Which came first, the pterosaur or the egg?

Posted by mjs76 at Jan 21, 2011 12:55 PM |
Leicester palaeontologist’s amazing discovery sheds light on sexual differences and breeding habits of ancient flying reptile.
Which came first, the pterosaur or the egg?

Female and male Darwinopterus (artwork: Mark Witton)

Pterosaurs were prehistoric flying reptiles. They lived at the same time as the dinosaurs, dying out around 65 million years ago during the K/T event. You’ll find pterosaurs in books about dinosaurs and on dinosaur websites but all the good reference material will point out that they weren’t actually dinosaurs.

However, just like dinosaurs, they have left some very cool fossils.

Darwinopterus – on the origin of a species

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Dr David Unwin in Guangzhou

Dr Dave Unwin from our School of Museum Studies is the University of Leicester’s pterosaur expert. Working with his colleague Junchang Lü from the Chinese Academy of Geological Sciences in Beijing, in 2009 he identified an important new speciesDarwinopterus modularis. This was a ‘missing link’ between the primitive ‘rhamphorhynchoid' pterosaurs with small heads and long tails and the more advanced ‘pterodactyloid’ pterosaurs with big heads and short tails. Darwinopterus had a big head and a long tail, providing remarkable evidence for the modular theory of evolution; essentially the idea that parts of the body evolve at different speeds and different times.

That first Darwinopterus fossil was found in the Tiaojishan Formation in China’s Liaoning Province, an area rich in Jurassic fossils. Local farmers supplement their income by selling fossils which they find on their land and it was a farmer who sold one particularly interesting fossil to the Zhejiang Museum of Natural History in Hangzhou. It was a pterosaur. It was Darwinopterus. It was laying an egg.

Laying. An. Egg.

You just can’t imagine how rare such a thing is – or how important.

Like a bird? Like a lizard?

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Darwinopterus hunting Anchiornis (artwork: Mike Witton)

Fossils can tell us about the morphology of prehistoric creatures of course but extrapolating behaviour is a lot more difficult and can only really be done by analogy with broadly similar living creatures.

Pterosaurs didn’t evolve into birds; indeed, some of them did their best to nip avian evolution in the bud by preying on early feathered dinosaurs like Anchiornis (which has also been found in the Tiaojishan Formation). But since pterosaurs and birds both developed flight, it’s not unreasonable to wonder if they may have shared other behaviour too. However, the few pterosaur eggs and baby pterosaurs which have been discovered suggest that perhaps their reproductive behaviour was more like lizards and snakes (which are collectively called squamates*).

Birds lay hard-shelled eggs, then usually incubate them and feed their otherwise helpless (‘altricial’) chicks. Whereas most oviparous squamates lay soft-shelled eggs and then depart, leaving their ‘precocial’ offspring to fend for themselves from Day One. To get an idea what pterosaurs did, we needed a fossil of a pterosaur and a pterosaur egg together.

Introducing Mrs T

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The 'Mrs T' fossil aka ZMNH M8802 (photo: Lü Junchang)

The Zhejiang Museum of Natural History’s new acquisition was given the name ZMNH M8802 but David and Lu christened her ‘Mrs T’ (T for, um, pterosaur). She has a skull about 15cm long and a wingspan of about 78cm. Her skeleton is pretty much complete and relatively undamaged apart from a broken left wing.

Between her legs, external to her body, is an egg. It’s only small – about 28mm long by 20mm across – but it tells us a great deal.

For a start, it tells us that Mrs T was female. Identifying the sex of fossils is largely a mug’s game because sexual characteristics are rarely fossilised. And where there are apparent skeletal difference between the sexes, palaeontologists have to work out whether they are dealing with the male and female of a single species or individuals from two closely related but distinct species.

