Birds: putting all their eggs in one "basket" for 120 million years

Boy, was this week good for those interested in bird evolution!

Like placental mammals, non-avian dinosaurs had two ovaries. Well, the females did, at least. Each of these ovaries was capable of producing a single egg independently, meaning mother dinosaurs could lay two eggs at a time. This interesting feature, laying eggs in pairs, is known as monoautochronic ovideposition (Grellet-Tinner et. al, 2006), and, interestingly enough, birds are incapable of this, having only one ovary.

The oviraptorosaur Citipati has provided much information on brooding and nesting behavior. Image D shows two eggs in typical monoautochronic position, as they would have been laid. From Grellet-Tiner et. al, 2006.

The evolution of the single ovary in birds is a very interesting, and oft overlooked, adaptation on the runway to flight. Having two ovaries is fine enough if you're a ground-dweller, but if you're a flying bird, the more weight you can lose, the better. It turns out that this feature has been present in avians long before modern birds made the scene. In a new study by the great Zhonge Zhou, two enantiornithes and a specimen of Jeholornis, one of the largest Mesozoic birds, were all found to have a single ovary (Zhou et. al, 2013).

Fossilized ovaries in a Cretaceous bird. From Zhou et. al, 2013.

Not only was it determined that these specimens had a single ovary, but each ovary contained ovarian follicles - structures within the ovary which hold developing eggs. These females were going to lay their eggs very soon! By counting the follicles, the clutch size of each of the specimens was determined - the largest clutch contained 20 eggs, much more than any modern bird is capable of producing. Another interesting discovery was that these specimens were not skeletally mature yet - their skeletons still contained some unfused bones, indicating they had not stopped growing yet.

This raises a few questions. If these birds already had a single ovary, characteristic of modern birds, why were they producing a greater number of smaller eggs at a younger age, a characeristic of non-avian dinosaurs? Clearly, they were not entirely like modern birds yet, still retaining some features present in non-avian coelurosaurs. The answer lies in the metabolism of these animals. Flight requires a huge amount of energy, and as birds evolved to be the masters of the air, they could expend much more energy on other functions, such as reproduction. As these birds were not expert fliers, and were still very early in their evolution, their metabolism reflected that of other non-avians.

The fossilization of internal organs is a rare and amazing phenomenon. In some cases, oddly-shaped rocks are confused for organs; in other cases, it's definitive that something soft was preserved. Because this is so rare, I was a bit unsure to accept the fact that these fossils truly showed ovaries; I assumed that they were gastroliths, which are known in many avian and non-avian dinosaurs, or possibly seeds, which have been found in the fossilized crop of a Sapeornis. Zhou and his colleagues also considered that the fossils may show stones or seeds, but their placement, size, and shape, convinced them otherwise.

Jeholornis sported teeth, clawed hands, and a long, bony tail, but that doesn't make it a lizard. Reconstruction by Matt Martyniuk.

Considering that enantiornithes had clawed hands and Jeholornis sported a long, bony tail and small teeth, it is amazing that they had a feature which is so commonly associated with modern birds. The days of envisioning Mesozoic birds as half-bird/half-lizard monstrosities are long behind us; we have to accept that birds were well on their way to their modern appearance even 120 million years ago.

References
Grellet-Tinner, Gerald, Luis Chiappe, Mark Norell, and David Bottjer. 2006. “Dinosaur Eggs and Nesting Behaviors: A Paleobiological Investigation.” Palaeogeography, Palaeoclimatology, Palaeoecology 232 (2–4) (March 22): 294–321. doi:10.1016/j.palaeo.2005.10.029.

Switek, Brian. 2013. “Exquisite Bird Fossils Reveal Egg-producing Ovary.” Nature (March 17). doi:10.1038/nature.2013.12616. http://www.nature.com/news/exquisite-bird-fossils-reveal-egg-producing-ovary-1.12616.