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VERTEBRATES > The Agnatha and evolution of the jaw > Flashcards

The Agnatha and evolution of the jaw Flashcards

1
Q

what are the Agnathans?

A

Jawless fish

gnathostomes (Jawed vertebrates)

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2
Q

Groups of agnathans

A
  • Ostracoderms - Extict
  • Lamprey - Extant
  • Hagfish - Extant
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3
Q

Ostracoderm features

A
  • 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.
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4
Q

Ostracoderms evolution

A
  • 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
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5
Q

Fossil cyclostome?

A
  • Tully monster (Tullimonstrum)
  • Has a notochord, arcualia
  • Teeth made of keratin
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6
Q

Evidence for Agnathans split from gnathostomes

A
  • 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
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7
Q

Lamprey mouth structure

A
  • Keratinous teeth
  • Oral hood
  • Oral gland secrete anticoagulant
  • Latch on and grind away at tissues and take up blood
  • Predaceous
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8
Q

Lamprey body structures

A
  • Notochord - support
  • Minute vertebrae / Arcualia (dorsally)
  • Dorsal fin ( no paired fin)
  • Structures are close together
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9
Q

Lamprey features

A
  • 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.
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10
Q

Lamprey eye

A
  • Well developed eyes
  • Pineal eye responsible for detecting light
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11
Q

Lamprey - Larval stage

A
  • Filter feeders
  • Can live in larval stage for 3/4 years
  • Produce mucus
  • Endostyle produces mucus (turns into thyroid gland in adult)
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12
Q

Lamprey reproduction

A
  • 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
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13
Q

what is an Anadromous lifecycle?

A

spend adult life in sea then move to freshwater to reproduce

sea -> freshwater

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14
Q

What is an Catadromous life cycle?

A

Lives adult life in freshwater then move to the sea to breed
Freshewater -> sea

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15
Q

Some Lampreys have become landlocked

US great lakes

A

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).
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16
Q

How do lampreys find spawning ground?

A
  • 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.
17
Q

UK species of lampreys

A
  • Brook lampreys: (don’t feed as adults).
  • River lampreys: coastal regions - estuaries.
  • Sea Lamprey: coastal, ocean, come back to streams to breed.
18
Q

Hagfish

A
  • 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
19
Q

Hagfish features and internal structure

A
  • 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
20
Q

Advantages of jaws

A

Allowed vertebrates to feed on a wide variety of prey

21
Q

Main hypothesis for why jaw evolved

A
  • Evolved as a way of improving ventilation
  • Ventilation hypothesis
  • Allowed more efficient ventilation
  • Water could be drawn in
  • Mallat (1996)
22
Q

Issues with the ventilation hypothesis for why jaws evolved?

A
  • 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
23
Q

How was the jaw formed?

A
  • Muscularised pharynx and branchial arches (gill bars)
  • Anterior gill arch (mandibular arch) became enlarged and formed the jaw
  • Next gill arch along supports jaw
24
Q

Developmental issues with the evolution of the jaw under the ventilation hypothesis

A
  • (it has been suggested that)Evolution of jaw can only occur when change from 1 to 2 laterally placed nasal sacs
25
Q

Paired fins are only seen in the jawed vertebrates (some exceptions on non-jawed). What is the advantage of paired fins

A
  • Pitch controlled by pelvic and pectoral
  • Yaw controlled by dorsal and anal
26
Q

Genes repsonsible for what similar developmental process in lamprey, bony fish and tetrapods?

A
  • 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
27
Q

Groups within Cyclostome?

A

Hagfish and Lampreys

28
Q

Hafish Sex ratio ?

A
  • 100 : 1
  • 100 females : 1 male
  • Female biassed
  • some are hermaphrodites
29
Q

What are Jawed vertebrates called?

A

Gnathostomes

30
Q

What does the development of the the jaw mean for prey capture in gnathostomes?

A
  • Can suck in prey and grasp it.
  • Selected to be large and used for feeding
31
Q

What is pitch and yaw for fish movement?

A
  • Pitch = movement up and down
  • Yaw = side to side

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