Several egg clutches (up to 35 eggs each) of sauropods were found on the site, counting with both complete and fragmented eggs. All the egg clutches were found in dig-out holes and were within 1–3 m of geothermal structures such as mounds, terraces, and geysers. The eggs are roughly spherical, with a diameter of circa 21 cm; the eggshells can be as thick as 7.9 mm up to as thin as 1.3 mm. And this variation in shell thickness can be seen on the same egg, with the thinnest portions usually being concentrated on a single region of the egg.
When all these surprising facts are taken together (the really thick eggshells, the huge variation in eggshell thickness and the hydrothermal setting), it can be seen that something rather unique was going on. This led us to study these eggs, investigating their functional morphology and their relationship to the environment.
The structural and functional properties of the eggshell are extremely important in determining the incubating and hatching success. Eggshell thickness determines the amount of mechanical protection against external forces and, together with the numerous pores that cross the eggshell, regulates the amount of gas exchange during incubation. Gas diffusion through the pores occurs according to the laws of diffusion and can be quantified as the eggshell conductance to water vapor (or water vapor conductance).
Moreover, the choice of a good nesting site is critical, as the parents cannot compensate post-hatching for a poor choice of
nesting environment. This maternal selectivity also affects hatching success and developmental rate. Nest moisture content and heat are the two environmental parameters most vital for hatching success.
Water vapor conductance
Water vapor conductance (GH2O) is commonly obtained for modern birds and reptiles by measuring live eggs in laboratory. In fossil dinosaur eggs, GH2O can be estimated from eggshell and pore system measurements. GH2O values can then be used to assess the moisture content in dinosaur nests and their environments, as well as the dinosaurs’ nesting strategies.
In order to obtain the water vapor conductance, the following parameters are required, obtained by simple measurement: egg radius, eggshell thickness (Ls), pore density, and pore diameter. All the other parameters can be obtained by simple equations (see the Table below): egg surface area, number of pores per egg, pore area, and total pore area per egg (Ap). Finally, the he water vapor conductance is calculated with the following formula: GH2O = Ap / (0.478·Ls). This equation is derived from Fick’s first law of gas diffusion across a porous barrier. The constant in the formula is linked to a 25 ºC nest temperature (commonly noted in extant reptiles).
The table above shows the mean value for Sanagasta eggs, but it should be noted that the thicker eggshells have the lowest GH2O values, while the thinner ones have the highest GH2O. The meaning of this will be explored in the section below.
The hot springs nesting resort
The sauropod colony chose Sanagasta as a nesting site specifically because it was a hydrothermal setting. Similar strategies are known from extant animals: the Pacific islands’ megapode birds and the Galapagos iguanas. This strategy avoids thermally heterogeneous nesting environments and helps maintain constant temperature and moisture content in egg clutches. However, geothermal waters and vapors may release acidic vapors that “corrode” the eggshell during incubation. This apparent hazard, however, was put to good use by Sanagasta’s sauropods.
The outer eggshell surfaces of their eggs were continuously “corroded”, resulting in increasingly thin eggshells. As this thinning process occurred, the GH2O consequently increased (remember how the GH2O formula works). Such a process was beneficial for the embryonic growth by maintaining an optimal gas exchange and water loss and concurrently facilitated the hatching process by allowing the hatching embryos to easily break through a thinner and more fragile material. Moreover, as lungs develop in the embryo the need for O2 increases, but, concomitantly, this gas becomes increasingly depleted in dug-out nests. A thinner shell with increased conductance values, as observed at Sanagasta, would have compensated and facilitated the exchange of respiratory gases during the late developmental (ontogenetic) stages.
As such we regard the presence of extremely thick eggshells in geothermal environments as a natural reproductive adaptation and a “symbiotic” relationship between the dinosaur parent lineage and this specific environment.
The preceding text is a summary of the following paper, where this research was published: GRELLET-TINNER, G.; FIORELLI, L.E.; SALVADOR, R.B. 2012. Water vapor conductance of the Lower Cretaceous dinosaurian eggs from Sanagasta, La Rioja, Argentina – paleobiological and paleoecological implications for South American faveoloolithid and megaloolithid eggs. Palaios 27: 35-47. [PDF]
A further interesting paper on Sanagasta’s dinosaurs is: Grellet-Tinner, G. & Fiorelli, L.E. 2010. A new Argentinean nesting site showing neosauropod dinosaur reproduction in a Cretaceous hydrothermal environment: Nature Communications 1: 32, doi: 0.1038/ncomms1031