The Agnatha and evolution of the jaw Flashcards
what are the Agnathans?
Jawless fish
gnathostomes (Jawed vertebrates)
Groups of agnathans
- Ostracoderms - Extict
- Lamprey - Extant
- Hagfish - Extant
Ostracoderm features
- Dermal exoskeleton
- Moveable plates around mouth
- Notochord
- Dorsal fin, some pectoral fins
- 10-50 cm
- Would have been more closely related to gnathostomes as they had mineralised tissues and paired fins.
Ostracoderms evolution
- Approx 500 MYA
- Coexisted with gnathostomes for 50MY -
- Unlikely that the evolution of jawed verts caused extinction because they coexisted together for so long.
- Extinct in late Devonian
Fossil cyclostome?
- Tully monster (Tullimonstrum)
- Has a notochord, arcualia
- Teeth made of keratin
Evidence for Agnathans split from gnathostomes
- Alpha and beta globins give evidence for this split
- Aplpha and beta globin genes aren’t shown in jawless vertebrates.
- Common ancestor of lampreys & hagfish, predates split so diverged before this
Lamprey mouth structure
- Keratinous teeth
- Oral hood
- Oral gland secrete anticoagulant
- Latch on and grind away at tissues and take up blood
- Predaceous
Lamprey body structures
- Notochord - support
- Minute vertebrae / Arcualia (dorsally)
- Dorsal fin ( no paired fin)
- Structures are close together
Lamprey features
- Nasohypophyseal opening (nostril). Able to detect chemical cues
- Adenohypophysis - adrenal gland
- Tidal ventilation - pull water in and out of gill opening - not very efficient.
- Velum: stops water from flowing into the mouth.
Lamprey eye
- Well developed eyes
- Pineal eye responsible for detecting light
Lamprey - Larval stage
- Filter feeders
- Can live in larval stage for 3/4 years
- Produce mucus
- Endostyle produces mucus (turns into thyroid gland in adult)
Lamprey reproduction
- Many spent adult life in sea then move to freshwater to reproduce (anadromous)
- Use oral hood to build a nest
- Female latches into a stone and produces hundreds of eggs
- Males fertilises eggs
- Adults die after mating
- Larvae burrow into silt and can stay there for up to 7 years
what is an Anadromous lifecycle?
spend adult life in sea then move to freshwater to reproduce
sea -> freshwater
What is an Catadromous life cycle?
Lives adult life in freshwater then move to the sea to breed
Freshewater -> sea
Some Lampreys have become landlocked
US great lakes
Live their entire lifecyle in the lakes:
- mature adults migrate into streams to spawn.
- Ammocotes larvae burrow in stream bed and metamorphose
- Emerge and migrate downstream
- Free swimming parasitic phase in lakes (normally live this stage in the sea).
How do lampreys find spawning ground?
- Use olfactory cues
- Detect chemicals released by larvae at spawning locations
- Females can detect mature males
- Can avoid areas if theres high mortality - due to chemical cues
- Cue made up of bile acids.
- Detected at low concentrations & produced in large quantities.
UK species of lampreys
- Brook lampreys: (don’t feed as adults).
- River lampreys: coastal regions - estuaries.
- Sea Lamprey: coastal, ocean, come back to streams to breed.
Hagfish
- Only found in marine environments
- Mainly deep sea
- Feed on carcases that fall to the bottom on the sea
- Scavengers
- They produce mucus when attacked - possibly use mucus to clog gills of prey fish. Possibly predacious?
- Can acquire nutrients through skin and gills
Hagfish features and internal structure
- Single gill opening - leads into chamber with branchial pouches
- Notochord
- Slime glands
- No dorsal fin
- Single nostril - uptake of water - pushed out through gill opening
- Have a vellum that splits gut and respiratory system
- Adenohypophysis
- Skin covered eyes
- Barbels for detecting
- Rudiments of vertebrae - arcualia
- Only verts with blood isosmotic to sea
Advantages of jaws
Allowed vertebrates to feed on a wide variety of prey
Main hypothesis for why jaw evolved
- Evolved as a way of improving ventilation
- Ventilation hypothesis
- Allowed more efficient ventilation
- Water could be drawn in
- Mallat (1996)
Issues with the ventilation hypothesis for why jaws evolved?
- Mandibular arch doesn’t form functional gill arch in any living vertebrate or fossil
- No evidence that the spiracle was ever a gill slit.
- Gill arches are slightly differently aligned in the jawless verts as they are in the jawed verts
- Cyclostomes = gills on inside ?
- Gnathostomes = gills on the outside ?
- Mandibular arch has different developmental origin & innervation
How was the jaw formed?
- Muscularised pharynx and branchial arches (gill bars)
- Anterior gill arch (mandibular arch) became enlarged and formed the jaw
- Next gill arch along supports jaw
Developmental issues with the evolution of the jaw under the ventilation hypothesis
- (it has been suggested that)Evolution of jaw can only occur when change from 1 to 2 laterally placed nasal sacs
Paired fins are only seen in the jawed vertebrates (some exceptions on non-jawed). What is the advantage of paired fins
- Pitch controlled by pelvic and pectoral
- Yaw controlled by dorsal and anal
Genes repsonsible for what similar developmental process in lamprey, bony fish and tetrapods?
- Genes responsible for dorsal fin of lamprey also responsible for midline & paired fins in bony fishes
- Same genes control limb development in tetrapods
- Same genetic mechanism in different location
Groups within Cyclostome?
Hagfish and Lampreys
Hafish Sex ratio ?
- 100 : 1
- 100 females : 1 male
- Female biassed
- some are hermaphrodites
What are Jawed vertebrates called?
Gnathostomes
What does the development of the the jaw mean for prey capture in gnathostomes?
- Can suck in prey and grasp it.
- Selected to be large and used for feeding
What is pitch and yaw for fish movement?
- Pitch = movement up and down
- Yaw = side to side
