Understanding Dinosaur Parenting Through Nest and Egg Fossils

It has long been known that dinosaurs laid eggs, but the discovery of adult dinosaurs sitting on nests, eggs arranged in a circle, repeated nests in the same place, and embryos and juvenile specimens has allowed for much more detailed research on reproductive behavior. Notably, the sitting posture of Oviraptoridae shows that avian parenting behaviors did not suddenly appear, but were partially developed during the non-avian dinosaur stage.

However, it cannot be said that all dinosaurs took care of their young for a long time. Large nesting sites indicate choices of breeding grounds, but may not provide evidence of care after hatching, and even when adults and juveniles are buried together, it is necessary to verify whether they were a family or a coincidental burial. This overview distinguishes what egg arrangement, shell porosity, embryo development, and adult postures signify, and what remains unknown. The distance between nests and the direction the eggs were laid also hint at how adults approached and moved around.

Ten examples show that there was not one single reproductive strategy among dinosaurs. Some species buried their eggs in sediment, while others likely exposed them and covered them with their bodies. There were also species that utilized large nesting sites collectively, and species that interpreted juveniles remaining in the nest after hatching as a form of care. The size of the eggs, their pores, the spacing between nests, and the development of juvenile limbs suggest varying costs of parental care.

The familiar word 'parenting' can too easily add emotion to fossils, so let's break down evidence of behavior. Sitting postures, nest defense, and providing food after hatching are separate behaviors, each requiring distinct evidence. Modern birds and crocodiles are good comparison subjects, but they do not provide a direct answer to the behaviors of past dinosaurs. Keeping multiple possibilities open is the best way to faithfully interpret fossils. The soil and temperature of nesting sites may have influenced not only egg development but also behaviors that caused returning to the same location.