A second egg found in Darwinopterus

Earlier we wondered if a second egg was present in the Darwinopterus specimen with an expelled egg. Now word arrives that indeed there is indeed a second egg present in the counterplate IVPP V18403 of the specimen ZMNH M8802, but it is not the one that appeared to be present (Fig. 1) in the plate. All three are shown below.

Figure 1. Animated GIF of female gravid Darwinopterus. One immature egg was expelled. The other two are inside. The eggs enlarge as they develop. Click to enlarge. 

Figure 1. Animated GIF of female gravid Darwinopterus. One immature egg was expelled. The other two are inside. The eggs enlarge as they develop. Click to enlarge. Images from Wang et al. 2015.

Note that two recognized egg shapes
are about the same size. An immature and poorly ossified embryo matching the proportions of the mother was traced inside the expelled egg here. The internal eggs did not preserve embryonic remains and all are crushed flat and without eggshell.

From the abstract
“The counterpart of a previously described non-pterodactyloid pterosaur with an egg revealed the presence of a second egg inside the body cavity of this gravid female. It clearly shows that pterosaurs had two functional oviducts and demonstrates that the reduction of one oviduct was not a prerequisite for developing powered flight, at least in this group. Compositional analysis of one egg suggests the lack of a hard external layer of calcium carbonate. Histological sections of one femur lack medullary bone and further demonstrate that this pterosaur reached reproductive maturity before skeletal maturity. This study shows that pterosaurs laid eggs even smaller than previously thought and had a reproductive strategy more similar to basal reptiles than to birds. Whether pterosaurs were highly precocial or needed parental care is still open to debate.”

Despite the mother’s size
there are also no lines of arrested growth (LAGs), Wang et al. report, “In these volant reptiles endosteal resorption is regarded as very extensive (Prondvai et al. 2012) and only the most recent chapter of a pterosaur’s life might be captured by the bones.”

Wang et al. also report, “The two eggs were the same length, but not the same width. Despite the different mass estimation of both eggs, with the one inside the body cavity being about 15.4% lighter than the other, all observable features suggest that they were at a similar developmental stage. This corroborates the idea that this pterosaur had two productive oviducts, which is the most common condition within extant reptiles.”

Wang et al. report, “Furthermore, the eggs of this gravid female were small and probably did not constitute a significant impediment for this animal to fly.” Actually they were similar in size to those of other known pterosaur parent/egg pairs in which the hatchling would have been 12% or one-eighth the size of the adult with adult proportions — just large enough to squeeze through the pelvic opening with that pliable, extremely thin, squamate-like shell.

Eggshell?
In order to test if the eggs of IVPP V18403 had calcium carbonate in the external layer, a fragment of the eggshell was submitted to Energy Dispersive Spectroscopy (EDS) done under SEM (Fig. 4). The analysis revealed no substantial traces of calcium carbonate nor showed any significant composition difference between the matrix and the eggshell. That may be so because eggshell is applied last, just prior to parturition (birth/egg laying). The expelled egg was immature, an abortion. Wang et al. report, “This indicates that the calcium carbonate from the eggshell was either removed during the fossilization process, was resorbed during embryogenesis (Grellet-Tinner et al. 2014), or that none was present at all. Another alternative is that these eggs were not in the calcifying developmental stage when the animal died.”

Hanging, for no apparent reason,
on the hypothesis of a pterosaur/dinosaur relationship, Wang et al. report in the mother there was no evidence of medullary bone and that a medullary layer must therefore be an evolutionary novelty in the dinosaur-bird lineage (probably true!). However, in the large reptile tree pterosaurs nest with lepidosauromorphs, not dinosaurs. So no medullary layer is to be expected and eggshells are expected to be very thin and parchment-like because pterosaurs, like many living lizards, retain the embryo within the mother until just prior to egg hatching.

Wang et al. also reported
(as noted above) “Histological sections of one femur lack medullary bone and further demonstrate that this pterosaur reached reproductive maturity before skeletal maturity.” Not necessarily, unless you expect medullary bone and its absence means lack of skeletal maturity to you. That’s jumping to the wrong conclusion based on ignoring the most complete cladograms available. There is another more logical answer that agrees better with the data (see above).

References
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
Wang X et al. 2015.
Eggshell and Histology Provide Insight on the Life History of a Pterosaur with Two Functional Ovaries. Anais da Academia Brasileira de Ciências (Annals of the Brazilian Academy of Sciences)
Printed version ISSN 0001-3765 / Online version ISSN 1678-2690. http://www.scielo.br/aabc
http://dx.doi.org/10.1590/0001-3765201520150364

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