Amniotes

Question 1

Amniote origins?

Answer 1

Oldest fossil which could be a stem amniote (320 mya), which means a sister taxon to all living amniotes. Cannot tell if its an amniote from the fossils.

Question 2

Amniote phylogeny?

Answer 2

Question 3

What features did the crown group amniotes have?

Answer 3

1. Amniotic egg
2. Internal fertilisation
3. Keratinised skin
4. Ventilation by a negative pressure aspiration pump

All been modified in different ways in the main clades, but none have been completely lost - all are functionally significant to amniote lifestyle.

Question 4

How is the amniotic egg characterised?

Answer 4

Characterised by 3 new extra-embryonic membranes which are completely absent in non-amniotes.

Amnion - forms the amniotic cavity, cushions the embryo in a buffered aqueous environment

Allantois - outgrowth of gut, forming sac used to store nitrogenous waste. Heavily vascularised, allowing gas exchange

Chorion - envelopes the exterior of the egg. Co joins the allantois in late development to act as a single large surface for gas exchange.

Protein rich aqueous albumen is secreted by mother’s oviduct. Egg shell, if present, is laid down in the oviduct around the shell membrane.

Question 5

What could have led to the origin of the amniotic egg?

Answer 5

1. Completing the transition to dry land:
   The tough, dry outer coat of amniotic coat reduces water loss
   Difficulty of nitrogenous waste is solved by the egg’s allantois in the form of urea. Otherwise the environment in the amniotic sac would suffocate the animals
2. Increased body size on land:
   SA:VOL ration limits the maximum size of an amniotic egg. Uncertain about the size of stem amniotes however.
3. Extended embryo retention:
   Retention of the embryo by the mother for a long time after internal fertilisation may have been present.

Question 6

How does amniote fertilisation occur?

Answer 6

Amniotic egg is fertilised internally, except in birds.

Question 7

What derived characters occur in the amniotes?

Answer 7

1. Keratinised skin
   Keratin isn’t waterprood, the cells between keratin and lipid in epidermis may be significant in osmoregulation
2. Ventilation mechanisms
   More efficient negative pressure aspiration pump for breathing. Tidal costal ventilation is primitive state, with many derived variations. Advanced unidirectional ventilation mechanisms in dinosaurs and birds

Question 8

What are reptiles?

Answer 8

Not a proper taxonomic group - paraphyletic as don’t include birds.

Question 9

What is ectothermy?

Answer 9

External environment is main source of heat the body.
High metabolic rates at low and high temperatures
Body temperatures can rise faster than the ambient temp
Behavioural thermoregulation is enhanced by change in blood flow
Cheap energetically, but restricted to smaller size
Spend much of time spending time out of harm’s way

Question 10

What is endothermy?

Answer 10

High rate of aerobic metabolism provides a warm environment
Metabolic rate follows environmental temperature