ZOO 2090 Lecture Notes - Lecture 3: Agnatha, Acanthodii, Fish Jaw
1 May 2018
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ZOO2090 – Evolution and Diversity of “Fish”
Extinct groups: placodermi, acanthodii
Agnathans: Jawless fish
• Mouth present, but no biting apparatus derived from pharyngeal arches
• Two extant groups: hagfish and lamprey (=cyclostomes)
• Groups were already highly specialized by 330 mya and display features likely
representative of ancestral Agnathans
• Conodonts and ostracoderms are groups of jawless “fish” that are now extinct
Hagfish (Myxiniformes)
• Scavengers of the deep sea (~70 species) that also prey on invertebrates
• They use a rasping tongue (cartilaginous plate bearing horny teeth0like processes)
• Characteristics:
o Somoconfomers
o Single nostril
o Multiple venous heart
o No jaws, fins or image forming eyes
o Large slime glands along body
o Cartilaginous skull but no vertebrate
Lampreys (Petromyzontiformes)
• Marine or freshwater inhabitants (~40 species)
• Many are parasitic using their round mouth to cling to live prey, other use mouth to cling
to substrate
• Characteristics:
o Metamorphosis of ammocoete larva
o Single nostril
o Seven gill openings
o Cartilaginous skull and vertebrae
o No jaws
o Fins (dorsal, unpaired)
o Image forming eyes unlike hagfish
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2
Conodonts
• Known only from hard tooth elements for a long period
• So abundant they are used for stratigraphic dating (540-230mya)
• Fossils founds in early 1980s highlight vertebrate characters
o Dentin and enamel
o Notochord
o Myomeres
o Postanal tail
• Feeding apparatus:
o Complex feeding basket in pharynx and large eyes (active feeders)
Ostracoderms
• Small jawless fishes that displayed lateral line system and an outer exoskeleton made up
of dermal bone plates
• Amour innovation
• Appeared in Ordovician, radiated most in the Silurian and were extinct by end of
Devonian
• Major groups:
o Pteraspidomorpha
o Anaspids
o Thelodonts
o Osteostracans
Gnathostomes: vertebrates with jaws
• Includes chondrichthyans (chimeras, sharks and rays) and osteichythans (bony
vertebrates; ray and lobe finned fishes plus tetrapods
• Placoderms were first group with jaws (now extinct)
Basic structure of a fish jaw:
• Major evolution in history of vertebrates occurs when fish obtained jaws
• …
Advent of Jaws:
• jaws seem responsible for new radiation during the Devonian period that led to decline of
Agnathans
• Enables new diets and food-handling techniques expanded predatory lifestyles
• Hyomandibula evolved to ear bones in vertebrates
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