Precambrian to Devonian Periods (Important Facts) {Part-5}

WHEN THE EARTH FORMED about 4,600 million years ago, its atmosphere consisted of volcanic gases with little oxygen, making hostile to most forms of life. One large supercontinent, Gondwana, was situated over southern polar region, while other smaller continents were spread over the rest of the world.

Middle Ordovician positions of present-day land-masses

 Constant movement of the Earth's crustal plates carried continents across the Earth's surface. The first primitive life-forms emerged around 3,400 million years ago in shallow, warm seas. The build up of oxygen began to form a shield of ozone around the Earth, protecting living organisms from the Sun's harmful rays and helping to establish an atmosphere in which life could sustain itself.

The Earth during the Middle Ordovician Period

• Examples of Precambrian to Devonian Plant groups:-

A Present day- ClubMoss(Lycopodium sp.)

A Present day Land Plant(Asparagus Setaceous)

Fossil of an extinct land plant(Cooksonia hemisphaerica)

• Examples of Precambrian to Devonian Trilobites:-

Acadagnostus
Group: Agnostidae ; Length: 8mm (1/3 in)

Phacops
Group: Phacopidae ; Length: 4.5 cm

Olenellus
Group: Olenellidae ; Length: 6cm

• Examples of Early Marine Invertebrates:-

Fossil Nautiloid(Estoniceras perforatum)

Fossil Brachiopod(Dicoelosia bilobata)

Fossil Graptolite(Monograptus Convolutus)

The first vertebrates appeared about 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 Period (409-369 million years ago), and the first land animals about 30 million years later.

• Examples of Devonian Fish

Rhamphodopsis
Group: Ptyctodontidae ; Length: 15cm

Pteraspis
Group: Pteraspidae ; Length: 25cm

Coccosteus
Group: Coccosteidae ; Length: 35cm