Sunday, November 29, 2015

[Paleontology • 2015] Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs


Knowledge about the types of nests built by dinosaurs can provide insight into the evolution of nesting and reproductive behaviors among archosaurs. However, the low preservation potential of their nesting materials and nesting structures means that most information can only be gleaned indirectly through comparison with extant archosaurs. Two general nest types are recognized among living archosaurs: 1) covered nests, in which eggs are incubated while fully covered by nesting material (as in crocodylians and megapodes), and 2) open nests, in which eggs are exposed in the nest and brooded (as in most birds). Previously, dinosaur nest types had been inferred by estimating the water vapor conductance (i.e., diffusive capacity) of their eggs, based on the premise that high conductance corresponds to covered nests and low conductance to open nests. However, a lack of statistical rigor and inconsistencies in this method render its application problematic and its validity questionable. As an alternative we propose a statistically rigorous approach to infer nest type based on large datasets of eggshell porosity and egg mass compiled for over 120 extant archosaur species and 29 archosaur extinct taxa/ootaxa. The presence of a strong correlation between eggshell porosity and nest type among extant archosaurs indicates that eggshell porosity can be used as a proxy for nest type, and thus discriminant analyses can help predict nest type in extinct taxa. Our results suggest that: 1) covered nests are likely the primitive condition for dinosaurs (and probably archosaurs), and 2) open nests first evolved among non-avian theropods more derived than Lourinhanosaurus and were likely widespread in non-avian maniraptorans, well before the appearance of birds. Although taphonomic evidence suggests that basal open nesters (i.e., oviraptorosaurs and troodontids) were potentially the first dinosaurs to brood their clutches, they still partially buried their eggs in sediment. Open nests with fully exposed eggs only became widespread among Euornithes. A potential co-evolution of open nests and brooding behavior among maniraptorans may have freed theropods from the ground-based restrictions inherent to covered nests and allowed the exploitation of alternate nesting locations. These changes in nesting styles and behaviors thus may have played a role in the evolutionary success of maniraptorans (including birds).

The nests of most dinosaurs, including duck-billed hadrosaurs, consisted of eggs covered under mounds of vegetation and dirt. The vegetation mound is not represented in this illustration to display the eggs. (Art by Julius T. Csotonyi.)

A duckbill dinosaur (left) next to its eggs buried in the ground, and a birdlike oviraptorid dinosaur (right) incubating its eggs in an open nest.
Illustration: Julius Csotonyi   DOI: 10.1371/journal.pone.0142829

The nest of a birdlike dinosaur from the Late Cretaceous of China.
photo: Kohei Tanaka

A diagram of an archosaur eggshell with high porosity (A) and low porosity (B). Thin slices of the eggs show Caiman latirostris (C), living open nester Pavo cristatus (D), and nonavian dinosaur Troodon formosus (E).

Fig 7. Evolution of nest types among archosaurs.
(A) Phylogeny of archosaurs with inferred nest types based on eggshell porosity and taphonomic evidence. Covered nests are the primitive condition for dinosaurs; open nests and brooding behavior were present among non-avian maniraptoran theropods but may have first appeared earlier. Although the eggs of early open nesters were still partially covered by substrate, open nests with fully exposed eggs likely arose among Euornithes. (B) Phylogeny of Neornithes with inferred nest types based on eggshell porosity (Emeidae) and literature (other birds). Open nests with fully exposed eggs are the primitive condition for modern birds, although secondary reversal to partial egg burial occurred independently in several clades.

Kohei Tanaka, Darla K. Zelenitsky and François Therrien. 2015. Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs. PLoS ONE.   DOI: 10.1371/journal.pone.0142829

Buried or Open? Ancient Eggshells Reveal Dinosaur Nesting Behaviors via @LiveScience