What Goes On Inside a Pregnant Pterosaur?

Earlier we looked at the Darwinopterus and egg association (Fig. 1). We noted that the embryo inside that egg had virtually the same proportions as the mother, matching the situation of other pterosaur embryos. Now let’s put that egg back inside the mother (Fig. 2) to learn what we can from a pregnant pterosaur.

A Pregnant Pterosaur
Prior to the moments before death, the Darwinopterus egg was within the mother. It was not full term. The embryo was not well ossified nor was it large enough to fill its shell. Here then is that egg placed back within the mother (Fig. 2).

Support from the Prepubes
Acting a pubic extenders, the prepubes of pterosaurs were anchors for important thigh adductor muscles, helping pull those slightly-to-greatly splayed legs inward for a more-or-less erect stance. Inside the prepubes would have supported the intestines, but when an egg was developing inside the mother, even they had to move out of the way (Fig. 2).

The size of the egg compared to the internal portion of the mother devoted to laying it, appears to indicate that there would have been room for only one egg at a time.

Then There’s Pterodaustro
Pterodaustro adults and eggs are known from the same formation, just not in intimate association — yet. When we place the embryo Pterodastro up against the most complete specimen known to date we get a 1:7 size ratio, a little larger than the 1:8 ratio seen in other pterosaurs. This may be due to the fact that the most complete specimen was not the largest specimen so far documented. Half-sized specimens were sexual according to Chinsamy et al. (2008), so we’re at least in the ballpark.

A Hypothetical Pregnant Pterodaustro
When we put the Pterodaustro egg back inside the mother (Fig. 4) we get a similar situation as in Darwinopterus (Fig. 2). There’s room for only one egg and the prepubes help support it.

In fenestrasaurs, including pterosaurs, the anterior pubes separate creating a wider embayment between them. The prepubes bridge that gap, becoming virtual pubic extensions, deepening the abdomen. Could it be that prepubes were as much tied into reproduction as to locomotion?

Prepubes (epipubic bones) on basal mammals are convergent structures that appear to support the pouch but may have other functions.

The only thing I find surprising is the lack of more pregnant female pterosaurs in the fossil record. We might look harder. Ossified embryo wing bones might be about the same size as gastralia. It may be that ossification and shell formation happened only during the last few days prior to laying, with the necessary minerals being leached from the inside walls of the mother’s own store of calcium, her bony skeleton.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Chiappe LM, Codorniú L, Grellet-Tinner G and Rivarola D. 2004. Argentinian unhatched pterosaur fossil. Nature, 432: 571.
Lü J, Unwin DM, Jin X, Liu Y and Ji Q 2009. Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society London B  (DOI 10.1098/rspb.2009.1603.)
Lü J, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011a. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
Lü J, Xu L, Chang H and Zhang X 2011b. A new darwinopterid pterosaur from the Middle Jurassic of Western Liaoning, northeastern China and its ecological implicaitions. Acta Geologica Sinica 85: 507-514.

wiki/Darwinopterus