dinosaur embryos

Linnaeus

This is not "new news", but it makes wonderful reading. I certainly would like to see these eggs and embryos!

https://embryo.asu.edu/pages/study-fossilized-massospondylus-dinosaur-embryos-south-africa-1978-2012

Adam Khor

There's been some really exciting dinosaur embryo finds in recent years, including what appears to be Gigantoraptor's:

Linnaeus

Any twin embryos known? Hen's eggs often contain double yolks; what about dinosaurs'?

I remember having read something about theropod eggs being elongated, and the eggs of herbivores being roundish. I wonder if this was because theropods in general had longer, more slender bodies. At any rate, no dinosaur eggs were huge, as many people imagine them to have been. Some species laid up to 40 eggs at a time...It would have been extremely uncomfortable for a female to produce 40 boulder-sized eggs! Dinos indeed hatched quite small, but grew at leaps and bounds.

Adam Khor

I've never heard of twin embryos from the fossil record. I bet it happened, tho; it's been reported for crocodiles as well:

http://www.dailymail.co.uk/news/article-2210041/Snap-Twin-baby-crocodiles-hatch-egg-China.html

Re: egg shape, the only reptiles (at least to my limited knowledge) that lay spherical eggs like sauropods are turtles. Interestingly, turtles lay large numbers of eggs and bury them. Sauropods were apparently the same. The spherical shape may simply be the most practical one if the mother is going to carry lots of them inside for a while; elongated eggs would be a problem.

Theropod nests such as those of oviraptorids are unusual in that they often hold many eggs; a lot more than would fit in the female's body, so chances are high that they behaved somewhat like ostriches, with several females laying their eggs in the same nest. So individually, they lay few eggs and the elongated shape was not so much of a problem.
Interesting fact; therizinosaurs were theropods but apparently lay spherical eggs, like sauropods.

Re: size, it's not so much about large eggs being an issue for the mother, but rather for the embryos themselves. The bigger the egg, the thicker the shell, and the more difficult to breathe for the embryo. Also, it would be much more difficult for the baby to push its way out from its egg.

Adam Khor

Come to think of it, there is one example of monozygotic twins from the fossil record... the famous two-headed Hyphalosaurus, not a dinosaur but a choristodere from the early Cretaceous.

Linnaeus

Mmm, that is interesting about the therizonosaurs. They were theropods, and had ferocious-looking claws; yet they were apparently herbivorous, at least predominantly. The fact that they laid roundish eggs makes me think. Could some plant element in the diet of the herbivores have been responsible for the spherical shape of their eggs? Likewise, could meat protein in carnivores' diets have resulted in elongated eggs? This may be just whimsical imagination on my part; but perhaps there is some basis to it?

Modern hens have been manipulated to lay an egg a day. I'm wondering if dinosaurs which had to produce numerous eggs for the survival of their species might have gestated and laid them over the space of several days, instead of carrying one heavy burden at a time. Again, this is just an idea.

Adam Khor

Linnaeus wrote: »

The fact that they laid roundish eggs makes me think. Could some plant element in the diet of the herbivores have been responsible for the spherical shape of their eggs? Likewise, could meat protein in carnivores' diets have resulted in elongated eggs? This may be just whimsical imagination on my part; but perhaps there is some basis to it?

Not likely; other herbivorous dinos (eg. Protoceratops) laid elongated eggs, whereas many modern turtles that lay spherical eggs are carnivorous.

Linnaeus

That's strange. So there seems to be no real, categorical criterion for determining the shapes of reptiles' eggs. What do you think could explain the two main forms, spherical and elongated?

Is the old theory about the raptors stealing eggs still valid? In older books about dinosaurs, you can see illustrations of slender sneaky theropods carrying away eggs in their arms...If they were really set on eating eggs, wouldn't they have done so on the spot? (Unless, of course, Mum and Dad were nearby, ready and willing to defend their nest!);)

Adam Khor

Linnaeus wrote: »

That's strange. So there seems to be no real, categorical criterion for determining the shapes of reptiles' eggs. What do you think could explain the two main forms, spherical and elongated?

Difficult to say about spherical eggs. Reading up on turtles, it appears that it is mainly the big ones that lay spherical eggs (sea turtles, softshell turtles, snappers), whereas some smaller species lay more elongated eggs; still, this is no rule written on stone and some smaller turtles lay round eggs too. It may be like I said before that a spherical shape is simply more practical if many eggs are going to be carried/laid in the same nest closely together. Sauropod nests resemble those of sea turtles in that the eggs were buried in large numbers, so there's that.

The more elongated eggs belonged generally to smaller, more slender dinosaurs, and are typical of theropods. What we know about most theropods thus far is that they were probably born quite developed (especially compared to say, hadrosaurs or prosauropods which were apparently born helpless and even in some cases unable to walk at birth).Maybe the elongated egg would probably allow for the theropod embryo to grow bigger and develop further than it would if the egg was rounder and smaller, while still being narrow enough for the mother to lay without major difficulties.

Therizinosaurs were very robust animals with large guts to digest vegetation, so maybe this explains the fact that they laid spherical or subspherical eggs; they would need more space to grow a wider ribcage and all that. Same with some large hadrosaurs whose eggs appear to be subspherical as well.

