Another Pterodaustro embryo reconstructed

I missed this one last June.

Grellet-Tinner et al. (2014)
report on a new Pterodaustro egg, concentrating on the eggshell morphology and its chemistry while strangely ignoring the contents. Here’s an egg they identify by its proximity to a loose Pterodaustro skeletal element and the egg’s similarity to the earlier pterosaur egg (Chiappe et al. 2004), which includes a Pterodaustro embryo.

And yet, there IS a nicely preserved embryo inside the 2014 egg! 
Here it is (Figs. 1, 2) traced using DGS and after reassembling the color graphic elements. Nearly every bone in the pterosaur is visible, down to the prepubes and wing tip unguals. The cranium, unfortunately, is below the two sets of free fingers, so it is difficult to restore without additional resolution (which I have requested).

Figure 1. Egg image from Grellet-Tinner et al. 2014, DGS tracing of elements reveals a complete Pterodaustro embryo with a small cranium and long rostrum.

Figure 1. Egg image from Grellet-Tinner et al. 2014, DGS tracing of elements reveals a complete Pterodaustro embryo with a small cranium and long rostrum. Click to enlarge. Grellet-Tinner et al. identified this egg not by its contents but by its proximity to Pterodaustro skeletal element and its similarity to the 2004 egg, which includes an embryo.

Earlier we looked at the first Pterodaustro embryo (Fig. 2) and discussed attempts by others at reconstructing the embryo without tracing the bones(!) here. Several years ago Laura Codorniu kindly sent me a high resolution image of the 2004 egg and embryo from which I produced that reconstruction. On agreement with her I have not published online that loaned egg image nor the tracing of it. (You might recall the 2004 egg was illustrated with a line drawing (Fig. 3), not a photo). Here (Fig.1) the egg photo is published (Grellet-Tinner et al. 2014. You’ll note the two reconstructions are essentially identical (Fig. 2) except for the slightly larger size of the 2014 specimen. Evidently it was in the egg a little longer and closer to hatching.

2004 specimen: MIC-V 246 – 6.0 x 2.2 cm, preserved in a concretion in two-dimensions with eggshell remains delimiting the egg outline around the embryo

2014 specimen: MIC-V 778 – 6.0 x 2.2 cm, the first partial three-dimensional egg, same deposit, but the authors note the matrix records offshore lacustrine environments characterized by very fine, laminated siltstones

Figure 2. Pterodaustro embryos compared. Note the 2004 specimen is a little larger with more robust wing finger phalanges and a larger sternal complex.

Figure 2. Pterodaustro embryos compared. Note the 2014 (778) specimen is a little larger with more robust wing finger phalanges and a larger sternal complex. Click to enlarge.

Let’s talk about the embryos first.
The two Pterodaustro embryos (Fig. 2) are essentially identical, but the 2014 specimen is slightly larger with more robust and longer wing phalanges, a more robust proximal tail, more robust free fingers, and a larger sternal complex. These elements suggest it was getting ready to hatch and to fly shortly after hatching.

Both embryos had the proportions of the adult Pterodaustro.
That embryo data supports the hypothesis of isometry during ontogeny found in other pterosaurs, like Zhejiangopterus. The embryos had the basic proportions of the adult.

Unfortunately
this hypothesis has been rejected by all other pterosaur experts that I am aware of, even in the face of incontrovertible evidence such as this. Bennett (1993-2007) is the traditional  proponent of allometry (or great change) during ontogeny, having written several papers on it.

The fact remains
There is no short rostrum in the two Pterodaustro embryos, much as some would like to imagine. Furthermore Phylogenetic analysis nests tiny pterosaurs as miniaturized taxa transitional between large taxa of one morphology and large taxa of another morphology. This happens often in the clade Reptilia and all of its subclades, like the origin of Dinosauria, Aves and Mammalia. It’s not a big deal. It’s common.

Pterodaustro adult with manual digit 3 repaired.

Figure 3. Pterodaustro adult to scale with the 2004 3mbryo and egg.

Now on to eggshells…
Pterosaurs have very thin eggshells. The Asian eggs (Wang 2004, Ji et al. 2004, Lü et al. 2009) were considered ‘leathery.’ By contrast the South American Pterodaustro eggs have a calcium coating, but still extremely thin. Did that calcium coating maintain an elliptical shape? Apparentely so.

Grellet-Tinner et al. 2014 report, “the only pterosaur eggs with embryos obtained from the Yixian Formation in China are soft-shelled, whereas those from Argentina display a well-mineralized calcitic shell. This conundrum may reflect a similar species-specific reproductive strategy observed in modern Geckonids, which lay both hard- and soft-shelled eggs.”

That’s a great explanation for the differences seen in pterosaur eggs.

Grellet-Tinner et al. 2014 report, “The calcareous eggshells of amniotes must allow the exchange of respiratory gases to support embryonic metabolism, while simultaneously regulating flux of water vapor. The thickness of the eggshell and the size and number of the pores, which both determine the conductance of the eggshell to respiratory gases and water vapor, are key features that facilitate oxygen supply to the embryo, allow it to excrete carbon dioxide, and prevent excessive water loss.”

Studies demonstrate
an inverse correlation between eggshell thickness and length of uterine egg retention. So, with such thin eggshells, pterosaur females likely held their eggs in utero, as many lizards do, depositing them shortly before hatching. That hypothesis is also in direct contrast to that of most pterosaur workers (like Grellet-Tinner et al) who continue to consider pterosaurs archosaurs, but are stymied by the lack of a thick calcium carbonate eggshell, like those of birds and crocs.

