Amniotic Egg

Suite of special membranes with various jobs (store nutrients, gas exchange, waste storage)

Shell (in some) for protection + waterproofing, can lay eggs in more exposed locations

Figure 34-16

ShellYolk saccontains nutrients

Embryo

Mammals dispensed with shell and embryo avoids desiccation by developing within mother

Dino EGGS

•Dino rookery-nests 23ft apart

•Lots of parental care

•Communal nesters

Birds and mammals have amniotic egg too!

Mammals also have…Mammary glands... nourish offspring with milk-Ramifications….????

•Offspring can grow fast and over a long period... •Don’t have to feed themselves (offspring can be born in locations that don’t support them directly)• Not dependent on abundance of prey •Mom can store energy in the form of fat to feed young

ANCESTRAL AMNIOTE Amniotic egg

Reptiles

Mammals

Synapsids

Early mammals split from Synapsids

Mammals only surviving lineage of Synapsids

Rest of Synapsids went extinct at end Permian extinction event (250mya)

WITHIN MAMMALS..

Eutheria=Placentals

Metatheria=Marsupials

Prototheria=Monotremes

Monotremes

•Eggs w/ soft shells

echidnas incubate their eggs in their pouch

platypuses incubate eggs in nest

•Milk-oozes out of skin onto fur

•Highly altricial

Marsupials•Born at very premature stage…(highly altricial-equivalent to 8-10 wk old human embryo) (no precocial young)

•Shorter period of diffusion of materials between mom and baby

•Pouch with nipples inside-permanently attached at start..ex utero development

What cool pouch facts did you learn about (open up pouch paper)??

Gas exchange through skin! Ectothermic! Immune system underdeveloped! No lymph system!

Advantages to pouch vs long pregnancy?Bad conditions? Toss offspring!

Note:•Is a kind of yolk sac which delivers nutrients •In some species there ARE placenta like organs but not clear whether nutrients are transferred.

Marsupials are much less diverse than placental mammals in terms of range of lifestyles, range of body shapes and where they live.

One hypothesis for why no fully aquatic or flying species has to do with reproduction….

Thylacosmilus (left / source) and Borhyaena (right / source). http://www.palaeontologyonline.com/articles/2012/fossil-focus-marsupials/

http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000436

Marsupials reached Australia 50 mya

Placentals were there in Australia but went extinct

Left Marsupials as only mammals

One group of mammals=Eutherian=Placentals have a Placenta! Very specialized membranes for transfer of nutrients….transfer of nutrients is high!

Figure 34-17

Yolk sac

Chorion

Amnion

Embryo

Allantois

Placenta

Mother’s uterus

Placentals or Eutherians

•Chorioallantoic placenta- “even more specialized tissues”-provide nutritional, respiratory and excretory exchange between mom and offspring

•So young can spend even more time developing in mom so…. more developed when born (altricial to precocial)

•Pathogen free! (well…)

No evidence that one evolved first…or that one is less advanced..Different solutions to different selective environments.

Why would selection favor a long gestation or short gestation?