The Paleozoic, which began approximately 0.54 billion years ago, saw the explosive diversification of multicellular organisms, the colonization of land by plants, the appearance of amphibians, and its end in the greatest mass extinction in history. It is an era in which interactions between organisms and the environment are clearly illustrated.
Basic Knowledge
The Cambrian explosion (approximately 0.54 billion years ago) was a phenomenon in which animal body plans diversified explosively over a short period of only several million years. Fossils from the Burgess Shale (Canada) and the Chengjiang biota (China) show that ancestors of most present-day animal phyla had already appeared. The evolution of eyes is also thought to have intensified predator–prey relationships and accelerated evolution.
At the end of the Ordovician (0.485–0.444 billion years ago), the first mass extinction occurred, eliminating approximately 85% of marine species. During the Devonian (0.419–0.359 billion years ago), plants advanced onto land, and vascular plants and ferns later formed forests. Fossil records also show the process by which fish, ancestors of amphibians, developed limbs and moved onto land, including intermediate fossils such as Tiktaalik.
During the Carboniferous (0.359–0.299 billion years ago), forests of large ferns spread and became the source material for today’s coal seams. Oxygen concentrations exceeded 30%, and giant insects, such as the dragonfly-like Meganeura with a wingspan of 75 cm, flourished. The end-Permian mass extinction (the P–T boundary, approximately 0.252 billion years ago) was the largest in history, eliminating approximately 96% of marine organisms and 70% of terrestrial vertebrates. Environmental changes caused by the Siberian Traps (large-scale volcanic activity) are considered the main cause.
What are the forests in which large ferns flourished during the Carboniferous used as today? Also describe the characteristic atmospheric oxygen concentration at that time.
Answer: The remains of Carboniferous plants were transformed into today’s coal seams (coal resources). Atmospheric oxygen concentrations at that time were approximately 30% or higher, far above today’s 21%, creating an environment in which giant insects could flourish.
- Cambrian explosion: approximately 0.54 billion years ago; explosive diversification of animal phyla
- End-Ordovician mass extinction: first major extinction; 85% of marine organisms became extinct
- Devonian: plants advanced onto land; ancestors of amphibians moved onto land
- Carboniferous: forests of large ferns → today’s coal seams; oxygen above 30%
- Flourishing of giant insects such as Meganeura was partly due to the high-oxygen environment
- End-Permian mass extinction (P–T boundary): the largest in history; 96% of marine organisms became extinct
- Cause of the P–T boundary extinction: the Siberian Traps (large-scale volcanic activity)
Points to Note
① Although it is called the Cambrian explosion, it was actually an evolutionary radiation that occurred over several million years. ② The Permian and Dyassic are synonymous (the former Japanese name is Dyassic). ③ Do not confuse the P–T boundary (Permian–Triassic boundary) with the K–Pg boundary (Cretaceous–Paleogene boundary). The former marks “the greatest mass extinction in history.”
Practice
- Name one representative fossil site showing the Cambrian explosion.
- State the era and boundary name at which the greatest mass extinction in history occurred.
- What did the plant remains of the Carboniferous leave behind to the present day?