What the world's oldest eggs reveal about dinosaur evolution

The results of the study show that hard-shelled eggs evolved early in dinosaur evolution with thickening occurring independently in several groups, but a few million years later other reptiles also developed hard-shelled eggs.

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What does this mean? Does this mean that early dinosaurs had live born offspring, but then a single species evolved the ability to lay eggs. But over a few million years, the other dinosaurs did also (evolve the egg laying ability?)

Is this within the expectations of evolutionary theory that egg-laying ability would develop in a single species, but then “spread” (the wrong term, I know) to other species over a few million years? If I’m reading this correctly, I’m quite surprised to see this change from liveborn to egg laying occur from species to species, over time.

@evograd @T.j_Runyon @T_aquaticus @John_Harshman

I think this is more about fossilisation. Hard shells leave behind fossils while soft shells don’t (I assume)… and since there are no fossils in the initial several million years, the eggs went through a processes of hardening (supposed to be selected for because predators couldn’t break them).

It might be true. However the “evolutionary” reason of being selected for because predators couldn’t break the harder shells seems to be an unverified just so story.

How would you like us to verify it?

name the predator. Check how hard the egg will have to be to resist its teeth.
Check whether Baby Dinosaurs are strong enough break eggs which are hard enough to resist said predators teeth…

That will atleast show the scenario is feasible.

Ok. How do we identify the predator? Then hopefully we have the right parts of the skull to estimate bite strength. You say do this and that like it’s easy. What about looking at dinosaurs closest living relatives? Can we look at them to draw inferences?

Nobody is forcing anyone to make up explanatory stories.
If its not possible to verify, just say the reason is unknown.
Would that be hard to do?

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There was no just-so story here. It’s called a hypothesis. Notice the words “ one possibility”. Also, from looking at living groups that is a very reasonable inference.

It’s possible

I would cal it a hypothesis of there is going to be any effort to prove or disprove it. Its just speculation, thats all.

Edit: Its entirely possible that there is no evolutionary advantage to having harder shells and its an side effect of some other adaptation.

Maybe a good project for my thesis?

Sure, if you think its worth it… tell me what you find out.

No, at least within the scope of this study, they showed that the ancestral state of dinosaurian eggs was quite thin, and that different lineages of dinosaurs independently thickened their eggs. The “ability to lay eggs” (on land) goes back to the first amniotes, much earlier than dinosaurs.

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No. It’s talking about eggs with calcified shells as opposed to eggs with flexible shells. Have you ever seen a snake egg? Egg-laying is the primitive condition in amniotes (and tetrapods, and vertebrates), and no dinosaurs had live birth.

No. What this implies is that several species independently evolved this condition, perhaps due to similar selection.

Thank you very much. I, obviously, misunderstood what they were saying. Now it makes good sense.

So, then, they are saying that early dinosaurs seemed to lay soft eggs, but one by one they began to lay eggs with hard shells?

Reisz is puzzled by the fact that “reptile and mammal precursors appear as skeletons in the fossil record starting 316 million years ago, yet we know nothing of their eggs and eggshells until 120 million years later. It’s a great mystery that eggs suddenly show up at this point, but not earlier.”

Read more at: What the world's oldest eggs reveal about dinosaur evolution

So, “knowing nothing of their eggs” potentially means that, because they were soft, there have been no remains found?

“What the world’s oldest eggs suggest about evolution” might be more spot on.

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Hard to tell what they’re saying based on a web site rather than the actual paper, but they do seem to be saying that several groups of dinosaurs, and some other taxa, independently evolved hard-shelled eggs. It’s suggested also that these were responses to egg-predation.

Yes, that’s the suggested explanation in that web article.

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Thank you. So, then, in this case, would it generally be assumed that this kind of transformation (independent, soft to hard shell) attributed (possibly) to predation avoidance, would be the result of natural selection vs. any of the other modes that cause species to evolve?

With actual evidence.

One for which you seem to think it’s not possible to provide evidence.

Yes. Predation would be the selective environment in which hard shells would be advantageous.

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