In this reconstruction, the duckbill dinosaur in the background has buried its eggs in a covered nest for incubation, while an oviraptorid dinosaur in the foreground incubates its eggs in an open nest. Julius Csotonyi

Researchers studying eggshells may have found a way to answer a longstanding question in dinosaur evolution: Did they bury their eggs like crocodiles or did they nest out in the open like most birds? Both, and you can tell by examining the tiny holes in the eggshells, a property called porosity. The findings were published in PLOS ONE this week.

Archosaurs are a group that includes living birds and crocodilians, as well as pterosaurs and dinosaurs. Since prehistoric nests aren’t well preserved, most of the information we have on extinct archosaurs comes from comparisons and inferences based on living archosaurs who either build covered nests that are incubated with external heat sources or exposed nests that require brooding by the parents. Researchers trying to figure out nest type typically look at different characteristics of the eggs and the nest setting.

For a statistically rigorous approach to predicting nest type, a team led by the University of Calgary’s Kohei Tanaka amassed a dataset of eggshell porosity and egg mass for over 120 living archosaurs and 29 extinct ones. The team found a strong correlation between eggshell porosity and covered or exposed nest types among living archosaurs.

That means eggshell porosity could be used as a proxy for nest type in extinct groups: Those producing high porosity eggshells were more likely to bury their eggs in nests, while animals producing eggshells with low porosity incubated their eggs in open nests

When they analyzed the data on extinct archosaurs, they found that covered nests were more common among more primitive dinosaurs. The transition of advanced theropods (the group that includes giant meat-eaters like the T. rex) from covered to uncovered nests likely allowed them to exploit alternative nesting locations. This shift lowered the odds of nesting failure because of predation, flooding, or torrential rainfall – and it may have contributed to the evolutionary success of maniraptorans, which includes oviraptors and birds.

Image in the text: Dinosaur nest. Kohei Tanaka