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The reproductive modes

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OK, Viviparity (live-bearing) evolved from Oviparity (egg-laying) to times via egg retention and eggshell thickness due to colder climates.

How Oviparity ended and Viviparity just begun the very first time?

Did the off-spring decided to stop Oviparity, and begins to Viviparity immediately for the next generations to come?

If this is an adaptation to colder climates evolves by transition, there must be a later or final step where the very thin egg is laid vulnerable to cold. (Maybe mother protects it by incubating like birds, or young is ready for cold before laying?)

I cannot find simple explanations, as well as videos and images.

 

Thank you!

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That is an interesting question.

 

Marsupials have pouch and a short gestation period, so there would be intermediate species and steps along that process.

Maybe pouches on the body for eggs evolving to eggs hatching in the uterus and the very immature foetus hanging on to the dam, to pouches and lactation to feed the foetus, to more mature stages and no pouches.

In fact the methods of nutrition of the foetus would have to have developed in unison.

https://en.wikipedia.org/wiki/Marsupial

 

Marsupials represent the clade originating with the last common ancestor of extant metatherians. Like other mammals in the Metatheria, they are characterized by giving birth to relatively undeveloped young, often residing in a pouch with the mother for a certain time after birth....

An infant marsupial is known as a joey. Marsupials have a very short gestation period (about four to five weeks), and the joey is born in an essentially fetal state. The blind, furless, miniature newborn, the size of a jelly bean,[35] crawls across its mother's fur to make its way into the pouch, where it latches onto a teat for food. It will not re-emerge for several months, during which time it develops fully. After this period, the joey begins to spend increasing lengths of time out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens, it will seek refuge in its mother's pouch for safety.
Joeys stay in the pouch for up to a year in some species, or until the next joey is born. A marsupial joey is unable to regulate its own body temperature and relies upon an external heat source. Until the joey is well-furred and old enough to leave the pouch, a pouch temperature of 30–32 °C (86–90 °F) must be constantly maintained.

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OK, Viviparity (live-bearing) evolved from Oviparity (egg-laying) to times via egg retention and eggshell thickness due to colder climates.

 

How Oviparity ended and Viviparity just begun the very first time?

 

Did the off-spring decided to stop Oviparity, and begins to Viviparity immediately for the next generations to come?

 

If this is an adaptation to colder climates evolves by transition, there must be a later or final step where the very thin egg is laid vulnerable to cold. (Maybe mother protects it by incubating like birds, or young is ready for cold before laying?)

 

I cannot find simple explanations, as well as videos and images.

 

Thank you!

Ovoviviparity is a mode where the egg is retained longer in the body (your intermediate of sorts), one obvious benefit is to protect the egg(s) from predation, not just thermoregulation. The process would have probably been gradual, as the amount of retention time increased over the genrations. Eventually species would have evolved that retained the egg the entire reproduction period. This would have provided the opportunity, or perhaps it co-evolved with increased retention time, of the evolution of the substitution of some and then more and then most of the nutrients in the egg for those directly from the mother.

That is an interesting question.

 

Marsupials have pouch and a short gestation period, so there would be intermediate species and steps along that process.

 

Marsupials have a different lineage than placental mammals having diverged in the cretaceous

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Ovoviviparity is a mode where the egg is retained longer in the body (your intermediate of sorts), one obvious benefit is to protect the egg(s) from predation, not just thermoregulation. The process would have probably been gradual, as the amount of retention time increased over the genrations. Eventually species would have evolved that retained the egg the entire reproduction period. This would have provided the opportunity, or perhaps it co-evolved with increased retention time, of the evolution of the substitution of some and then more and then most of the nutrients in the egg for those directly from the mother.

Marsupials have a different lineage than placental mammals having diverged in the cretaceous

Was the OP just about mammals? it wasn't clear there is even talk of birds in the OP.

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Was the OP just about mammals? it wasn't clear there is even talk of birds in the OP.

Your right, I am not sure now that you point it out. Although viviparity is mostly associated with placental mammals in which their particular form of viviparity has a common evolutionary origin.

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Your right, I am not sure now that you point it out. Although viviparity is mostly associated with placental mammals in which their particular form of viviparity has a common evolutionary origin.

Even so, it is likely to be about mammals. So marsupials are one of the types of mammals.

This explains it better than I can.

http://www.palaeontologyonline.com/articles/2012/fossil-focus-marsupials/

Edited by Robittybob1

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Marsupials are obviously viviparous. The only oviparous animals (edit: nonsense of course, it should be mammals) are monotremes. Viviparity was assumed to be evolved from oviparity via egg-retention and has evolved multiple times in scaled reptiles (Squamata).

 

However, in this study (Early origin of viviparity and multiple reversions to oviparity in squamate reptiles, R. Alexander Pyron and Frank T. Burbrink, Ecology Letters, (2013)) the authors suggest that viviparity originated 174 million years ago, but many members switched back to egg laying and back to live birth again. This does support the notion that there is some flexibility in terms of egg retention and release.

Edited by CharonY

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"The only oviparous animals are monotremes."

 

Typo. You meant; "The only oviparous mammals are monotremes."

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Yep, brainfart. Left for further generations to see.

Edited by CharonY

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Occasionally have babies born in their amniotic sacs. I think our own ancestors were soft shell which was probably an easier jump than a switch from hardshell would have been.

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