Page 66 - Ultimate Visual Dictionary (DK)

PREHISTORIC EAR TH

Precambrian to MIDDLE ORDOVICIAN POSITIONS
OF PRESENT-DAY LANDMASSES
Devonian periods America Greenland China
North
South Australia
America
WHEN THE EARTH FORMED about 4.6 billion years ago, its
atmosphere consisted of volcanic gases with little oxygen,
making it hostile to most forms of life. One large supercontinent,
Gondwana, was situated over the southern polar region, while
other smaller continents were spread over the rest of the world.
Constant movement of the Earth’s crustal plates carried
continents across the earth’s surface. The first primitive life-forms
emerged around 3.4 billion years ago in shallow, warm seas.
The build up of oxygen began to form a shield of ozone South
around the Earth, protecting living organisms from the Africa India
Africa
Sun’s harmful rays and helping to establish an atmosphere
in which life could sustain itself. The first vertebrates appeared about Scandinavia North East
Europe Africa
470 million years ago, during the Ordovician period (510–439 million years ago),
the first land plants appeared around 400 million years ago during the Devonian Siberia Central Asia
period (409–363 million years ago), and the first land animals about 30 million years later.

EXAMPLES OF PRECAMBRIAN TO DEVONIAN PLANT GROUPS

A PRESENT-DAY CLUBMOSS A PRESENT-DAY FOSSIL OF AN EXTINCT LAND PLANT FOSSIL OF AN EXTINCT SWAMP PLANT
(Lycopodium sp.) LAND PLANT (Cooksonia hemisphaerica) (Zosterophyllum llanoveranum)
(Asparagus setaceous)

EXAMPLES OF PRECAMBRIAN TO DEVONIAN TRILOBITES

ACADAGNOSTUS PHACOPS OLENELLUS ELRATHIA
Group: Agnostidae Group: Phacopidae Group: Olenellidae Group: Ptychopariidae
Length: ⅓ in (8 mm) Length: 1¾ in (4.5 cm) Length: 2½ in (6 cm) Length: ¾ in (2 cm)

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