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.