Granted, it's only been seen once, in a captive colony. But the researchers found a female angulate tortoise that retained one or two eggs inside of her during a particularly hot summer. She only laid the eggs a day before the hatchlings pipped.
Between viviparity and ovovoviviparity, what are the advantages and disadvantages to each as a form of gestation?
I'm not sure why but the writer of serina seems to think it would be really really hard for birds to move from hard shelled eggs to this method of reproduction.
They have it take like 200 million years and even when it happens the egg is still involved in someway. like the Dolfinch embryo starts developing in a tiny soft shelled egg that only attaches to its mother's blood supply a little ways into development.
I keep seeing people say that the advantage dinosaurs had over mammals during the Mesozoic was egg-laying, as they could produce a bunch of small young in a short period of time.
If oviparity is the superior reproduction, then how come viviparous mammals were the successful ones and outcompeted oviparous mammals like monotremes? And how come the oviparous birds and reptiles didn't take back all their megafaunal niches during the Cenozoic?
i'm not familiar with the life cycle of most sharks to a detailed extent, and i'm aware that the distribution between R to K varies considerably between species, but why are at least some characterized by low numbers of pups, yet no sharks that i'm aware of have any parental involvement? my best guess atm is that it has something to do with the internal gestation and live birth of all shark species, which may provide advantage over oviparity in high-risk enviroments, but the mystery deepened when i learned that although most sharks are ovoviviparous, some are viviparous.
whale sharks produce several hundred eggs which are gradually fertilized as health permits. shortfin mako are also ovoviviparous, but only produce 4-18 young. however, hammerheads, blue, and bull sharks are viviparous. bull sharks produce 1-13 pups, which, as i'd expect from viviparity, yield lower numbers of young due to the higher metabolic demands of having a placenta(edit-is this correct?). so what evolutionary pressures selected for this, and why is it found in such unrelated species? i'm not great at understanding phylogeny though, so maybe they are more related than i know. i tried looking into this but got a bit lost. some insight here would be appreciated. these unusual few species don't seem to have any particular advantage over the typical ones though, so i'm confused how this developed, especially only in a few sharks.
i'll stop here before this gets too long and meandering, but i'd appreciate some help, as well as any source recommendations. sharks are today's random research binging topic, but this riddle has me a bit stumped. thanks!
There's a lot of evidence suggesting the Jurassic metriorhinchid crocodylomorphs were viviparous, due to them being incapable of moving around on land.
If that's the case, does this mean Dougal Dixon's vortex and porpin are plausible after all?
(I know I've brought this up before, but I've rarely gotten a good answer.)
One of the most important distinctions between amphibians and their reptilian descendants - and the adaptation that allowed reptiles to dominate the land, while amphibians could not - is the calcified or amniotic egg. This allowed reptiles to reproduce on land, rather than in the water. However, this could also be done by skipping the egg entirely and transitioning to live birth. What if evolution had taken this route instead? What would modern life look like?
Could anyone recommend a paper/book discussing the evolution (and examples of going both ways in one lineage?) between the birth methods? Some of the posts in the last week have me wondering.
I'm specifically curious about whether or not they experience anything similar to menstruation in any form, or if not, why not? What happens if their eggs go unfertilized?
I remember people nowadays saying that Dougal Dixon's Pelagornids from After Man are no longer plausible, due to science marching on and revealing that archosaurs are incapable of viviparity due to the thick shells of their eggs.
But what about thalattosuchians (those marine crocodylomorphs from the Jurassic)? They show no signs of land capability, and thus they're thought to be viviparous. So does mean the viviparous penguins are plausible after all?
Weird question, I know, just bear with me for a moment...
One planet-concept that's been booted around in my brain for quite some time is that of a cold, dead city-planet, orbiting an equally dead white dwarf star. Before I had truly mapped out or considered the implications of the setting, my brain supplied the idea that the beasts of this world would be viviparous, bearing and birthing their young in a manner similar to Terran mammals. the organisms themselves have a biochemistry mostly the same as that on Earth. This leads into my question.
Amniotic fluid is one of the more prominent elements of the mammalian birthing process, a remnant of our aquatic, water-surrounded ancestry repurposed to cushion the unborn and lubricate the birth canal in preparation for birth itself. However, the extreme, inescapable cold on my planet means that having a water-based analog to amniotic fluid would possibly result in the fluid freezing solid around the birth canal after the mother's water breaks, or even causing the fluid-soaked child to die of frostbite while still halfway out if birth takes more than a few minutes. Either way, an unfavorable prospect. In our own world, seals and other mammals retreat to warmer biomes to breed, or give birth underwater(which, by merit of being a liquid, can't be below the liquid's freezing point). Building a fire is only an option for sapient or pre-sapient life, so that isn't an option for the vast majority of this biosphere.
