Vertebrates Evolution

Phylum Chordata• Vertebrates are a subphylum

of the phylum Chordata• This lecture will build up to

vertebrates and mammals by moving through the Chordata Phylum, from the most inclusive traits and simpliest organisms to the to most specific traits and complex organisms

Phylum Chordata

• This phylum includes– All vertebrates– Urochordata

(tunicates)– Cephalochordata

(lancelets)– Myxini (hagfish)

Characteristics of Chordates Many appear only during embryonic

development1. Notochord - Chordates named for

this structure; a long flexible rod that appears during embryonic development (some adults retain it)

• Provides skeletal support• Most adults retain only a

remnant of this• In humans it becomes the

gelatinous disks between our vertebrae

Characteristics of Chordates Many appear only during embryonic

development

2. A dorsal, hollow nerve cord - forms from a plate of ectoderm that rolls into a hollow tube; develops into the central nervous system

Characteristics of Chordates Many appear only during embryonic

development3. Pharyngeal clefts -

grooves that separate a series of pouches along the sides of the pharynx.

• Usually develop into slits that allow water to enter and exit the mouth without going through the digestive tract

• Become gill slits in fish or become part of the ear in terrestrial animals

Characteristics of Chordates Many appear only during embryonic

development

4. A muscular tail posterior to the anusMany adults lose this during embryonic

development

Invertebrate Chordates

• 2 subphyla - Urochordata (tunicates) and Cephalochordata (lancelets)

• Think of these 2 groups as simpler versions of vertebrates

• Tunicates - most primitive; like other chordates during larval stage, lose chordate traits as adults

Craniates• Chordates that have a

head• Most ancient of these

are the hagfish• Hagfish are marine,

bottom dwelling scavengers

• No vertebrae• Do have a skull made

of cartilage

Vertebrates

• Craniates with a backbone

• Evolved during the Cambrian when some Craniates developed

• more complex nervous systems

• more complex skeletons

• more extensive skull

• backbone composed of vertebrae that encloses the spinal cord

Vertebrates

• Lampreys are the oldest line of vertebrates

• Marine or freshwater• Cartilage pipe surrounds

the notochord• No jaws - have a rasping

mouth that bores holes in the sides of fish - live on the blood of their host

Gnathostomes

• Vertebrates with jaws– Evolved from the

pharyngeal slits– 2 major classes:

• Class Chondrichtyes

• Class Osteichthyes

Gnathostomes - Class Chrondrichythyans• Sharks, rays, and their relatives• Flexible endoskeletons made of

cartilage• Streamlined bodies• Will sink if they stop swimming

because they are denser than water

• Have changed little over 300 million years of evolution

Shark Reproduction• Eggs fertilized internally• Some are oviparous - lay

eggs that hatch outside the body

• Some are ovoviviparous - fertilized eggs stay in body, hatch in uterus

• Some are viviparous - baby develops inside the uterus and gets nutrients there

Horn shark egg case

Gnathostomes - Class Osteichthyes

• The bony fishes• Most numerous of all the

vertebrate groups• 2 main classes

– The ray-finned fish– The lobe fin fish

• Includes lungfish - freshwater, air gulpers

• Have an ossified (bony) endoskeleton

• Covered in scales• Have a swim bladder - air sac

that controls buoyancy

Most fish ray finned

Tetrapods - Gnathostomes

with limbs & feet• Evolution of limbs and

feet from fins ~ 360 mya

• Loss of gill slits• Fusion of pelvic bones

to backbone• All of these adaptations

lost or altered in various groups of tetrapods

Tetrapods - Class Amphibia• Includes salamanders, frogs, and caecilians• Still closely tied to water• Rely on skin for gas exchange with environment• Not all have legs• Some have an aquatic larval stage with a terrestrial adult

life• Eggs do not have a shell• External fertilization• Complex social behavior

Tetrapods - Amniotes

• Tetrapods with a terrestrial adapted, amniotic egg

• Consists of mammals and reptiles (including birds)

Tetrapods - Amniotes• Evolution of the amniotic egg very important for life on land

– Shell retains water– Now animals can live away from water– Amniotic eggs have an extramembryonic membrane that functions in

gas exchange, waste disposal, and transport of nutrients to embryo

Tetrapods - Reptiles• Have scales that contain

keratin– No longer breath through skin

like amphibians• Get oxygen through lungs• Lay eggs on land• Internal fertilization• Most are ectothermic -

control body temp with behavior, not metabolism

Tetrapods - Reptiles• Extinct reptiles include:

– Dinosaurs (land dwelling)– Pterosaurs (flying)– Plesiosaurs (marine)

• Modern reptiles include:– Turtles– Tuataras– Lizards– Snakes– Alligators– Crocodiles

tuatara

brown basilisk

Giant Galapagos

Tetrapods - Birds• Lay amniotic eggs (like reptiles)• Have keratin containing scales on

their legs (like reptiles)• Endotherms - maintain a warm

internal body temp • 4-chambered heart• High metabolism• Larger brains than reptiles

Tetrapods - Birds

• Most have bodies adapted for flights– Light, hollow bones– Relatively few organs– Wings– Feathers

Amniotes - Mammals

• Amniotes that have hair and produce milk from mammary glands

• Endothermic • Most born, not hatched• All have internal

fertilization• Large brains• Teeth

Mammals• 3 groups:

– Monotremes - egg laying, have hair, produce milk, playpuses, spiny anteaters

– Marsupials - give birth to embryo, completes development in a pouch while nursing, wombats, kangaroos, Tasmanian devils

– Placental mammals (eutherians) - longer pregnancy, deer, mice, tigers, primates

Mammals - primates• All have

– hands & feet that grasp– Large brains– Short jaws– Forward-looking eyes– Flat nails– Well-developed parental care– Complex social behavior

Mammals - primates• 3 subgroups

– Lemurs, lorisese, pottos

– Tarsiers– Anthropoids

(monkeys, apes, humans) lemurs

tarsier

ape

Mammals - humans • Increase brain volume• Shorter jaw• Less of a size difference between the

sexes• Different family structures

Jane Goodall

Diane Fossey

Quick CheckFor each new clade,

list the primary feature that separates it from

the clade before it.