Unit 2: Paleozoic Life

The Cambrian Explosion of Life

The Paleozoic Era begins with the Cambrian Period (541 Ma). The start of the Cambrian marks one of the most important events in the history of life: the Cambrian Explosion. This was a "short" geological period (over 10-20 million years) of incredible, rapid diversification of animal life.

What Changed?

After 3 billion years of simple life, almost all major animal phyla (basic body plans) appeared in the fossil record.

• Hard Parts Evolved: For the first time, animals developed shells, skeletons, and hard exoskeletons. This is why the fossil record suddenly becomes so rich.
• New Lifestyles: Complex ecosystems with active predators (like Anomalocaris) and burrowing animals emerged.
• Key Fossil Groups: The most famous animals of the Cambrian are the Trilobites. Other groups include brachiopods, mollusks, and early chordates (our ancestors).
• The Burgess Shale: A famous fossil site in Canada that preserved soft-bodied "weird wonders" of the Cambrian, showing the full extent of this evolutionary "experimentation."

Causes of the Cambrian Explosion (Triggers):

• Environmental: Rise in atmospheric oxygen (from the GOE) allowed for larger bodies and active (aerobic) lifestyles.
• Ecological: The "arms race" between new predators and prey drove the evolution of defenses like shells and new behaviors.
• Genetic: The evolution of complex Hox genes (genetic toolkit) allowed for the construction of new, complex body plans.

Ordovician Life

The Ordovician Period (485 - 443 Ma) followed the Cambrian. It was a time of continued diversification, known as the Great Ordovician Biodiversification Event (GOBE). Life expanded, especially in shallow marine environments.

Key Life Forms:

• Trilobites: Remained abundant and diverse.
• Brachiopods: Became one of the most dominant shelled animals on the seafloor.
• Graptolites: Colonial animals that floated in the water; excellent index fossils for dating rocks.
• Nautiloids: Early, large predators with straight or coiled shells (ancestors of squid/octopus).
* Bryozoans and Corals: The first large reef-building communities appeared.

Episodic Mass Extinctions of Cambrian Trilobites

The Cambrian was not a smooth ride. The fossil record shows several "biomeres"—cycles of diversification followed by sharp mass extinctions, particularly affecting trilobite populations.

These extinctions (like the "Ptychaspid biomere") are thought to be caused by rapid, short-term climatic changes. One leading hypothesis is the sudden upwelling of cold, oxygen-poor (anoxic) water from the deep ocean onto the shallow continental shelves where trilobites lived.

Ordovician Climatic Change and Mass Extinction

The Paleozoic Era was punctuated by 5 major mass extinctions (the "Big 5"). The first of these occurred at the end of the Ordovician Period (~443 Ma).

The End-Ordovician Extinction:

• Severity: The second-largest extinction of all time. Around 85% of all marine species were wiped out.
• Cause: A sudden, intense global ice age (glaciation).
• Mechanism:
  1. Phase 1 (Cooling): A large supercontinent, Gondwana, drifted over the South Pole. This allowed massive ice sheets to form, causing sea levels to fall dramatically. This destroyed the shallow, warm-water habitats where most life existed.
  2. Phase 2 (Warming): The ice age ended abruptly. The ice sheets melted, and sea levels rose rapidly, flooding the new low-lying habitats with water that was low in oxygen.
• Victims: Brachiopods, trilobites, corals, and graptolites were devastated.

Middle Paleozoic Life: Radiation of Fishes

Life recovered in the Silurian and Devonian periods (the "Middle Paleozoic"). The most significant evolutionary event during this time was the diversification of fish. The Devonian Period (419 - 359 Ma) is known as the "Age of Fishes."

Key Groups in the Radiation:

• Agnatha (Jawless Fishes): The first fish. They were covered in bony "armor" plates and lacked jaws (e.g., Pteraspis).
• Placoderms (Armored Fishes): The dominant predators of the Devonian. These were the first fish with jaws, a massive evolutionary innovation. They had bony head-shields (e.g., Dunkleosteus).
• Chondrichthyes (Cartilaginous Fishes): The first sharks evolved.
• Osteichthyes (Bony Fishes): This group split into two important lines:
  • Ray-Finned Fishes: The ancestors of 99% of all fish today (tuna, cod, etc.).
  • Lobe-Finned Fishes: These fish had fleshy, muscular fins supported by bones. This fin structure was the pre-adaptation for walking on land. One group of lobe-finned fish would eventually evolve into amphibians and all other four-limbed land vertebrates (tetrapods).

Early Land Plants and Impact of Land Vegetation

While fish ruled the seas, another revolutionary event was happening on land: the invasion of the land by plants. This began in the Silurian and truly took off in the Devonian.

The First Land Plants:

• The first plants were small, simple, and moss-like, living at the water's edge.
• The first true vascular plants (with internal "pipes" for water) evolved in the Silurian (e.g., Cooksonia). They were just a few centimeters tall.
• By the late Devonian, the first forests had appeared, with tree-sized plants like Archaeopteris (which had a woody trunk and fern-like leaves).

Impact of Land Vegetation:

The "greening" of the continents permanently changed the planet in profound ways:

• Atmosphere: Photosynthesis on a massive scale drew down atmospheric CO₂ and pumped out even more O₂.
• Climate: The reduction of CO₂ (a greenhouse gas) caused global cooling, which helped trigger the Late Devonian mass extinction.
• Soil: Plant roots broke up rock and created soil for the first time (a process called pedogenesis).
• Rivers: The new soils and root systems stabilized river banks, changing them from wide, shallow "braided" rivers to the meandering rivers we see today.