Earth Science: Geological Eras – Paleozoic Era
Our detailed fossil records only begin at around 542 million years ago, in the Paleozoic era. All major animal groups, excluding mammals and birds, evolved in this era, ending in a great extinction about 250 million years ago.
The first proto-organisms with a cell-like structure appeared about 3.5 billion years ago. after which the first organisms formed relatively quickly. These single-celled, aquatic organisms, some of which had developed a nucleus, dominated life on Earth for more than two billion years.
About one billion years ago the first multicellular organisms developed, along with sexual reproduction. The resulting marine organisms, such as annelids, cnidarians, and articulates, were later to become the foundation of evolution in the Paleozoic era.
From the Cambrian explosion to mass extinction
At the beginning of the Cambrian period about 570 million years ago, the first invertebrates, including trilobites and graptolites, evolved within a period of only 50 million years. The first terrestrial organisms were algae, lichen, and bacteria, which spread along the edges of shallow ponds, adding oxygen to the atmosphere. Small air-breathing animals followed, including arthropods (resembling millipedes) with hard outer skeletons that prevented them from drying out.
The first vertebrate’s were oceanic fish, existing during the Ordovician period 470 million years ago. Later, during the Devonian period, placoderm fish co-existed with ray-fins, ancestors of most modern-day fish. It is thought that fish developed lungs and “legs” as a climate change reduced water levels and evolving into the first four- legged vertebrates, amphibians, from which all other vertebrates later originated.
The warm, moist climate during the Carboniferous period allowed for an abundance of vegetation—tall horsetails, club mosses, liverworts, and ferns—to florish in the swamps. Several amphibians became terrestrial and began producing eggs (protected by hard shell on land). This group later evolved into reptiles, which occupied every possible ecological niche during the Permian period.
Reptiles even managed to fly, such as the pterodactylus. However, the masters of the air continued to be insects, some of which evolved into giants, for example, the Meganeura, a dragonfly with a wingspan of up to 27.5 inches (70 cm). Meanwhile the oceans were inhabited by lamp shells (brachiopods), snails, mussels and clams, bony fish, sharks, oraminifers, and numerous ammonites.
BASICS
THE MOST dramatic mass extinction in geological history occurred 250 million years ago at the end of the Permian period.
Almost all marine organisms and more than three-quarters of terrestrial organisms were destroyed.