Dinosaur Eggs

University of Bristol

Theropod nests

Two main types of theropod nests are recognised, those of Oviraptortype, and those of Troodon type.

Oviraptor has been found several times in association with eggs. When it was found in 1923 at the Flaming Cliffs (at Bayn Dzak), Mongolia, it was thought that the eggs belonged to another dinosaur, Protoceratops and that Oviraptorwas 'caught in the act' of stealing them (Andrews 1932). Oviraptor philoceratops was named meaning 'egg thief fond of ceratopsian eggs'.

It is somewhat ironic that after the 1993 Joint Mongolian Academy of Sciences/American Museum of Natural History expedition to the Flaming Cliffs, the eggs are now thought to belong to Oviraptor(Dong 1995; Norell 1995).

In the Nemegt Valley of Mongolia, Oviraptor >embryos were found, confirming who laid the eggs.

More evidence comes from Bayan Mandahu, Mongolia, where an Oviraptor skeleton has been revealed lying over a clutch of eggs (Dong 1995; Norell 1995). Perhaps the animal was brooding the eggs, keeping them warm with its own body heat. The nest and brooding mother were then evidently buried in a sandstorm.

Sketch reconstructions of the brooding Oviraptor and its nest

The clutch consists of six visible eggs and broken egg fragments. It is estimated that about 22 eggs were in the nest originally (Norell 1995). The eggs were laid concentrically in pairs and are inclined to the ground at low angles (13-16 degrees) and slope away from the nest centre (Andrews 1923; Dong 1995; Norell1995).

Egg arrangement in an Oviraptor nest.


The eggs and eggshell fragments from the Campanian Two Medicine Formation, previously assigned to Orodromeus, are now assigned to Troodon because of the discovery of Troodon embryos (Varricchio 1997).

Evidence for nesting behaviour comes from eight clutches, where all but two show disturbance (they are broken and/or eroded). In one nest there is a clutch of 24 asymmetrical eggs, standing almost vertically in a shallow bowl-shaped pit with a distinct rim. The rim is approximately 10 cm high, surrounding a 1 m diameter nest (Varicchio 1997). The egg bottoms are situated in a micritic limestone, which indicated the sediment was loose at the time of laying. The eggs are packed tightly together, leaning inwards.

Partial remains have been found in contact with eggs (although very fragmentary) which have been interpretated by some as evidence of brooding. Troodon could have well brooded its young, being only 100 pounds in weight, well below the 300 pound weight of an ostrich which broods its eggs.

Reconstruction of Troodonnest.

There is no evidence for post hatching care (altricial) and embryo bones suggest the animal was mature on hatching (Horner 1988; Stark 1994; Geist 1996), indicating it could leave the nest. This is known as precocial behaviour.

Post-hatching care (precocial behaviour) by Troodon.

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