Wednesday, 9 September 2015

Pterosaur with 2 eggs?

Understanding reproduction in extinct animals can be extremely difficult as we lack the necessary information of soft tissues. However, there are certain things we can learn from fossils, and particularly well preserved fossils are even better.

3D preserved Hamipterus eggs with soft impressions
showing a soft pliable egg (Wang et al. [4]).
For example, we know that pterosaurs were egg-layers like other reptiles. In a phylogenetic context, pterosaurs are located in between crocodiles and birds in terms of modern animals, which both lay eggs, meaning it was most likely pterosaurs would have too. Thanks to fossils, we know that they did. While pterosaur egg fossils are rare, they do exist. The first pterosaur eggs were found in China[1-2], and Argentina[3] and come from Early Cretaceous pterodactyloids. The eggs have embryos preserved, and the material from Argentina can even be identified as Pterodaustro. Further pterosaur eggs have been found, and we know that pterosaur eggs were softer (much like those found in snakes) rather than hard calcareous shells like those seen in birds, thanks to one of the first finds that suggested pterosaur shells were leathery[2], and some 3D preserved eggs with impressions in them, from the Hamipterus bonebed in China[4].

Kunpengopterus counterslab and drawing showing 2 eggs
in black from Wang et al. [5]
This is a recent study that I saw presented at Flugsaurier 2 weeks ago, where Alex Kellner mentioned it in his talk on pterosaur reproduction. A pterosaur described a few years ago has been recently restudied, and is potentially teaching us a bit more about pterosaur reproduction in a paper published in July. Wang et al. [5] discovered some interesting features in the counterslab of the pterosaur Kunpengopterus. The counterslab is not fantastically preserved, in fact the images make it hard to see things, but what they have pointed out are not one but two eggs present with the individual. One egg is visible just below the pelvis, as if it had been expelled shortly before or after death, while the other is still visible within the body cavity. In close up images, you can see two rounded egg-shaped structures in these areas. The first egg, the one below the pelvis, was identified initially when this specimen was first described. The second one, however, is new. For the first time, it seems that pterosaurs could have 2 eggs in their body at once.
Outline of eggs from A) the pelvic region, and B) inside the body cavity from Wang et al. [5].
So what does that mean? Who cares if they have 2 eggs? Wang et al. [5] further go into the significance of this find. They suggest that this pterosaur was actually a pregnant female, pregnant with 2 eggs when it died. Having 2 eggs at once is a sign of having 2 functioning oviducts, which is interesting because while most extant reptiles have 2, birds have only 1 functioning oviduct. In birds, this has been thought of as relating to decreasing mass for the evolution of flight, especially as non-avian dinosaurs appear to have 2 functioning oviducts as well. The thought was that having only 1 functioning oviduct was essential to decrease mass in order to achieve flight. However, that seems not to be the case if pterosaurs were able to have 2 oviducts, while being the largest animals ever to achieve powered flight.

This new study provides a lot of interesting information about pterosaur reproduction that we previously didn't know, and of course for me, it's interesting when it comes to the evolution of flight. I'm always interested when people suggest that flight couldn't have evolved without things like the reduction of an oviduct. I think this study shows that mass reduction to achieve flight is not as straight forward or "black and white" as we may have previously thought. Different taxa do different things, and what may be valid in birds, is not valid in pterosaurs, and vice versa.

1. Wang and Zhou. 2004. Pterosaur embryo from the Early Cretaceous. Nature 429: 621. 
2. Ji et al. 2004. Pterosaur egg with a leathery shell. Nature 432: 572. 
3. Chiappe et al. 2004. Argentinean unhatched pterosaur fossil. Nature 432: 571-572.
4. Want et al. 2014. Sexually dimorphic tridimensional preserved pterosaurs and their eggs from China. Current Biology 24: 1323-1330.
5. Wang et al. 2015. Eggshell and histology provide insight on the life history of a pterosaur with two functional ovaries. Annals of the Brazilian Academy of Sciences.