Thus far, I can see three ways to get around this, though I have concerns about all three.
- the fluid is laced with glycogens or other natural antifreezes. Simple, but easily drinkable water is scarce on this planet, so I don't know if water-based fluid is even a good idea.
- the water is replaced with various fatty oils with a much lower freezing point than water. Again simple, but possibly much more energy/nutrient-intensive to make as opposed to water.
- the amniotic fluid isn't a fluid at all, but a thick, viscous mucus with a much lower freezing point to both avoid freezing and conserve water. Suitably "alien," but may have other issues of its own and again may be rather intensive to produce atop the other demands of pregnancy.
Which of these three do you think is the best? Is there any option I'm missing? My thanks in advance for any answer.
I'm planning a new world and one of the characteristics of it is that (almost) all the fauna is taken from the Permic and Triassic periods, that means, synapsids ruling the world, small dinosaurs, giant amphibians, rhyncosaurians and all that jazz. But in this world there are humans, virtually identical to real world in almost every metric, plus some extra "races" or species. But there is one particular thing in which these humans are different: as they didn't eveolve from placental mammals but from cynodonts, they oviparous, they lay eggs.
What I want to discuss here is what the consequences of an egg-laying humanity would be, as most threads I found in the internet tend to not be good enough. So go around, share your ideas about what consequences would this have.
For my own case I decided to go with a more reptilian egg. Human eggs, while hard, are softer than bird eggs. Humans also don't sit on themas birds, but put them in "nests" that are usually a small reserved space in houses. There, there's usually a carpet in the ground to avoid the egg directly touching it. The eggs are covered in mants and cloaks to maintain it warm, and the mother verifies it each day or so. Usually the number is around 3 eggs; sometimes more but that's rare. The egg also hatches earlier than a human fetus, at six months, so it is smaller. But for the first three months the baby has to live in the nest.
About the societal and cultural impact of this I hadn't though thag much, but among other things I suppose that, as usually women lay more than one egg per litter; as such the number of twins would be bigger. This can mean more emphasis on sibling relationship in this world, maybe with some moral codes related to protect your litter siblings and betraying them being a very big taboo. This has also implicancies for royalty. A hereditary monarchy in which the succesor is the oldest son cannot work, so maybe in this world elective monarchy would be more common, as well as the goverments may be more aristocratic, theocratic or oligarchic than monarchical (If we understand monarchy as the rule of one person).
What are your thoughts?
Like I know the Dinosauroid concept has A LOT of issues already but I don't think I've seen this one discussed ever. Having a hard (waterproof) egg shell makes it much harder to transition to Placental viviparity.
here's how /u/ArcticZen explained it to me a while back when I was asking a question about Sernia and birds becoming pregnant.
>The biggest issue is their calcified shells. Viviparous sharks, snakes, and lizards have soft eggs, such that it's comparable to a placenta, but bird eggs are essentially reinforced due to the prolonged and continued use of oviparity - it just made sense for them, sense it prevents eggs from drying out or taking in too much moisture in terrestrial habitats.
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>a placenta is basically an internalized soft shell egg. Both allow for the diffusion of gas and fluids from the outside to the developing fetus. The one distinction is that a placenta will attach to a mother to facilitate nutrient transfer, whereas a soft-shelled egg will supply the developing fetus with nutrients from the start. This is consequently why most animals hatching from soft-shelled eggs will tend to be small, while newborn mammals (like elephants and whales) can be born weighing hundreds of kilograms.
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>You'd basically need to lose the shell, which has become an integral part of the egg structure at this point. It's basically requiring bird reproduction to undo an intense specialization.
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>The other would be connecting what is then a soft-shelled egg to the mother bird to facilitate survival of the egg, similar to the umbilical system mammals use. Bird development is more complex than reptiles or fish and would require it. There's no benefit to birds laying soft-shelled eggs in the meantime though, only downsides, but it's a necessary step in order to allow for viviparity. You've got this figurative adaptational valley that needs to be crossed between two peaks. You can make the journey, but it's going to take a long time.
I keep hearing that archosaurs (dinosaurs, crocodilians, and pterosaurs) are incapable of viviparity due to their hard eggshells.
Well, I have two questions.
1: I keep hearing that the marine crocodilians like Metriorhynchus might have been viviparous due to being incapable of walking on land. Is this much to support this theory, and if so, does it contradict the idea that dinosaurs (birds included) were incapable of viviparity?
2: I read somewhere that pterosaur eggs were softer than bird eggs, and more similar to lizard eggs. Does this mean pterosaurs may have been capable of viviparity?
