Geoscience Reference
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wait-until-they're-all-out one. But sometimes evolution works in mysterious ways,
going counter to the expectations of us geologically short-lived primates.
Did dinosaurs ever engage in cuckoo-like behavior, laying their eggs in with
the eggs of other species in a nest, and letting some other species' parents do all of
the work of raising the young? Thus far, we only know of one instance in which
an egg assemblage contained embryonic bones of more than one species of dino-
saur, which was found in Late Cretaceous rocks of Mongolia. The bones showed
that one dinosaur was probably Oviraptor (closely related to Citipati ) and the other
was Byronosaurus , a dinosaur more closely related to Troodon . One would think a
discerningmother Oviraptor wouldhavebeenabletotellwhethershewasbrooding
and feeding a Byronosaurus child. But such is the insidious nature of nest parasit-
ism in modern birds: most bird parents don't know they have a changeling in their
midst until it is too late. However,this one instance ofpossible parasitism is hard to
test further, as eggs also could have easily been transported and deposited together
in the same place by currents. One way to test this hypothesis, though, would be
through trace fossils, such as two distinctive sets of hatchling tracks in a nest. Have
hatchling tracks ever been found in a nest? Not exactly, but they have been found
very close to nest sites, as will be explained later.
Another type of interpretable hatchling trace fossil would be an exit hole
(“hatchingwindow”)inaneggshell,madebyhatchlingsastheyemergedfromtheir
temporary confinement. Sea turtles, for example, have a temporary extension of
their beakswhenborn,whichisapplied like acanopenerfromtheinside oftheegg
to open it. Sea-turtle researchers can thus pick up an empty sea turtle egg from a
previous year and instantly tell whether its former occupant successfully hatched,
died in the egg, oran egg predator got to it before hatching. