes

Jawless Cartilaginous Ray-finned Lobe-finned Reptiles Lancelets Tunicates fishes fishes fishes fishes Amphibians (with birds) Mammals

6 Amniote eggs 5 Four limbs

4 Bony appendages 3 Swim bladder or lung(s)

2 Jaws 1 Backbone

• Fishes – First vertebrate lineage to evolve – Most fully aquatic of vertebrates – Rely on gills for oxygen exchange • Earliest jawless fish fossil dates to 530 million years ago • Two lineages survive to the present – Lampreys – Hagfishes

• Some lampreys are parasites of fish • Hagfishes are marine bottom feeders – Feed on worms and carcasses on the seafloor – About 60 species of Hagfish exist • Frightened hagfish emits a compound that combines with water to form a gelatinous slime • Harvested for their skin – “Eel skin” belts and wallets actually made of hagfish skin © Cengage Learning 2016 25.4 Evolution of Jawed Fishes

• First jawed vertebrates evolved 420 million years ago • Jaws evolved from gill arches – Skeletal elements that support gills • Jawed fishes: first animals with scales • Scales – Hard, flattened structures that cover the skin • Most jawed fishes have movable paired fins

location of spiracle (modified gill slit)

jaw support

Stepped Art © Cengage Learning 2016 Evolution of Jawed Fishes

• Placoderms – Lived during the Devonian period – Bony armor covered their head and neck – No teeth, but instead, sharp bony plates – Some were very large • Acanthodians – Arose during the same time period as placoderms – Centimeters long – No bony armor © Cengage Learning 2016 © Cengage Learning 2016 25.5 Modern Jawed Fishes

• Cartilaginous fishes – Mostly marine fishes with a cartilage skeleton – Jaws include teeth that grow in rows • Teeth are continually shed and replaced – Separate sexes – Eggs develop in an egg case inside the mother’s body • Ruptures to release young into the environment – Cloaca functions in reproduction and to remove wastes

• Sharks – Some are predators – Some are scavengers that suck up invertebrates – Others strain plankton from seawater • Rays – Flattened body with large pectoral fins – Feed by filtering out plankton – Barbed venomous tail defends against predators

• Lineages of bony fishes – Ray-finned fishes – Lobe-finned fishes • Bone replaces cartilage in the adult skeleton • Gill slits hidden beneath gill cover • Ray-finned fishes have thin, membranous fins – Swim bladder (sac) volume can be adjusted to affect buoyancy

• Lobe-finned fishes have thick, fleshy fins – Bones support the inside of the fins • Two lineages of lobe-finned fishes – Marine coelacanths • Thought to be extinct until 1938, when a living coelacanth found in the Indian Ocean – Freshwater lungfishes • Have lungs • Oxygen diffuses from the lungs into the blood

anus intestine stomach liver heart gills

A Goldfish (a type of carp). Note the flexible fins B Anatomy of a perch. The swim bladder allows supported by thin rays. As in most bony fishes, a bony the fish to adjust its buoyancy (its tendency to float). cover hides the gills. (A) Ultrachock/Shutterstock.com; (B) © 2016 Cengage Learning

© Cengage Learning 2016 25.6 Amphibians – First Tetrapods on Land • Amphibians – Scaleless, land-dwelling vertebrates – Typically breed in the water – First tetrapods – Evolved from fishes about 395 million years ago • Changes supporting the move to land – Skeletal changes – Lungs became larger and more complex – Eyelids evolved to keep eyes from drying out

• Modern amphibians – Carnivores • Eat insects and worms • Examples of modern amphibians – Salamanders – Caecilians – Frogs – Toads • Frogs and toads undergo metamorphosis

3 Early amphibian (Icthyostega) with well- developed ribs, and thick limbs with distinct digits.

2 2 Fish (Tiktaalik) with sturdier weight- bearing pectoral ﬁns, wristlike bones, and enlarged ribs. 1

1 Fish (Eusthenopteron) with bony ﬁns. left, © Cengage Learning; right #1 & 3, © P. E. Ahlberg; right #2, Illustration by © Kalliopi Monoyios

• Amphibian diversity declining – Thin, scaleless skin makes them more susceptible to parasites, pathogens, and pollutants – Habitat loss: another influencing factor

• Amniotes – Live their lives entirely on land – Adapted to life in dry places – Have lungs, and a waterproof skin – Separate sexes – Produce eggs in which young are hatched • Reptile clade includes: – Turtles, lizards, snakes, crocodilians, and birds

