Saturday 3 January 2015

Bird eggs from a Late Cretaceous colonial nesting site in Argentinean Patagonia.

In the 1980s a large collection of Avian eggs were uncovered at the campus of the National University of Comahue at Neuquén City in Argentinean Patagonia. These ages were located on a single bedding plane in the Late Cretaceous Bajo de la Carpa Formation, which has also produced several Crocodyliforms (Neuquensuchus universitas, Notosuchus terrestris, Comahuesuchus brachybuccalis, Cynodontosuchus rothi and Wargosuchus australis) an early Snake (Dinilysia patagonica), two species of non-Avian Theropod Dinosaurs (Alvarezsaurus calvoi and Velocisaurus unicus) and two species of early Birds (Neuquenornis volans and Patagopteryx deferrariisi). Several of the eggs contained embryonic remains, of what appeared to be Enantiornithine Birds, making these eggs the first discovered from the Cretaceous that could be confidently assigned to Birds by the presence of embryos.

In a paper published in the journal PLoS One on 17 April 2014, a team of scientists led by Mariela Fernández of the Instituto de Investigaciones en Biodiversidad y Medioambiente re-examine the Neuquén eggs, and make comparisons to other, subsequently discovered, Cretaceous Birds’ eggs, as well as the eggs of related Dinosaur groups and modern Birds.

The Bajo de la Carpa Formation is a quartz-rich yellow sandstone, with poorly sorted, low sphericity, subangular-to-subrounded grains, which is typical of sandstones produced in arid, windswept environments (sandstones produced in marine or other aquatic environments typically have well sorted, rounded grains). This is interpreted to have been formed in a dry continental interior, however it also has a secondary carbonate cement, which is likely to have formed in a waterlogged environment, and which is taken to be indicative of seasonal streams ot ephemeral standing water.

A total of 65 eggs were found at the National University of Comahue site; their locations at the excavation site were carefully mapped, revealing that the majority were separated from their neighbours by just over one egg length (consistent with the densest modern Avian colonies). The eggs were arranged in a band orientated roughly north-south, that probably extends beyond the excavated area to the North. Almost all of these eggs were buried singularly, with one pair and one group of three being discovered. Almost all of the eggs were preserved in an upright position, with the polar end inserted into the ground, those that were not appeared to have toppled from this position onto their sides. This arrangement strongly suggests that the eggs were preserved where they were laid, rather than secondarily (as might happen if, for example, they had been washed from a nesting site by a flood), which is consistent with the fossil vertebrate remains recovered from the Bajo de la Carpa Formation, almost all of which were preserved as articulated skeletons.

Close-up of site location and in situ egg map. Top: overview of the Universitary campus showing the location of several paleofaunal elements and the gridcorresponding to the nesting colony (shaded purple box). Bottom: grid showing the location of each mapped egg; circles represent upright eggs, ovals represent eggs slightly inclined vertically and the oval that lies with its long axis parallel to the substrate represents anegg found in that position. Fernández  et al. (2014).

Many of the eggs lack their larger pole, suggesting that they hatched successfully prior to burial, though Fernández et al. also note that the position in which they were buried also means this end would have been exposed to any predators or scavengers targeting the eggs would probably also have attacked this end. The shells of the eggs are about 180 mm thick, and the volume of three complete eggs was calculated to be 19.5, 18.77, and 17.18 cm3, respectively. This corresponds roughly to the egg volume and shell thickness of living Plovers, and while it is difficult to make an exact match between egg size and the size of the adult Bird, this suggests that the eggs were more likely to have been laid by the smaller of the two Bird species known from the area, Enantiornithine Neuquenornis volans.

In situ eggs within the Comahue campus. In situ association in bedding plane (a), inset of single egg in position (b), egg half-buried insediment (typical for almost all eggs collected) (c), close up lateral view of same egg showing degree of asymmetry (d). Fernández et al. (2014).

Fernández et al. were also able to estimate the water vapour conductance of the eggs (i.e. the rate at which water would be lost through the shell due to evaporation) as 4.14 mgH₂O/day·Torr. This is within the distribution range of modern Birds, but is very low, suggesting that the Birds were able to nest in very dry, low humidity areas without their eggs drying out due to water loss through evaporation. Studies of a variety of Dinosaur eggs have suggested that many of these had significantly higher water vapour conductance values, suggesting that their eggs would have been more prone to drying out than those of modern Birds or the Bajo de la Carpa Birds, and that they would therefore have needed to incubate their eggs in significantly more humid environments, though within the Dinosauria as a whole, species closely related to the Birds, such as the Troodontid Prismatoolithus levis, had lower water vapour conductance values, within the range of modern Birds, but not so aridity tolerant as the Bajo de la Carpa Birds. Interestingly these Dinosaurs are known to have practiced care of the incubating eggs, in common with Birds, suggesting that these two traits might be connected.

Partial egg with typically fractured but well preserved eggshell. Some Avian bone fragments are visible insidethe egg (arrows). Fernández et al. (2014).

The nesting strategy of the Bajo de la Carpa Birds appears to have been unlike anything seen in modern Birds. The eggs were highly elliptical and apparently mostly laid singularly in simple scrapes, with the pointed end of the egg sticking into the ground. Among modern Birds singular eggs are typically quite rounded, with elliptical eggs being found in Birds that lay their eggs in clutches. This is to do with the way in which Birds incubate the eggs with their own body-heat; a rounded single egg allows for maximum contact with the parental body, while elliptical eggs in clutches enable the parent to maintain contact with a higher number of eggs. The upright position of the egg is also unlike that seen in almost all Birds, and would appear to have prevented the parent turning the egg, which in a modern Bird would greatly raise the egg mortality rate (when Chickens eggs are not turned around 85% fail to hatch, and the chicks that do survive are much weaker than normal). Among modern Birds only a single species, the Megapode, lays its egg vertically and this has a very different strategy to the Bajo de la Carpa Birds, laying clutches of rounded eggs with very porous shells (i.e. shells which allow the exchange of water with the environment and which can only survive in moist places) in piles of rotting vegetation, and relying on the heat generated by the decomposition of the vegetable matter to incubate the eggs. However a similar nesting strategy has been seen in Troodontid Dinosaurs, suggesting that this may have been a trait the earliest Birds shared with their nearest relatives.

See also…

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Birds evolved from Theropod Dinosaur ancestors in the Jurassic, and have a fairly extensive Mesozoic fossil record, with around 120 species described from around the world. The fossil record of Bird’s eggs is...

The two surviving groups of Archosaurs, Crocodilians and Birds, show very different reproductive strategies.  Crocodilians have paired ovaries and produce large clutches of small eggs, which mature slowly prior to being laid, and make a relatively small investment in parental care. Birds have only a single ovary, a...

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