Shifts in eggshell thickness are related to changes in locomotor ecology in dinosaurs

Abstract

Birds share an array of unique characteristics among extant land vertebrates. Among these, external and microstructural characteristics of extant bird eggs have been linked to changes in reproductive strategy that arose among non‐avian theropod dinosaurs. More recently, differences in egg proportions recovered in crown birds relative to other dinosaurs were suggested as possibly linked to avian flight, but dense sampling close to its proposed origin was lacking. Here we assess the evolution of eggshell thickness in a targeted sample of 114 dinosaurs including birds, and test the relationship of eggshell thickness with potential life history correlates and locomotor mode using phylogenetic comparative methods. Only egg mass and flight are identified as significant predictors of eggshell thickness. While a high correlation between egg mass and eggshell thickness is expected, that relationship is much stronger in flying taxa, which show a significantly higher slope and lower residual variance than flightless species. This suggests stabilizing selection of eggshell thickness among theropods, as recovered for other traits in extant birds (e.g. genome size, metabolic rate). Within living birds, Eufalconimorphae present an apomorphic increase in relative eggshell thickness which remains unexplained, as few morphological synapomorphies of this clade have been identified.

Ancient proteins resolve controversy over the identity ofGenyorniseggshell

“…slightly thicker than that of mid-sized Paleognathae (e.g. emu, rheas, cassowaries - (23)(24)(25)). Using eggshell fragment curvature and a range of equations relating body mass (estimated from skeletal remains of Genyornis ) to egg size, the eggs' dimensions can be estimated as ~125x155 mm (11,19) or ~97x125 mm (17,18).…”

“…Phylogenetic comparative methods for eggshell thickness scaling A dataset for eggshell thickness (µm) and egg mass (g) was compiled for 602 species of extant and subfossil birds from Maurer et al ( 75), Juang et al (76), and Legendre and Clarke (25). Two additional megapode species ( Alectura lathami and Leipoa ocellata ) were sampled from Grellet-Tinner et al (30).…”

“…Two additional megapode species ( Alectura lathami and Leipoa ocellata ) were sampled from Grellet-Tinner et al (30). Egg mass was estimated from egg length and width using the equation of Hoyt ((77); see also (25)). Both traits were log-converted (common logarithm (78)) prior to analysis.…”

Microstructural and crystallographic evolution of palaeognath (Aves) eggshells

“…However, the Palaeogene palaeognaths Lithornithidae might have been a fully volant clade (Torres et al, 2020;Widrig and Field, 2022). Proportional to their body size, absolute sizes of eggs and eggshells of Palaeognathae are usually large and thick, respectively (Grellet-Tinner, 2006;Birchard and Deeming, 2009;Legendre and Clarke, 2021).…”

“…Species diversity of Palaeognathae is much lower than that of Neognathae (Prum et al, 2015) but it is critical for avian egg research; due to their lower diversity, it is feasible to investigate the egg features of all major clades of Palaeognathae. More importantly, considering that most Palaeognathae are flightless and that flight influences egg mass in Dinosauria (Legendre and Clarke, 2021), which influences eggshell thickness (Ar et al, 1979; Legendre and Clarke, 2021), the eggs and eggshells of Palaeognathae might be more appropriate modern analogues for those of flightless non-avian dinosaurs than those of volant Neognathae.…”