the Tetrapod invasion of land

Question 1

Sarcopterygii (gnathostomata class)

Answer 1

key traits :
- enamelled teeth
- flesh, lobed fins
- cosmoid scales
- 26,742 extant species
- only 8 extant fishes

Question 2

Coelacanthimorpha (Sarcopterygii subclass)

Answer 2

• appeared in the Devonian, max diversity in the triassic
• 2 living species
• 3 lobed tail supported by hollow spine
• unconstricted, unossified notochord
• double gular plate
• spiny dorsal fin
• cranio-vertebral joint

Question 3

Dipnotetetrapodamorpha (Sarcopterygii subclass)

Answer 3

• Dipnomorpha (includes lungfish: 6 living species, all fw)
  
  • all living spp are FW: 1 Aus, 1 S amer. 4 African
  • once considered origin of tetrapods
• Tetrapodomorpha ( all extinct except infraclass tetrapod)
• large predatory fishes
• symmetrical tails (functioning swim bladder?)

Question 4

monophyletic group

Answer 4

consists of a common ancestor plus all descendants of that ancestor

Question 5

paraphyletic group

Answer 5

does not contain all the descendents of a single ancestor

Question 6

polyphyletic group

Answer 6

containing descendents of different ancestors

Question 7

Tetrapodomorphs

Answer 7

• peaked in late devonian, extinct in permian … except tetrapods
• robust limb skeleton : hip / shoulder girdles ad rotational shoulders
• could be large : up to 6m in Rhizodontiformes
• dorsally placed eyes
• 1 pair of external nostrils (incurrent)
• choana (excurrent nostrils moved to palate)
• many thought to ambush predators

Question 8

breathing frequency

Answer 8

• 1 per 3-10 mins in obligate air breathers
• 1 per hour for water breathers with lungs
• fewer breaths = fewer trips to the surface

Question 9

water : respiratory medium

Answer 9

• high density (heavy)
• high viscosity
• low o2 areas
• available everywhere
• easy to expel metabolic wastes

Question 10

air : respiratory medium

Answer 10

• low density (1/800th water)
• low viscosity (1/30th water)
• o2 always available
• requires surface access
• hard to expel metabolic wastes

air is more o2 rich : 21% in air vs < 1% in water
diffusion from air to blood is also faster

Question 11

air breathing fish

Answer 11

• > 370 known spp (49 families) : capacity to obtain o2 from air
• has evolved many times (~50)
• aquatic vs amphibious
• facultative vs obligate
• air breathing organs :
  
  • derived from gut (lungs, gas bladder, stomach, intestines)
  • head and pharynx (gills, mouth, pharynx, opercles)
  • skin
• in higher actinopterygians, lungs -> gas bladder

Question 12

Origins of lungs

Answer 12

air breathing
- to cope with seasonal dryness?
- exploit new habitats / release competitions?
- to survive low o2 waters?
* lungs probably evolved in marine species - not usually hypoxic
* habitats of lungfish not necessarily hypoxic
- Farmer (1999) : to avoid myocardial hypoxia
* exercise stimulates air breathing more than aquatic hypoxia
* death from exercise results in heart faliure
* o2 sensors afferent to gill
* early fish active, high o2 environment
secondary loss of lung?
- air access not possible at depth
- aerial predation

Question 13

Circulation: Chondrichthyes & Actinopterygii

Answer 13

positioned behind gills
4 chambers in series:
1. sinus venosus
* a reservoir to collect blood
* assures easy filling
2. atrium
3. ventricle (pump)
4. conus / bulbus arteriosus
* conus, muscular in sharks
* bulbus, elastic in bony fish
sinoatrial and atrioventricular valves maintain unidirectional flow

Question 14

features central to the evolution of tetrapods

Answer 14

• pectoral (and pelvic) fins
• respiratory system
• circulatory system
• also : reproductive, digestive system, sensory system

Question 15

Origins separate circulation

Answer 15

• to survive low o2 waters?
• atrium fully divided
• ventricle functionally divided
• oxygenated blood -> reduced gills
• systemic blood -> gills then lung