What can bird eggs tells us about dinosaurs?

Biologists are fond of referring to birds as “living dinosaurs”; they are, after all, descendents of that lineage and so they share many features in common, like being feathered, flight, being warm blooded and laying eggs. And it’s in this latter respect that, as he explains to Chris Smith, David Varricchio, from Montana State University, has been studying the eggs of one group of birds - the big ones, like ostriches and emus, which belong to a group called the palaeognaths - to explore what the eggs of today can tell us about the eggs - and hence behaviours - of the dinosaur era…

David - Among dinosaurs - so I'm talking about non avian dinosaurs, the ones that want extinct 65 million years ago there is a great diversity in egg form, and especially in the micro structure, the microscopic components of the eggshell. Some of them relate to animals like birds today, and then some are just kind of unique. So a lot of what we want to do is try to understand the functional significance of that microscopic eggshell and also the taxonomic. So who do these eggs belong to? Because for many dinosaur eggs or most dinosaur eggs, we can't really tie them to a specific dinosaur.

Chris - Is this in a way, a bit like we, we know that some birds lay eggs that have cone shaped structures so that the egg ends up lying in a crook of a cliff in the right sort of way and not drowning the chick inside. Is it that kind of thing, interpreting how the animal may have behaved and how it would've reproduced based on egg structures and we are using what we can learn from today to try and apply that in retrospect?

David - Exactly, yeah. One thing we look at is porosity. Some dinosaurs had very porous eggs that likely reflect being buried during incubation; but then, diving in further, it's, it, it's to ask, you know, the crystalline structures, these microscopic structures, do they say something more about the nature of the nests?

Chris - And what eggs can you look at to give you these sorts of clues then?

David - The undertaking of the study was to really look at a group, in this case it's the palaeognath birds, the modern birds that fall under that group and to see if there's patterns there that are, you know, that reflect the evolutionary relationships of the birds and also maybe eventually if they reflect different environments or nesting behaviours of these birds. So palaeognaths, there's sort of two major groups of birds. One is very diverse and that's most birds that you might see in your backyard. And then palaeognaths are the ostriches, emus, rheas, kiwis, so these are all flightless birds, typically fairly large. And then there's another group, the tynimos, which is kind of like a quail like group of birds from South America.

Chris - Why did you look at those groups in particular and not the average birds you find flying around in your garden?

David - Well, in part we wanted to be able to look at the more the entirety of the group, so that was one reason. So they're a relatively small group relative to the other branch, the neognath birds, which is, you know, some 9,000 species. So that was too much for us to do. The other idea was that some of these birds are fairly large like ostriches and emus and cassowaries and produce fairly large eggs with thick eggshell and they're flightless. So maybe there's some connection there in a behavioural sense or a nesting sense with dinosaurs.

Chris - So how did you do the study then? Was it a question of going to these birds and obtaining eggs and then just looking at them?

David - We basically ask around for samples from museums or collections or emu ranchers and things like that and get, get our eggshell samples from that way.

Chris - And how did you interrogate the eggs?

David - So we're really looking at the microscopic aspects of the eggshell making thin sections. So you basically break off a piece of eggshell, cut a very thin slice, and to look at the cross section of the eggshell. And then we use a technique called EBSD - electron back scatter diffraction.

Chris - So having considered this complete group in its entirety, because you could, what has that then enabled you to deduce about dinosaurs that we couldn't have inferred from the fossil record that we already had before?

David - That's a good question. I would say <laugh>, we came to the conclusion that it's still challenging. I guess I would say that, you know, within these modern birds, there's sort of three main types of eggshells that we can kind of broadly define. Two of those show up in in some of these carnivorous dinosaurs. And these three types though aren't unique to one sort of specific group within the paleognaths. These styles of eggshell have evolved multiple times within palaeognaths, so that it tells us that eggshell evolution is complex, similar structures are evolving multiple times among different animals. I guess for me that kind of satisfies my dinosaur self in that we do see some diversity from fairly closely related animals back in the fossil record with our dinosaurs. And that kind of says, oh, that that's okay, that's kind of normal. To see those kind of transitions in, in an evolutionary sense, I think it's also useful in a functional sense to say that these animals in the fossil record had basically the same structures as some of these paleognaths.

Chris - It kind of says nature's a wonderful thing in the sense that it tends to find a solution and, and just use that solution multiple times. If it doesn't hang onto it, it just reinvents it again and again. But that's sort of what you're saying, isn't it, that you're getting convergences with what these birds are doing, but also probably what dinosaurs did?

David - Yes, there's a lot of convergences. I, you know, I think at this point it, it sort of sets us up to then ask further questions and to say, can we tease out additional functional or maybe physiological aspects that are reflected in the eggshell? So for example, ostriches, which have a really big egg, have a very similar microstructure with, with big moas from New Zealand. And so you can ask, well, is there some advantage to those, that structure? And we actually see that structure in true odontid dinosaurs back in the Cretaceous too. So in many ways this study is very observational. We're kind of just sampling a bunch of things and trying to see if there's patterns that we can then address.