The biomechanical, chemical and physiological adaptations of the eggs of two Australian megapodes to their nesting strategies and their implications for extinct titanosaur dinosaurs

Abstract

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.

Electron backscatter diffraction (EBSD) analysis of maniraptoran eggshells with important implications for microstructural and taphonomic interpretations

“…The overall crystallographic arrangement is consistent in all cases, as reported in fossil and extant maniraptoran eggshells (Figs 1–2; Dalbeck & Cusack ; Trimby & Grellet‐Tinner ; Grellet‐Tinner et al . , , ; Moreno‐Azanza et al . ; Eagle et al .…”

“…The rugged grain boundaries in the squamatic zone of avian eggshells are also observable in other published EBSD images (Dalbeck & Cusack ; Grellet‐Tinner et al . , , ; Dauphin et al . ).…”

Different incubation patterns affect selective antimicrobial properties of the egg interior: experimental evidence from eggs of precocial and altricial birds

“…Considering the high diversity of eggshell microorganisms (Grizard et al, 2014;van Veelen et al, 2018), including pathogens that are able to invade egg contents (Wang et al, 2018), birds have evolved many defence mechanisms to minimize the risk of microbial trans-shell infection (D'Alba and Shawkey, 2015). Among them, variation in the eggshell microstructure (D'Alba et al, 2014;Grellet-Tinner et al, 2017), eggshell pigmentation (Ishikawa et al, 2010), deposition of egg white antimicrobial proteins (AMPs) into the eggshell membranes and cuticle (Bain et al, 2013;Gautron et al, 2011;Wellman-Labadie et al, 2008a), and cuticle nanostructuring (D'Alba et al, 2016(D'Alba et al, , 2017 have been documented to significantly reduce the eggshell microbial load. Among behavioural mechanisms, partial and full incubation of a clutch have been shown to significantly reduce the abundance (Bollinger et al, 2018;Cook et al, 2005a;D'Alba et al, 2010a;Ruiz-De-Castaneda et al, 2011 but also the diversity and community structure of the eggshell microbiota (Grizard et al, 2014(Grizard et al, , 2015Lee et al, 2014).…”

Dinosaurios saurópodos de la Formación Los Llanos (Cretácico, La Rioja, Argentina): ocurrencia, caracterización sistemática y tafonómica, bioestratigrafía, análisis paleoambientales y sus implicancias geológico-regionales

“…Alectura lathami y Leipoa ocellata son dos especies constructoras de montículos pertenecientes a esta curiosa familia de aves, que por regla general incuban sus huevos utilizando fuentes externas de calor. Aunque ambas utilizan montículos, sus estrategias de nidificación presentan diferencias ya que A. lathami incuba en montículos con alta humedad relativa, predominantemente constituidos por hojarasca, mientras que L. ocellata incuba en montículos más secos, compuestos por arena y suelo de zonas desérticas y muy escasa vegetación, de tipo "spinifex" (Grellet-Tinner et al, 2016). A nivel de la microestructura de la cáscara ambas especies presentan poros en forma de Y, pero solo A. lathami cuenta con intercone-…”

“…xiones laterales horizontales simples o dobles entre poros adyacentes (Grellet-Tinner et al, 2016). En base a observaciones recientes, estas interconexiones facilitarían la difusión lateral de gases en el caso de oclusión de los poros en la superficie del huevo.…”