VE Lecture 3

Fish Evolution from Agnathans to Modern Groups

Overview of Fish Evolution

  • Main Topics:
    • Fish evolution from agnathans to modern groups
    • Origin of the vertebrate jaw
    • Vertebrates of Orcadian Lake
  • Suggested Reading: Benton chapters 3 and 7

Key Phylogenetic Groups

  • Agnatha (Jawless Fish):
    • First appearance in early Cambrian
    • Types include: Myxinoidea (hagfishes), Petromyzontida (lampreys), and various extinct groups (e.g., Conodonta, Heterostraci)
  • Gnathostomata (Jawed Fish):
    • Distinguished by the presence of jaws, arising from anterior gill arches
    • Major subgroups include:
    • Placoderms (Silurian and Devonian)
    • Chondrichthyes (sharks and rays)
    • Osteichthyes (bony fish and tetrapods)

Evolutionary Timeline

  • Palaeozoic Era:
    • Notable periods for fish from Cambrian to Devonian
    • Ordovician and Silurian:
    • Emergence of armored jawless fish
    • Features include large dorsal and ventral head shields, multiple paired gill openings
    • Devonian Period (Age of Fishes):
    • Marked by radiation of fish diversity

Key Fish Groups

Agnathans
  • Characteristics:
    • Jawless, some with bony head shields
    • Examples: Osteostracans
  • Diversity:
    • Various forms with different adaptative traits, such as armored bodies and multiple gill openings
Jaws: A Dietary Revolution
  • Development of Jaws:
    • Emerged in Silurian, enabling more efficient feeding
    • Hints at evolutionary advantages leading to increased complexity in feeding strategies
Placoderms
  • Features:
    • Equipped with armor plating; notable for large bone-enclosed heads
    • E.g., Dunkleosteus, known from late Devonian fossil records
Sharks (Chondrichthyes)
  • Evolutionary Traits:
    • Presence of an endoskeleton made of calcified cartilage
    • Unique dentine and enamel dermal scales
    • Early shark fossils date back to late Ordovician
Bony Fish (Osteichthyes)
  • Categories:
    • Actinopterygii (Ray-Finned Fish), includes most modern fish
    • Sarcopterygii (Lobe-Finned Fish), includes ancestors of tetrapods
  • Evolutionary Innovations:
    • Lungs and swim bladders leading to independent fin movement
Coelacanths and Lungfish
  • Lungfish:
    • Possess lungs and gills; capable of aestivation during dry periods
    • Fossils show early diversification in marine and freshwater environments
  • Coelacanths:
    • Recognized as living fossils; arose in mid Devonian
    • Known for unique fin structures adapted for land traversal

Orcadian Lake Vertebrates

  • Palaeoenvironment:
    • Found in an ancient subtropical lake ecosystem
    • Preserved diverse fish fossils including agnathans, placoderms, and both ray- and lobe-finned bony fish
  • Significance:
    • Offers insight into early vertebrate ecosystems and evolution during the Devonian.

Conclusion

  • The evolution of fish reflects a complex interplay of morphological adaptations and environmental changes, laying the foundational diversity seen in modern aquatic life.