10.1 4.2.2 geological timescale  (Page 2/3)

Dinosaur

• Coenozoic : Mammals developed further, including primates. Development of birds and flowering plants is evident. Global cooling occurs (i.e. the most recent ice ages, which caused a drop in sea levels. This allowed development of land bridges between NorthAmerica and Asia, also between India and Sri Lanka and between Australia and the islands to its north. These land bridges assisted greatly in the migrationof species to new land masses).

Hominid evolution started during the Coenozoic. This will be discussed more fully in Grade 12.

Two types of flowering plants (Angiospermae)

Critical evaluation

The geological column summarises a commonly accepted evolutionary theory about life’s history. As with most scientific theories, not all scientistsagree about the theory. Criticisms include the following:

• The sequence (order) of fossils given in the geological column is not found anywhere on earth. Instead, there are many places where ‘older’ layers are found on top of ‘younger’ layers.
• There are many gaps in every sequence. To explain this, palaeontologists suggest that about 2/3 of all the kinds of organisms that ever lived could not have left any fossils.(Palaeontologists are scientists who study fossils and try to use then to reconstruct the past.) It is reasonable to expect that many soft-bodiedcreatures would not have been fossilised. However, it is strange that there are gaps in the fossil records even for creatures with hard shells and strongbones.
• The dating methods used to date rocks might not give valid dates. This is discussed in the section about fossils and dating.

The “missing link” between dinosaurs and birds

It’s been believed for many years that modern birds developed from reptilian ancestors, as dinosaur fossils often showed AVIAN or ‘bird-like’ features. There was thus much excitement when the first fossil of a “missing link” between birds and dinosaurs wasfound, the so-called Archaeopteryx. Around 10 such fossils have been found to date. Read the information below:

Left is a specimen of Archaeopteryx , and right is an artist’s impression of the skeleton in an upright position. Note the very fine feather impressions, including the flight feathersof the wings. Despite its obvious avian nature, Archaeopteryx has a hand virtually identical to other dinosaurs such as Velociraptor .