Amniotes Flashcards
what are the four key derived features of crown group amniotes?
Internal fertilisation
keratinised skin
amniotic egg
ventilation by negative pressure air pump
what are the 3 new extra non-embryonic membranes (external to and derived from the embryo) that are absent in non-amniotes?
amnion
allantois
chorion
draw the amniotic egg to help!
Amnion - forms amniotic cavity, cushions embryo in buffered aqueous environment
Allantois - outgrowth of gut, forming sac to store N waste. heavily vascularised, allowing for gas exchange. in placental mammals, allantois forms part of umbilical cord.
Chorion - envelopes interior of egg. co joins allantois in late development to act as gas exchange surface - choir-allantoic membrane. in placental mammals, the chorion forms the fatal part of the placenta.
yolk sac is 4th membrane, present in amniote and non amniotes
What is the egg white secreted by?
the egg white is secreted by the mother’s oviduct
a protein rich aqueous albumen
Discuss the three main adaptive scenarios suggested for the origin of the amniotic egg
completing the transition to dry land
increased body size on land
extended embryo retention
1) amphibians can lay eggs - in moist places only. so the tough and dry outer coat and reduce water loss and prevent ammonia from diffusing outwards
evolution of allantois provides safe storage of N waste in the form of urea. difficulty of dealing with this horrible waste may be overstated because the protein metabolism of embryos is primarily anabolic.
2) evolution of chorion and allantois may have permitted an increase in size due to SA available for gas exchange. uncertain whether stem amniotes near transition were large or small …
3) extra embryonic membranes may have allowed mother to keep the embryo after internal fertilization to enable mother to exchange nutrients. not sure whether embryo retention is certain in amniotes
Internal fertilization
what is used to internally fertilise the amniotic egg?
what are the exceptions?
an intermittent phallus is used - except in tuataras and most birds - they use cloacal apposition instead.
highly divergent adult morphologies, but initially patterning events shows a shared sequence that reflects derivation from common ancestor.
limb like developmental origin of external genitalia - ancestral condition of crown group amniotes.
what is the stratum corneum?
Organelles that accomplish keratinisation by forming a thick cornified outer layer.
Keratin itself does not provide waterproofing. How is waterproofing provided?
what might be the main role of keratin?
By lipids sequestered between the keratinised cells of the stratum corneum. Balance between keratin and lipid in epidermis may be significant in osmoregulation.
main role of keratin is probably structural - providing lightweight support consistent with the more active terrestrial lifestyle . facilitated by more efficient ventilation systems of amniotes.
Contrast the aspiration pumps used by amphibians and amniotes.
Amphibians use a positive pressure buccal pump while amniotes use a negative pressure aspiration pump.
what are the ventilation mechanisms used to increase the volume of air drawn in, in addition to tidal cost ventilation?
Ribcage is fused to the shell in turtles, evolved to breathe by pulling on the viscera.
Crocodiles have evolved a diaphragm by a hepatic piston mechanism, where the liver acts as a plunger.
Saurischian dinosaurs and avian descendants evolved an air sac system to permit unidirectional flow through the lungs.
Discuss the advanced unidirectional ventilation mechanism
Birds and non avian dinsauurs use a balloon like system of anterior and posterior air sacs connect to the lungs. air sacs act like bellows, alteranlty storing air in one pahse, and releasing it to the lungs in the next phase of the cycle.
What is meant by reptiles
they are ectothermic amniotes and can be grouped by physiological convenience. they are not a proper taxonomic group.
Reptilia to be synonymous with Sauropsida - a stable clade. Reptilia include all living reptiles, and birds, and various extinct sauropsids.
What is ectothermy and endothermy
For ectothermic animals, the external environment is the main source of heat to the body. low rate of aerobic metabolism at rest.
endotherms.- high rate of aerobic metabolism provides an internal heat source, even at rest.
Some living reptiles and extinct reptilia are/ were endotherms. birds are endotherms.
