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How giant oviraptorosaurs sat on their nests

Ben Creisler

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How giant oviraptorosaurs sat on their nests without crushing eggs





TANAKA, Kohei; ZELENITSKY, Darla; LÜ, Junchang; YI, Laiping; PU,
Hanyong; CHANG; XU, Li; LI, Hua.

In the past 20 years, the abundance of well-preserved specimens of
oviraptorosaur eggs and nests has shed light on their nest types and
incubation behaviors. Although large oviraptorosaur species have yet
to be found associated with their nests, prior discoveries of smaller
species (Oviraptoridae) of adults in brood-like positions atop egg
clutches suggest that these dinosaurs sat on their nests during
incubation. This is in contrast to the results of water vapor
conductance studies of oviraptorosaur eggs, which have suggested that
incubation occurred in covered nests, like extant crocodilians or
megapodes, rather than in open nests with a brooding adult as in most
birds. Here we reexamine oviraptorosaur eggs in order to resolve this
discrepancy in their nest types and associated incubation behaviors
and to determine how these features are affected by changes in body
size. Eggshell porosity, shell strength, and clutch morphology were
examined for eggs and nests of small (Elongatoolithus and
Macroolithus; egg mass 200-840 g) and large forms
(Macroelongatoolithus; egg mass 2960-6590 g) recovered primarily from
Cretaceous deposits of Jiangxi and Henan provinces in China. Adult
body mass for these egg sizes was estimated using clutch volume
(clutch size × egg volume) and shows a range of 60 to 4000 kg.
Estimations of the eggshell porosity from all egg sizes were
consistent with the eggs of birds that brood in open nests. This
contrasts with the results of prior water vapor conductance studies,
which have suggested incubation in covered nests. Examination of
clutch morphology revealed that all oviraptorosaur clutches are laid
in a circular fashion with the long axis of the eggs arranged radially
relative to the center of the nest. There is a positive allometric
change in clutch morphology in that small eggs (Elongatoolithus and
Macroolithus) are packed with a small central open space, whereas
large eggs (Macroelongatoolithus) are arranged in a large ring with a
large central space. Estimations of egg strength from shell thickness
indicated that unlike smaller eggs, the shell of large eggs
(Macroelongatoolithus) may not have been strong enough to directly
withstand the adult body mass. These results indicate that
oviraptorosaurs incubated their eggs in open nests regardless of adult
size, and that small oviraptorosaurs could have sat atop packed eggs,
whereas large species may have distributed most of their weight across
the large central opening while on the nest.