Australian Science Illustrated — Issue 54 2017

A

small, rat-like creature

ended up settling a debate

between palaeontologists and

geneticists. Via DNA from

modern animals, geneticists had

calculated that the placental

mammals, whose embryos are

nurtured through the placenta,

originated 160 million years ago,

but palaeontologists thought

that the fossils pointed at a later

date 125 million years ago.

In 2011, the fossil of the only

15 g Juramaia placental animal

was discovered, proving

geneticists right with an age of

160 million years. According to

scientists, the small creature

led an active life between

the branches of trees, catching

insects, etc. On the ground below

Juramaia, large dinosaurs

roamed the world.

High metabolisms, sharp

senses, and quite large brains

made the mammals a very

successful group of animals –

even in Juramaia's era. Still,

almost 100 million years passed,

before a huge meteor strike

seriously sped up the evolution

of mammals.

Small Mammal was Early

Early

mammals gave us:

Hair

Ears

Placenta

Mammary glands

Heart with 4 ventricles

A larger brain

Early terrestrial

vertebrates

gave us:

Arms

Legs

Fingers

Toes

Chest

Neck

Juramaia’s incredibly well-

preserved teeth had things

in common with modern

placental mammals.

Shoulder broke loose from the head

1 The ancestors of terrestrial animals disengaged their shoulders from their skulls, "growing" necks, which might have made

it easier for them to lift their heads above the water to breathe.

Reinforced skeleton supported body

3 It was probably a wealth of untouched food resources such as plants and insects, which finally persuaded animals to

go ashore. Without water to support the body weight, the
animals had to develop a stronger spine and larger ribs.

First legs walked on the ocean floor

2 Arms and legs developed from the fish's fins, and their orginial purpose was probably to push the animals

forward among aquatic plants or along the ocean floor.
Pederpes was the first to have limbs, which could carry it ashore.

PEDERPES

TEETH

BONEY FISH

160 million years ago

CARL BUELL/AMNH

CLAUS LUNAU

CLAUS LUNAU ZHE-XI LUO/CARNEGIE MUSEUM OF NATURAL HISTORY