One thing you would expect in a female animal is a larger pelvic girdle, and indeed the relative size and shape of this has been used to suggest that previous specimens were probably male or probably female. Then along comes Mrs T who is 100% definitely female and her pelvic girdle neatly matches those of ‘female’ Darwinopterus fossils like the Yizhou Museum’s YH-2000 while being considerably different to Zhejiang’s own ZMNH M8782 which is believed to be male. So now we have a clear indicator of what a female Darwinopterus skeleton looks like and, by extension, what a male skeleton looks like. And we can with reasonable confidence extend the model to other pterosaurs.

Vive la difference

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Skulls of male (top) and female Darwinopterus (artwork: Mike Hanson)

At the other end of any pterosaur is another distinctive feature believed to indicate which sex the beast is (or rather, was) – the crest. There are 133 known species of pterosaur and about 40% of them have a bony crest of some shape on top of their head. In fact in some pterodactyloid species these crests achieved quite ludicrous proportions which make you wonder how the poor things were ever able to fly in a cross-wind without breaking their neck. It has long been assumed that these crests served some sort of display purpose (mainly because it’s difficult to see what else you could do with them) and that they were evidence of sexual dimorphism.

Sexual dimorphism simply means that the male and female of a species look different. To give an obvious example, female blackbirds are brown. Whereas every robin looks the same as every other robin. In most sexually dimorphous creatures it’s the male which is larger and more colourful, but there are exceptions; birds of prey for example, which would seem more likely candidates than many other groups to be analogous to pterosaurs.

What palaeontologists have to consider is whether crestless fossils might simply be younger or even a different species entirely. And where incomplete fossils are all we have, there is always the possibility that the thing had a crest but it was snapped off at some point after death.

Nevertheless, Darwinopterus seems to back up the sexual dimorphism theory. Mrs T is a complete, well-preserved skeleton with no crest – and we know she was not only female but sexually mature. YH-2000 is also crestless, whereas the putative male ZMNH M8782 has a crest. This is all very good stuff indeed.

Boiled or scrambled?

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Mrs T's egg (photo: Lü Junchang)

What about the egg itself? What can that tell us? Well, it’s pretty small for the size of the animal. We can calculate that it weighed about 6.1g whereas Mrs T herself weighed somewhere between 110g and 220g. A comparably sized modern bird is the kestrel (wingspan 76cm, female weighs 220g) which produces an egg measuring 39mm by 31mm and weighing about 21g. Not just one egg either, but four or five of the things.

About 8% of the kestrel’s egg is hard shell, which is necessary when eggs have to support the body of a brooding adult, but Mrs T’s egg seems to have a thin, flexible, ‘parchment’ shell like that of a lizard or snake. This allows the egg, when buried, to absorb moisture, increasing the mass over time to an estimated 11g which would give a hatchling of about 4.7g.

All of which points to Darwinopterus (and by extension, until we see evidence to the contrary, probably other pterosaurs) behaving more like modern reptiles than birds: burying their egg(s) in soft matter and then leaving them, with hatchlings having to fend for themselves. So those dodgy B-movies in which huge pterosaurs grab a struggling caveman or explorer and try to feed him to a nest full of hungry babies are, incredibly, even wider of the mark than previously thought...

It’s worth noting in passing that squamates tend to lay quite a lot of eggs whereas Darwinopterus, on this evidence, only laid one per brood. Which is interesting. And not terrifically helpful in times of population crisis. On the other hand, lizards and snakes don’t have to fly around with the things inside them.

RIP Mrs T

Finally, how do we reckon Mrs T met her end? In their paper, published this week in the highly prestigious journal Science, David and Lü suggest that the broken wing is a clue. Unable to fly, the mum-to-be fell into a body of water, drowned and sank to the bottom. Then, as her waterlogged carcass decayed, the egg was forced out of her oviduct by some sort of natural pressure.

A sad end for Mrs T, but a unique opportunity for pterosaur experts to learn more about the habits and behaviour of these amazing prehistoric creatures.

*Today’s word is ‘squamate’. Try to get it into a sentence.