The Loma del Pterodaustro lake deposits of Central Argentina have produced large numbers of the Pterosaur Pterodaustro guinazui, which is interpreted as being a filter feeder with a lifestyle similar to modern Flamingos, with jaws equipped with specialist filter feeding equipment and living in large colonies in an environment effectively closed to most other vertebrates. As well as adult specimens, these deposits have produced a number of young juveniles and a single eggs specimen, crushed, but with a preserved embryo inside. To date Pterosaur eggs have only been described from one other site, the Early Cretaceous Yixian Formation of China.
In a paper published in the journal Geoscience Frontiers on 7 June 2014, Gerald Grellet-Tinner of the Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja and OrcasIsland Historical Museums, Michael Thompson of the School of BiologicalSciences at the University of Sydney, Lucas Fiorelli of the the Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja and the Consejo Nacional de Investigaciones Científicas y Técnicas, Eloísa Argañaraz of the Centro deInvestigaciones Paleobiológicas at the Universidad Nacional de Córdoba, LauraCodorniú of the Consejo Nacional de Investigaciones Científicas y Técnicas and the Departamento de Geología at the Universidad Nacional de San Luis, and Martín Hechenleitner of the the Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja and the Consejo Nacional de Investigaciones Científicas y Técnicas, describe a second preserved Pterosaur egg from the Loma del Pterodaustro lake deposits.
This second egg does not contain an embryo, but by comparison with the first egg suggests that has the same dimensions, and since the Loma del Pterodaustro deposits have never produced any fossils other than Pterodaustro guinazui, Grellet-Tinner et al. feel confident in assigning this specimen to the same species.
Comparison between the new egg specimen and the previously discovered, flattened specimen. (a) Photo of the new 3-dimensional egg in the original claystone matrix with its contouroutlined. (b) Pterodaustro guinazui embryo with the preserved eggshell outlined. (c) The outline of the pointed section of the first egg brought to scale and properly orientated matches perfectly the 3 dimensional mold and egg. (d) Traced contour of the original eggshell tofacilitate comparison between the 2 specimens. Grellet-Tinner et al. (2014).
The new specimen comprises a polar region of the egg, plus adjacent eggshell, measuring 25 mm in width and 30 mm in length. From this Grellet-Tinner et al. calculate that the egg would originally have had a volume of 14.7 cm3, and a mass of 15.46 g. The shell of the egg is preserved without apparent alteration, and is made of calcium carbonate and shows three separate shell layers, with a total thickness of about 50 mm. Layer 1 (the innermost layer, and the one that would have been laid down first) is about 30 mm thick, and is comprised of horizontal tabular calcitic crystals. Layer 2 (the middle layer) is formed of shorter, narrower columns of tabular crystals, and is perforated by numerous vesicles, averaging 2 mm in width. Layer 3 (the outermost layer) is only 2 mm thick, and is composed of amorphous material.
Entire cross-section of the new eggshell with possible fossilized membranatestacea mixed with clay minerals at its base. The eggshell thickness averages 50 mm. Grellet-Tinner et al. (2014).
The shell of the new egg is considerably thinner than would be expected in a Bird’s egg of similar size, which would be predicted to be 179 mm thick. No pores could be found in the shell, which implies that either it had none, that they are all missing from the eggshell fragments examined in a scanning electron microscope by pure change (which is judged to be extremely unlikely) or that they were all concentrated at a the missing pole of the egg (which seems unlikely, but is known to have been the case in Troodontid Dinosaurs, and therefore cannot be ruled out).
Pores are important in a mineralized eggshell, as they help oxygen to reach the embryo and carbon dioxide to escape, while at the same time regulating the loss of water from the shell. Grellet-Tinner et al. calculate from the size of the egg that if the embryo had developed at the same speed as that of a Bird, then it would have hatched 21.3 days after it was laid, and during this time could afford to have lost no more than 14% of the water stored inside to evaporation. Due to the thin shell of the egg it would have been expected to lose water at a rate of 12.8 mg/d.Torr (expand), which means that had it been incubated at a temperature of temperature of 30°C (typical for Crocodilians and Turtles), then the nest would have needed a minimum vapour pressure of 23.9 Torr in order to retain sufficient water to survive, while had it been incubated at a temperature of 36°C (typical for modern Birds), then the nest would have needed a minimum vapour pressure of 36.5Torr (these are minimum values, and it is likely the humidity in the nest would have been higher than this, to avoid fatal moisture fluctuations).
Most modern Birds lay their eggs in dry nests, however Grebes (diving waterfowl closely related to Flamingos) incubate their eggs in moist nests, and have similar estimated gas conductance’s to the Loma del Pterodaustro egg (though they do have pores), and the preserved nest of an early Flamingo from the Miocene of Bardenas in Spain is also thought to have been moist. Mirandornithes (Grebes and Flamingos) also have an outer layer of amorphous calcium carbonate around their eggs, something very similar to the outer layer of Loma del Pterodaustro egg, but not seen in any other Bird. This suggests that the analogy between the lifestyle of the Pterosaur Pterodaustro guinazui and Flamingos may also be good for its reproductive behaviour.
Interestingly the preserved Pterosaur eggs of the Yixian Formation of China do not appear to have been mineralized at all, but leathery like the eggs of many modern reptiles. This is particularly puzzling as the Yixian Pterosaurs are thought to have been fairly closely related to Pterodaustro guinazui. While this is problematic if Birds are used as modern analogies for the lifestyles of Pterosaurs (all Birds produce mineralized eggs), modern Geckos can produce mineralized and non-mineralized eggs, often in quite closely related species living in different environments. Furthermore the mineralized eggs of modern Geckos (and of fossil Gecko eggs from the Cretaceous of from France) typically produce shells about 30 mm thick, also comparable to the Loma del Pterodaustro egg.
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