Geoscience Reference
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thenextstepforeitherherorbothparentsshouldhavebeennestbuilding.Although
this supposition might be taken for granted, what is surprising to many people is
that actual trace fossils of dinosaur nests are only known for a few dinosaurs, and
the details of how these were made are still a bit murky. The known nest builders
are:
• The ornithopod
Maiasaura
from the Late Cretaceous in Montana.
• The theropod
Troodon
, also from the Late Cretaceous in Montana.
• Unidentified therizinosaurs from the Late Cretaceous of Mongolia, which
may have even formed nesting colonies.
• Titanosaur sauropods (not yet identified precisely) from the Late Creta-
ceous in Argentina and Spain.
• The prosauropod
Massospondylus
from the Early Jurassic in South Africa.
• The ceratopsians
Psittacosaurus
from the Early Cretaceous in China, and
Protoceratops
from the Late Cretaceous in Mongolia, although their nests
are interpreted more on the basis of many same-sized hatchlings, tightly
packed in a small space.
Otherdinosaursthatwearesuremadenests,eventhoughwedonothavedirect
evidenceofaneststructure,aretheLateCretaceoustheropods
Oviraptor
and
Citip-
ati
fromMongolia.Inafamouscaseofdinosaur-parentingmisattribution,paleonto-
logist Henry Fairfield Osborn, of the American Museum of Natural History, found
an
Oviraptor
skeleton on top of an egg clutch during an expedition to Mongolia in
1923. Nonetheless, he figured the eggs were those of the small ceratopsian
Proto-
ceratops
,whoseboneswereabundantinthearea,too.InOsborn'sscenario,
Ovirap-
tor
died and was buried while raiding the nest of another dinosaur, not while taking
care of its own eggs. This idea was disproved much later, and through a closely re-
lated dinosaur,
Citipati
. In one of many spectacular finds from American Museum
of Natural History expeditions to Mongolia in the 1990s, a nearly complete speci-
men of
Citipati osmolskae
—missing only its head—was found in a sitting position
above a clutch of long oval eggs. Even better, the clutch was oriented radially. This
