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.