But we’re going to concentrate on the second reason. Fish and amphibian eggs didn’t, and don’t to this day, have waterproof shells (think of frog eggs) so they’d dry out if laid on land. And as you’re about to see, a waterproof egg laid on land presents its own problems that had to be solved before the embryos inside could survive in eggs laid on land.<\/li>\n<\/ul>\n <\/p>\n
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![\"\"](\"https:\/\/featheredphotography.com\/blog\/wp-content\/uploads\/2018\/08\/curlew-egg-8198b-ron-dudley.jpg\" "\\"curlew-egg-8198b-ron-dudley\\"")
<\/p>\n
What was needed to make that transition was something like this – a waterproof shell. A friend and I found this apparently scavenged Long-billed Curlew egg on Antelope Island about 12 years ago.<\/p>\n
The waterproof shell of modern birds and reptile eggs prevents the eggs from drying out. However, the shell also makes it more difficult for respiratory and metabolic wastes like carbon dioxide and urea to diffuse out of the egg and away from the developing embryo. Those wastes are toxic to the developing embryo.<\/p>\n
So the development of the waterproof shell all by itself wouldn’t be sufficient to allow early vertebrates to fully invade the land without being tied to the water for reproduction.<\/p>\n
<\/p>\n
<\/p>\n
![\"\"](\"https:\/\/featheredphotography.com\/blog\/wp-content\/uploads\/2024\/11\/amniotic-egg.jpg\" "\\"amniotic-egg\\"")
<\/p>\n
Chicken egg nine days after fertilization – Wikimedia Commons<\/em><\/p>\nThis is what was required to solve all the problems – what is today called the amniote egg or land egg of modern birds and reptiles. The shell, plus four extraembryonic membranes, save the day.<\/p>\n
\n- Shell – prevents the egg contents, including the developing embryo, from drying out.<\/li>\n
- Yolk sac (vitellus) – food source for the developing embryo. This membrane isn’t new to land vertebrates. Early and present fish and amphibians had (have) it too.<\/li>\n
- Amnion – a fluid filled sac that encloses the embryo in an aqueous environment, protecting it from shocks and adhesions.<\/li>\n
- Allantois – stores metabolic wastes, isolating them from the embryo. Later on it also acts as a respiratory surface for the exchange of oxygen and carbon dioxide.<\/li>\n
- Chorion – the outermost membrane enclosing the entire system. It’s just below the shell (it’s the membrane you notice when you peel a hardboiled egg.<\/li>\n<\/ul>\n
Note: Late in embryo development the allantois and chorion fuse to form the chorioallantoic membrane. Lying just below the porous shell, this vascular membrane serves as a provisional “lung”, allowing gas exchange through the shell.\u00a0<\/em><\/p>\n <\/p>\n
There you have it. The development of the amniote egg is considered a pivotal moment in the evolution of vertebrates, perhaps the most significant event in vertebrate evolutionary history.<\/p>\n
So in my view, we ought to know about it.<\/p>\n
Ron<\/p>\n
<\/p>\n
Note: All this explains how egg laying (oviparous) terrestrial vertebrates like reptiles and birds invaded and exploited a terrestrial environment but what about amphibians and mammals?<\/em><\/p>\n\n- Amphibians. They’re vertebrates but they don’t have a shelled egg. As their Class name suggests, amphibians are only partially terrestrial. Many can live on land as adults because they have small lungs to supplement the oxygen they obtain through their skin. But they’re tied to the water because the skin of most must be kept wet to keep it permeable to respiratory gases. But more importantly for us in this discussion, their eggs are shell-less so they must be laid in water to keep them from desiccating.<\/em><\/li>\n
- Mammals. They’re vertebrates but with the exception of the Monotremes (platypus and echidnas) they don’t lay eggs. However, they do produce eggs – they’ve just evolved novel ways of nurturing their developing embryos in an aqueous environment inside the mother, so no shell is required.<\/em><\/li>\n<\/ul>\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
Perhaps the most significant event in vertebrate evolutionary history. We wouldn’t be here without it. (2 images)<\/p>\n
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