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

Abstract

The water vapor conductance (G H2O ) of the neosauropod eggs from the Lower Cretaceous Sanagasta nesting site in La Rioja Province, Argentina, was examined and compared with other Cretaceous Argentinean oological material. The 2900 mgH 2 O/day?Torr G H2O of the Sanagasta eggshells confirms an extremely moist nesting environment and supports field observations of dug-out nests in a geothermal setting. The observed thinning of the outer eggshell surface during incubation increases gas conductance and concomitantly decreases eggshell mechanical resistance during the late ontogenetic stages, thus facilitating embryonic development and hatching. The Sanagasta and Entre Ríos Province faveoloolithid eggs display the highest and comparable G H2O values and share several morphological and diagenetic characters, indicating comparable nesting strategy in geothermal settings. However, the faveoloolithid Yaminué and La Pampa Province specimens cluster together with lower G H2O values closer to the megaloolithid eggs. The G H2O of the megaloolithid egg Megaloolithus patagonicus was reconsidered and new results are now congruent with other reported megaloolithid G H2O values. Additionally, we hypothesize that Y-shaped pore canals of M. patagonicus, which upper sections reach only the top third or half eggshell thickness and, a wider section in the middle would not compromise the overall egg mechanical resistance like vertical pores connecting directly the outer to the inner eggshell surfaces. Such pore spatial arrangement and geometry would enhance, as the eggshell thins during incubation, a greater G H2O , G O2 and G CO2 and facilitate embryonic development in high moisture nesting contents. Overall, data suggests that neosauropod nesting and brooding behaviors were dependent on elevated moisture nesting environments.

Micro-CT scan reveals an unexpected high-volume and interconnected pore network in a Cretaceous Sanagasta dinosaur eggshell

“…Although the lateral canal network is well developed in Z 3 -2 , it greatly reduces in Z 1 (electronic supplementary material, S1), where pore canal obstructions should not have represented a drawback because shell thinning leads to a net increase (double) in the shell's conductance in a few tenths of a millimetre ( figure 3c). Therefore, the presence of a highly complex lateral network at the outermost twothirds of the eggshell reinforces previous hypotheses that suggest that the Sanagasta eggs were able to maintain a sufficient gas and water vapour diffusion through their shell during burial incubation even in the event of partial obstruction [1,2,5]. The extremely thick Sanagasta eggshells were well adapted to minimize the effects of chemical erosion and obstruction of pores by possessing the most complex funnel-like pore canal system so far described.…”

“…Although clearly the percentage of shell thinning does not scale isometrically with shell thickness, inner-shell thinning has been observed in the Sanagasta eggshells [5]. Moreover, the geochemical analyses of the Sanagasta nesting sediments coupled with the wide range of eggshell thicknesses [2,21] independently support the hypothesis that the hydrothermal fluids progressively and greatly eroded the outer surface of the 7.9 mm thick eggshell during incubation [2,5,21,22]. This chemical erosion represents the main factor contributing to the shell final thickness of 1.2 mm [2,5,22] that allows an embryo to successfully break through at hatching time.…”

“…The results (electronic supplementary material, S2) indicate that the shell conductance grows only a little through Z 3 -2 compared with the exponential increase experienced when shell thickness reduces below approximately 2.2 mm thick (Z 1 ). To compare our results with previous studies [5], we calculated egg G H2O by using the equation derived from Fick's first law of gas diffusion (see Material and methods). Here, we considered only the outermost tangential section for each thickness, thus assuming single straight pore canals (figure 3c, coloured orange in the online version; electronic supplementary material, S2).…”

“…Despite the difference in egg size, several morphological features of Z 1 are strikingly reminiscent of the Auca Mahuevo titanosaur eggshells [12], including their Y-shaped pore canals (figure 3d). Typically, these eggshells are thinner than 1-1.2 mm at the time embryos hatch because their inner surface thins during embryogenesis [12 -19] and the outer surface is eroded as a direct consequence of their incubation in burial conditions with ambient heat and moisture [1,5,12,20] in contrast with the classic avian contact incubation. In addition, calcium absorption from the inner surface of the eggshell is typically 4-8% during embryonic development in birds, but it is much higher (12 -21%) in megapodes, which bury their eggs [15 -17,19] like the titanosaurs [1].…”

“…Knowing that the Sanagasta egg clutches were deposited in close proximity to hydrothermal structures such as vents, springs and pools [1,2,5,21,22], water and muddy material could have easily obstructed several pores at times [16]. Although the lateral canal network is well developed in Z 3 -2 , it greatly reduces in Z 1 (electronic supplementary material, S1), where pore canal obstructions should not have represented a drawback because shell thinning leads to a net increase (double) in the shell's conductance in a few tenths of a millimetre ( figure 3c).…”