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Approximate date category originated

about 160 million years ago

Defining characteristics

feet

ankles

jaws

teeth

placenta

The earliest organism with this feature

Juramaia sinensis

http://www.bio.sdsu.edu/faculty/archibald/Archibald01Eutheria.pdf

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Approximate date category originated

About 65 million years ago

Defining characteristics

Monotremes

Lay eggs

Milk secreted from belly

Marsupials

Higher metabolic rates

Nipples that provide milk

Give birth to live young

Pouch called marsupium

The earliest organism with this feature

Steropodon galmani

Kangaroo

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Approximate date category originated

210 million years ago

Defining characteristics

lateral temporal fenestra (a hole in the lateral surface of the skull behind the orbit)

modern synapsids are all warm blooded

glandular skin

differentiated teeth

two occipital condyles (bony knobs forming a joint between the head and the neck)

The earliest organism with this feature

ophiacodontid

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Approximate date category originated

320 million years ago

Defining characteristics

tetrapods

lay eggs on land

retain fertilized egg inside the mother

en egg with an amnios (sac inside of the egg that has nutrients)

The earliest organism with this feature

Basal amniotes

http://www.biologyreference.com/A-Ar/Amniote-Egg.html

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Approximate date category originated

370 million years ago

Defining characteristics

ectothermic

tetrapods

start out as larvae living in water

gills as a larvae morph into adult air-breathing organism with lungs

permeable skins

The earliest organism with this feature

Ichthyostega

http://www.amphibianark.org/education/what-are-amphibians/

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Approximate date category originated

375 million years ago

Defining characteristics

“Four feet”

Head separated from the body by a neck

Pelvic girdle fused to backbone

Except for fully aquatic species, they don’t have gills

Pharyngeal clefts instead give rise to ears, certain glands, and other structures

Full set of ribs

The earliest organism with this feature

Tiktaalik

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Approximate date category originated

about 420 million years ago

Defining characteristics

female spawns a lot of eggs and then the male inseminates the eggs

fins are webs of skin supported by bones

radials which link external bones to the internal skeleton

The earliest organism with this feature

Sarcopterygii

http://www.seawater.no/fauna/chordata/Actinopterygii.html

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Approximate date category originated

450 million years

Defining characteristics

Jaws with teeth

Fore brain enlarged

Lateral line system- organs that form a row along each side of the body

The earliest organism with this feature

Placoderms

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Hollow bundle of nerve fibers that transverse dorsally the longitudinal axis of chordates and runs above the notochord and gut

Approximate date category originated

500 million years ago

Defining characteristics

embryonic feature unique to chordates

develops into the central nervous system: the brain and the spine

derives from ectoderm that rolls into a hollow tube during development

The earliest organism with this feature

lancelet branchiostoma

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Animals with skulls
Cephalization/craniates

Approximate date category originated

480 million years ago

Defining characteristics

obviously having a skull

neural crest (embryonic tissue)

all develop a notochord

all possess an endoskeleton

circulatory system of arteries, veins, capillaries, and a chambered heart

The earliest organism with this feature

jawless fish

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Approximate date category originated

545 million years ago

Defining characteristics

Tail that extends posterior to the anus

Contains skeletal elements and muscles

Helps propel aquatic species

The earliest organism with this feature

Lancelets

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Openings in the pharynx that develop into gill arches in bony fish and into the jaw and inner ear in terrestrial animals

Approximate date category originated

500 million years ago

Defining characteristics

in most, present only during embryonic development and then become inner ear and jaw bones

some invertebrate chordates use the pharyngeal slits to filter food out of the water that enters the mouth

The earliest organism with this feature

fish embryo

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Approximate date category originated

550 million years ago

Defining characteristics

Longitudinal, flexible rod located between the digestive tube and the nerve chord

Composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue

Provides skeletal support

The earliest organism with this feature

Cephalochordata (Lancelets)