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Dinosaur Eggs

University of Bristol
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Ornithopod nests

The first evidence for nesting colonies came in 1979, when a number of nests were found together (Horner, 1979). Sites included Egg Mountain and three additional sites at Egg Island at the north end of the Willow Creek Anticline (2 km north of Egg Mountain).


Eggs from 'Egg Island' were originally thought to belong to the hypsilophodontid (small gazelle-like herbivore) Orodromeus. Re-examination of embryonic material indicates it actually belongs to the theropod Troodon.


Maiasaura peeblesorum

Three nesting horizons are thought to exist, all in the area of the Willow Creek Anticline, Teton Country, Montana. There is one lower horizon and two upper horizons. In the lower horizon, eight nesting sites were found, two of which contained babies (Horner, 1979). The two upper nesting horizons have also yielded skeletal remains. On all three horizons there are desiccation cracks, indicating that the surface was exposed for some time before being covered with sediment.

The nests generally consist of bowl-shaped excavations about 2 m width and 0.75 m deep. It was conjectured that vegetation was used to cover these nests (Coombs 1989), as abundant plant remains were found around the eggs. The vegetation may have been used to incubate the eggs, fermenting as in a compost heap. This is a technique used today by birds such as modern scrub fowl, which remove and add plants to keep the temperature constant.

The pattern and number of eggs within the nests is unknown as many are broken into eggshell fragments (Horner, 1979). Nests have been reported containing many skeletal remains of maiasaur babies. One nest in the lower horizon has been reported with 11 hatchlings.

The nest also provides information on dinosaur behaviour. The individuals ranged from around 30 cm long (known from femur lengths) to up to 1m long. This indicates the individuals did not leave the nest until they were longer than 1 m (Horner 1979, 1994). They may have stayed in the nest while their parents fed them. When they were more mature they could join the rest of the social group. This is altricial behaviour, seen in some birds today, such as blackbirds.

Evidence for care in the nest comes from tooth wear in the hatchlings, indicating they were fed while in the nest (Horner 1979, 1994). The presence of highly fragmented and crushed eggshells indicates a high degree of use of the nest by the hatchlings.

The nests are roughly spaced 7 m apart, about the length of an adult Maiasaura and has been suggested that this is evidence of a dinosaur nesting colony (Horner, 1982). Maiasaura could move between the nests, watching out for predator activity. The different nesting horizons suggest that the site was used for a number of years.


Below; a reconstruction of Maiasauranests


Below;a reconstruction of a Maiasaurahatchling.


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