Linnaeus wrote: »

Is the old theory about the raptors stealing eggs still valid? In older books about dinosaurs, you can see illustrations of slender sneaky theropods carrying away eggs in their arms...If they were really set on eating eggs, wouldn't they have done so on the spot? (Unless, of course, Mum and Dad were nearby, ready and willing to defend their nest!);)

Oviraptor has long been declared innocent of all charges, as the original specimen found with a nest was found to be a guarding parent, not an egg-robber. However, there is nothing to suggest that it couldn´t or wouldn´t steal and eat other dinosaur's eggs if opportunity arose. After all, Oviraptor is known to be at least partially carnivorous (a lizard's skeleton was found within its abdominal cavity), and possibly fed its young with meat as well (as the skulls of baby troodontids were found in an Oviraptor nest, although it is not certain how they got there. It's been suggested that the troodontid was a nest parasite similar to cuckoos and that it laid its eggs on the oviraptor's nest so that it would feed its young (or that the baby troodontids would feed on the oviraptor's scraps). Another possibility is that the troodontid skulls were washed into the nest by a flood.
Either way, lots of birds today will happily eat eggs, including crows, vultures, toucans, cuckoos, grackles and many more. In the wild one does not simply say no to such an easy source of protein. Surely small carnivorous dinosaurs such as Oviraptor and Velociraptor would've eaten eggs if they could find them.

May as well mention a personal theory of mine concerning egg robbers. Whereas oviraptors seem to have been varied in their feeding preferences (with Oviraptor being carnivorous or at least omnivorous, and earlier forms such as Caudipteryx and Incisivosaurus being herbivorous), there's a group of Cretaceous theropods whose diet remains a mystery thus far, and those are the alvarezsaurids.

Alvarezsaurids were generally small, fleet-footed, with very small teeth, a long tongue (if the hyoid bones are anything to go by), and very short, robust arms with only one developed claw.
They have been interpreted as insectivores that used their clawed arms to strip the bark off trees or dead logs and feed on insects, woodpecker style. But somehow I don´t think this is a good explanation.

Look at the skeleton of these creatures:







Their arms are so stubby that they would have to hug a tree in order to do any work on it. And if it was attacking termite and ant nests as has also been suggested, again, it would have to be very close to it (would certainly get bitten a lot!).
If this animal was a specialized insect eater, why not develop long, powerful arms with huge claws, like an anteater? We know therizinosaurs developed these traits (albeit probably for other reasons) so its not impossible that such a specialized anteater dinosaur ever evolved.

Personally, I think alvarezsaurs were not specialized insect eaters, but rather specialized egg robbers. Consider this:

- They evolved from larger, more omnivorous ancestors whose remains have been found, and became smaller over time. This may be because, as they became more specialized, food would be more difficult to find and thus, they couldn´t afford to get very large.
- Their remains are often found near the nesting places of other dinosaurs; this is a pattern we see in both Asia and South America.
- Their small arms have strong claws that curve inwards; they may have been great for holding an egg close to the body, safely tucked so that it wouldn´t break if for some reason (angry parent chasing?) it had to flee at high speed.
- They could flee at high speed. They have very long legs and are considered one of the fastest kinds of dinosaurs. Very useful for a hypothetical egg robber.
- Once in a safe place, the alvarezsaur could use its claws to hold the egg while it used its snout and teeth to break it on one end, and then use its long tongue to lick the nutritious fluids inside.

Perhaps most interesting is the fact that sometimes you find two or more species of alvarezsaur coexisting together, but when they do, they come in different sizes.
This would also make sense as the larger alvarezsaurs could feed on larger dinosaur eggs, whereas the smaller ones would raid the nests of smaller species, thus avoiding competition.

I think all of this would explain the alvarezsaur's strange anatomy much better than the insectivorous idea. I even approached a professional paleontologist about this, and he was (apparently) rather uninterested, saying that "there are not so many egg-specialists out there in our times". Which I think is a terrible argument, as today's ecosystems are not dominated by large, ground-dwelling, egg-laying reptiles.
Back in dinosaur times, there would've been a constant supply of food for an egg specialist.

So if I was going to nominate any group of dinosaurs as specialized egg-robbers, it wouldn´t be the oviraptors, but the alvarezsaurs.

Linnaeus

Sorry for my delay in responding to your very interesting post. I have not been well lately.

An intriguing idea, that the Alvarezsaurs could have snatched up other dinosaurs' eggs and run away swiftly with them. Alvarezsaurs were certainly lightweight theropods which were blessed with the gift of speed and, probably, with quick wits. Their legs were sturdy and supple enough to allow for quick escapes. But observing the form of their arms and claws, I have my doubts about their ability to grasp objects without breaking or perforating them. Those arms were short and stubby. Instead of the flexible hands with at least three "fingers" that most raptors and other small theropods had, the Alvarezsauridae had only "clippers" (essentially just one developed claw, as you mentioned, plus another mini claw), with limited grasping power at best. I think that they would have done a clumsy job of trying to grab eggs which might have been laid by dinos considerably larger than themselves. The result may have been a sticky mess: more yolk spilled on the Alvarezsaur's body than sucked up by his mouth.

At any rate, I'm planning to make some putty models of various Alvarezsauridae, in order to experiment with their grasping ability.

I'm not trying to argue against Alvarezsaurus being an egg-eater, however. He may just have pierced the eggs on-site with his vestigial claws, and eaten them right there and then.

I think that if fossilized egg shells were to be more carefully analyzed, their diverse organic elements could be determined and thus it would be easier to determine what kind of dino produced the eggs. If, for example, we found fragmentary remains of egg shells near the skeletal remains of raptors, such a laboratory analysis would demonstrate whether the shells belonged to eggs laid by the raptors themselves, or rather to eggs deposited by other species and possibly stolen by the maniraptors.

Adam Khor

Alternatively, they may have worked in pairs or groups as a team- whereas one of them distracted the parent(s), the others raided the nest. Modern day Nile monitor lizards are known to do this when stealing crocodile eggs.

Sorry to know you've been unwell.