Mesozoic

Mesozoic (251-72 million years ago)

The Mesozoic Era is often referred to as the 'age of reptiles' because throughout the marine and terrestrial habitats, reptiles (especially dinosaurs) were dominant. This era is divided into three major periods: Triassic, Jurassic and Cretaceous.

The major events of this era included significant changes in the climate, evolutionary activity and land mass of Earth.

• The climate of the Mesozoic fluctuated dramatically between cooling and warming periods.
• Until this point, the Earth existed as one giant land mass called Pangaea. During the Mesozoic era, this land mass broke up, and by the end of the era, the continents as we know them today drifted into their current positions.
• The extinction of nearly all animal species in the former Paleozoic era led to rapid evolution of many new life forms.
• Dinosaurs appeared on land, becoming the major terrestrial vertebrates for nearly 135 million years.
• Pterosaurs (flying reptiles) dominated the skies and aquatic reptiles such as ichthyosaurs were found in the oceans.
• During the Triassic, Archaeopteryx, a link between reptiles and birds was thought to evolve.
• The earliest birds appeared during the Jurassic period having evolved from an order of dinosaur called theropods.
• Dinosaurs became extinct around 65 million years ago, at the end of the Cretaceous period.
• The dominant land plant species of the time were the seed-producing plants known as the gymnosperms. These include the cycads and conifers.

Fact:

The existing sequoia trees found mainly in the United States were also thought to have evolved in the Mesozoic.

Sequoia trees in California, USA.

South African fossil examples: The mammal-like reptile, Lystrosaurus was by far the most dominant terrestrial vertebrate during the Early Triassic period. Specimens of the Lystrosaurus were unearthed in the Balfour and Katburg formations in the Karoo. The discovery of Lystrosaurus in the Coalsack Bluff in the Transantarctic Mountains helped confirm the theory that the continents of the Earth were once joined, as Lystrosaurus had already been found in the Early Triassic fossil record of Southern Africa, India and China.

Lystrosaurus skeletal diagram.

"Living fossils" in South Africa: Coelacanths were thought to have gone extinct towards the end of the Cretaceous period but were re-discovered in 1938 off the northern coast of South Africa in KwaZulu-Natal. The coelacanth is thought to have evolved into its current form over 400 million years ago and is nicknamed a 'living fossil' because knowledge of the species was previously based on fossils, as it was thought to have gone extinct. Coelacanths and amphibians share a common ancestor- the lobe-finned fish. Lobe-finned fish have fleshy fins unlike fins in other fish which are joined to the body by a single bone. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs in the earliest tetrapod land vertebrates which were amphibians. Living lobe-finned organisms include the coelacanths and lungfish.

Coelacanths, which represent a link between fish and amphibians, were though to have gone extinct at the end of the Cretaceous period.