Eggshell pores
are important features of amniote eggs. They allow the exchange of gases and water between the developing embryo and the environment. Since pores are open to the atmosphere for gas exchange, they are visible on the eggshell surface sometimes without a microscope. However, distinct from archosaur eggshells, Pterodaustro eggshells have no pores, another clue that they were held in utero by their lizard-derived mothers.

Grellet-Tinner et al. 2014 report, “Although the shell of MIC-V 778 is superbly preserved, the scanning electron micrographs do not reveal any pores, for which there could be a number of explanations: (i) pores may not have been evident in a scanning electron micrograph of a radial fracture of the eggshell, which is possible but not likely; (ii) pores were not present or noticeable at the ontogenetic stage at which the egg died; or (iii) all pores were located in the larger pole like in extinct troodontids.”

Unfortunately the G-T assumed that Pterodaustro was an archosaur. But that’s a mistake. Pterosaurs were lepidosaurs. And most, if not all of them retain the egg in utero longer than archosaurs do.

Eggshell thickness
Grellet-Tinner et al. 2014 report, “The thickness of the shell of MIC-V 778 is approximately 50 micrometers, which is less than one third of the thickness expected for the eggshell of an avian egg of similar size (179 micrometers). As eggshell conductance is inversely proportional to eggshell thickness the MIC-V 778 calcareous eggshell would have a conductance more than three times higher than for an avian egg of equivalent size.”

“if considering reptilian incubation (hard-shelled eggs of crocodilians, geckos, and turtles generally do not lose water during incubation), and given the relative egg: female mass ratio of pterosaurs, it is very unlikely that pterosaur eggs would lose water during incubation. Thus, nest humidities would need to be even higher than our calculations suggest.”

Like inside a mother?

Apparently not.
Grellet-Tinner et al. 2014 report, “Such a nesting environment is likely to result in the egg being incubated in a nest with a very high moisture content during some of the incubation time, conditions matching those of grebes or primitive flamingos living in similar  eological/environmental settings. The eggs of P. guinazui and grebes, which incubate their eggs in moist nests, are of similar size and have similar estimated gas conductances that are about three times higher than in other avian eggs of similar size. The difference between these two species is the way in which the higher conductance is achieved. In P. guinazui, high conductance is achieved by having a thin eggshell (short pore length), whereas in grebes high eggshell  conductance is achieved through having a larger number of pores through an eggshell that is as thick as that in other birds.”

Another outer layer on the pterosaur egg.
Grellet-Tinner et al. 2014 report, “Another curious feature of MIC-V 778 is the ubiquitous 2 micrometer outer layer of calcium carbonate, an eggshell feature unknown in reptiles but observed in a few avian taxa, notably Mirandornithes (grebes and flamingos). Termed the “accessory layer or surface crystal layer”, it may function in avian eggs that are incubated in relatively wet or muddy nests to prevent water from occluding the pores through the eggshell and thus prevent a reduction in the eggshell conductance to oxygen.”

Since pterosaurs are not birds, this is a convergent appearance of the surface crystal layer. So at least some reptiles (pterosaurs) had this new layer.

Grellet-Tinner et al. 2014 report, “the sum of all the taphonomic and biological evidence gathered for MIC-V 246 and MIC-V 778 allow us now to reconsider this original hypothesis in favor of incubation in nests built in palustrine ecosystems, where the water could have intermittently permeated in the nests under a semi arid climatic regime.”

Or the egg could have been retained by the mother…

Taphonomic transportation of the egg
Grellet-Tinner et al. 2014 report: “A grey-brown matrix encrusts the egg, but no sediment was present inside the egg when discovered, implying that the egg was complete and not broken at the time of burial. This preservation is most unusual considering the fragility of the specimen and the thinness of its eggshell, so even a short distance of transportation by underwater debris flow seems unlikely.” 

And yet some of the legs bones were broken and other bones displaced (Fig. 1). The tail extends beyond the shell boundaries.

It is also rare
in that locality to find complete Pterodaustro specimens. Most are fragments and loose bones. I know of only one complete specimen. As in the Hamipterus find that includes a nesting colony disassociated bones and associated pterosaur eggs, the possibility must be considered that the eggs found had not been laid yet, but were inside the mother when she was killed, then as the carcass rotted, was transported and buried, the egg became disassociated with the mother and her disassociated bones. I’m no ob-gyn, but something tells me the pterosaur uterus likely also produced a mesohaline environment.

Grellet-Tinner et al. 2014 report, “our results demonstrate that the nesting paleoenvironment of this pterosaur species was closely linked to a mesohaline [salt concentrations between 5 and 18 ppt,] lacustrine ecosystem in a basin governed by regional tectonic subsidence, a setting characteristic for the feeding and reproduction of modern flamingos.”

Or the eggs could have been held in utero.
See what happens when the first domino tells you ‘pterosaurs are archosaurs’?

One final thought
Darren Naish and other detractors are fond of saying that I see things with DGS that aren’t there. This Pterodaustro embryo is evidence to the contrary. All the bones looked like the other Pterodaustro embryo and both looked like those of the adult.

References
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Bennett SC 1993b. Year classes of pterosaurs from the Solnhofen limestone of southern Germany. Journal of Vertebrate Paleontology. 13, 26A.
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen limestone of Germany: year classes of a single large species. Journal of Paleontology 69, 569–580.
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