Geol 204 - march 11th

Overview of Paleozoic Fish and their Evolution

• The Devonian Era is often referred to as the "age of fishes," characterized by a diverse array of aquatic life.

Plankonic Fish and Placoderms

• Placoderms: An early group of jawed fish characterized by bony armor and, in many cases, powerful jaws. They were apex predators in Devonian seas.

  • Example: Dugongsteus, known for a more squat profile, adapted for speed and possibly the first real fast-swimming predator in oceans.

  • Previously misunderstood as being more eel-like in restoration, newer models correct this.

Chondrichthyes (Cartilaginous Fish)

• Chondrichthians: Includes sharks and their relatives, characterized by cartilaginous skeletons rather than bony structures.

  • Presence in Devonian, but not yet apex predators.

  • Evolved into more important predators during the Carboniferous and Permian periods.

  • Modern sharks differ significantly from their Paleozoic ancestors.

Evolution of Ray-Finned Fish

• Ray-Finned Fish: Including nearly all modern fish; this group became increasingly diverse after the Devonian.

  • Dominance of ray-finned fish today is significant, with them comprising about half of vertebrate diversity since the Carboniferous.

Lobe-Finned Fish and Evolution

• Lobe-Fins: Characterized by a more complex bone structure in their fins and considered the ancestors to all tetrapods.

  • Examples include modern lungfish, which can burrow into mud during dry conditions and wait for rain.

  • These fish retain many characteristics from their Devonian forms.

Coevolution and Defenses

• As predatory fish evolved, prey species like trilobites adapted new defenses, including larger eyes and protective spines, leading to a coevolutionary arms race.

Late Devonian Extinctions

• The Late Devonian extinctions are marked by two major events:

  • Kielfosser Event: An earlier extinction phase related to reef community collapse.

  • Engenburg Event: Occurred at the Devonian-Carboniferous boundary, with significant loss of biodiversity, particularly in reef systems and coral communities.

• The extinction was characterized by a decrease in replacement rates rather than an increase in background extinction rates.

Reef and Community Collapse

• Reefs that supported vast biodiversity, characterized by many species, collapsed leading to numerous extinction events, especially among chondrichthians, placoderms, and ammonoides.

• Major reef-building groups like tabulate and stromatoporoid corals faced significant loss.

Effects of Terrestrialization

• The evolution of land plants contributed significantly to mass extinction events in the Devonian:

  • Development of roots allowed for soil creation, changing nutrient cycles and affecting marine environments.

  • This nutrient runoff led to algae blooms and subsequent hypoxic events in waters, killing off reef organisms.

Emergence of Early Terrestrial Ecosystems

• Rapid diversification of land plants started with non-vascular forms like mosses and liverworts.

  • Essential adaptations included the development of stomata for gas exchange and cuticles to prevent desiccation.

• The transition towards vascular plants (tracheophytes) allowed for further colonization away from water, including the emergence of woody plants and true trees.

Arthropods and the Transition to Land

• Arthropod groups, especially myriapods (millipedes, centipedes), hexapods (insects), and arachnids (spiders, scorpions) began to venture onto land.

  • Many evolved adaptations such as chitinous exoskeletons to prevent desiccation and jointed limbs for movement.

• Some ancient arthropod groups, like certain eurypterids, adapted to living on land.

Evolution of Tetrapods

• The evolution of tetrapods occurred from lobe-finned fishes that could breathe air and traverse land for short periods.

  • Key adaptations included the development of limbs with joints that allowed for movement on solid ground.

  • Tiktaalik serves as a transitional fossil showcasing adaptations for both aquatic and terrestrial life.

Summary of Major Events and Impacts

• Colonization of land by plants and animals significantly altered ecosystems.

• The Late Devonian results in significant changes in marine biodiversity due to increased nutrient flow from land.

• Evolutionary changes supported the development of more complex land ecosystems in subsequent periods.