• Dinosaurs are a reptile group – Birds branched off from a dinosaur lineage during the Jurassic – Dinosaurs became extinct by the end of the Cretaceous • Endotherms – Maintain body temperature by adjusting metabolic heat production • Ectotherms – Body temperature varies with the environment

• Lizards and snakes – Most diverse group of modern reptiles – About 9,000 species exist – Covered with overlapping scales – Periodically shed their skin • Komodo dragon – Largest lizard – Venomous bite • All snakes are predators with teeth

• Turtles – 300 species – Bony, keratin-covered shell attached to skeleton – Toothless – Freshwater turtles eat fishes and invertebrates – Land turtles (tortoises) eat plants • Crocodilians – Predators that spend much of their time in water – Have a four-chambered heart, like birds

• Feathers – Filamentous keratin structures derived from scales • Feather functions – Help birds maintain temperature • Slow the loss of metabolic heat – Helps keep bird dry by shedding water – Color plays a role in courtship – Play a role in flight

• Most birds are surprisingly lightweight – Helps them become airborne – No bladder or teeth • Bird has a much larger brain than a lizard – Flying requires coordination and good eyesight • Reproduction – Fertilization occurs through the cloaca – Female lays eggs with nutrients which sustain the embryo

• 10,000 living bird species – About half are perching birds • Examples: jays, sparrows, starlings, robins, and cardinals • Hummingbirds – 450 species – Capable of flying backwards • Many birds make a seasonal migration • Penguins and ratite birds cannot fly – Emus, ostriches, kiwis, rheas, cassowaries © Cengage Learning 2016 25.10 Mammals – Milk Makers

• Females produce milk to nourish offspring • Mammals have hair or fur made of keratin • Endotherms • Four-chambered heart • Larger skull and brain for their body size than other vertebrates • Lower jaw consists of a single bone • Four different types of teeth – Incisors, canines, premolars, and molars

• Two branches split from the amniote lineage – One branch gave rise to reptiles and birds – One branch (synapsids) gave rise to mammals • Therapsids (snynapsid subgroup of mammals) – Became dominant creatures on land by the end of the Permian – Largest extinction event known occurred 250 million years ago • 70 percent of land species disappeared

• Cynodonts survived the extinction event – Endotherms with insulating fur or hair – Gave rise to mammals during the Jurassic • Three surviving mammal lineages – Monotremes • Egg-laying mammals – Marsupials • Pouched mammals – Placental mammals • Placenta transfers nutrients to growing embryo

• Monotremes – Five species: four are echidnas (spiny anteaters) and the platypus • Marsupials – 300 species live in Australia and surrounding islands – Examples: kangaroos, koalas, Tasmanian devils – 100 species live in Central and South America – Opossum lives in North America

Jack Dermid

A Kangaroo with a juvenile in its pouch.

• Placental mammals – Encompass 5,000 species – Dominant mammals in land habitats – Only mammals that live in seas – 40 percent are classified as rodents • Rodent teeth specialized for gnawing – Bats have 1,200 species • Only mammals capable of sustained flight – Moles and shrews • Adapted to burrowing

ChiRodentiaroptera (bats(rats,) mice, squirrels, porcupines)

Carnivora (dogs, cats, bears, Soricomorpha weasels, seals, and walruses) (moles and shrews)

Cetacea (dolphins, whales)

Artiodactyla (even-toed mammals: deer, cattle, goats, pigs, hippos)

Perissodactyla (odd-toed mammals: Primates (lemurs, monkeys, apes, humans) horses, zebras, rhinos)

Clockwise from top left, © Abel Tumik/Shutterstock.com; © Kirsanov/ Shutterstock.com; © Maxim Kulko/Shutterstock.com; © Vishnevskiy Vasily/Shut- terstock.com; © Eric Isselée/Shutterstock.com; © prapass/Shutterstock.com; © olly/Shutterstock.com; © Vishnevskiy Vasily/Shutterstock.com. © Cengage Learning 2016 Modern Mammalian Diversity

• Placental mammals (cont’d.) – Whales and dolphins • Adapted to life in the sea – Carnivora • Examples: dogs, cats, bears, wolves, foxes, weasels, seals, sea lions, and walruses – Cetaceans • Large mammalian grazers • Diet of plant material – Primates • Includes humans © Cengage Learning 2016 Points to Ponder

• Are all chordates vertebrates? Are all vertebrates chordates? • Were dinosaurs more similar to birds or reptiles? • Why is it necessary for amphibians to produce very large amounts of eggs?