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[dinosaur] Australian megapode nesting strategies applied to titanosaurs





Ben Creisler
bcreisler@gmail.com

A new paper:


G. Grellet-Tinner, S. Lindsay and M.B. Thompson (2017)
The biomechanical, chemical and physiological adaptations of the eggs of two Australian megapodes to their nesting strategies and their implications for extinct titanosaur dinosaurs.
Journal of Microscopy (advance online publication)
DOI: 10.1111/jmi.12572
http://onlinelibrary.wiley.com/doi/10.1111/jmi.12572/full




Megapodes are galliform birds endemic to Australasia and unusual among modern birds in that they bury their eggs for incubation in diverse substrates and using various strategies. Alectura lathami and Leipoa ocellata are Australian megapodes that build and nest in mounds of soil and organic matter. Such unusual nesting behaviours have resulted in particular evolutionary adaptations of their eggs and eggshells. We used a combination of scanning electron microscopy, including electron backscatter diffraction and energy-dispersive X-ray spectroscopy, to determine the fine structure of the eggshells and micro-CT scanning to map the structure of pores. We discovered that the surface of the eggshell of A. lathami displays nodes similar to those of extinct titanosaur dinosaurs from Transylvania and Auca Mahuevo egg layer #4. We propose that this pronounced nodular ornamentation is an adaptation to an environment rich in organic acids from their nest mound, protecting the egg surface from chemical etching and leaving the eggshell thickness intact. By contrast, L. ocellata nests in mounds of sand with less organic matter in semiarid environments and has eggshells with weakly defined nodes, like those of extinct titanosaurs from AM L#3 that also lived in a semiarid environment. We suggest the internode spaces in both megapode and titanosaur species act as funnels, which concentrate the condensed water vapour between the nodes. This water funnelling in megapodes through the layer of calcium phosphate reduces the likelihood of bacterial infection by creating a barrier to microbial invasion. In addition, the accessory layer of both species possesses sulphur, which reinforces the calcium phosphate barrier to bacterial and fungal contamination. Like titanosaurs, pores through the eggshell are Y-shaped in both species, but A. lathami displays unique mid-shell connections tangential to the eggshell surface and that connect some adjacent pores, like the eggshells of titanosaur of AM L#4 and Transylvania. The function of these interconnections is not known, but likely helps the diffusion of gases in eggs buried in environments where occlusion of pores is possible.



Lay Description

Megapodes are galliform birds (the group that includes chickens) endemic to Australasia. They are unusual amongst modern birds in that they bury their eggs for incubation in diverse substrates and using various strategies. Alectura lathami and Leipoa ocellata are two Australian megapode species that build and nest in mounds of soil and organic matter in Australia. Such unusual nesting behaviours have resulted in particular evolutionary adaptations of their eggs and eggshells. We used a combination of high-end microscopy to determine the fine structure of the eggshells and, for the first time, micro-CT scanning to map the structure of pores. We discovered that the surface of the eggshell of A. lathami displays a nodular ornamentation similar to those of extinct titanosaur dinosaurs from Transylvania and Argentina that also buried their eggs for incubation. We propose that these eggshell nodes are an adaptation to an environment rich in organic acids from their nest mound, protecting the egg surface from chemical etching and leaving the eggshell thickness intact during the 50–60 day long incubation process. We suggest the internode spaces in both megapode and titanosaur species act as funnels, which concentrate the condensed water vapour between the nodes. This water funnelling in megapodes through the layer of calcium phosphate reduces the likelihood of bacterial infection by creating a barrier to microbial invasion. In addition, the accessory layer of both species possesses sulphur, which reinforces the calcium phosphate barrier to bacterial and fungal contamination. Micro-CT scanning reveal pores through the eggshell that are Y-shaped in both megapode species, like titanosaurs of Transylvania and Argentina Furthermore, A. lathami displays unique mid-shell connections tangential to the eggshell surface and that connect some adjacent pores, also similar the eggshells of titanosaur. The function of these inter-connections is not known, but likely helps the diffusion of gases in eggs buried in environments, where occlusion of pores is possible.

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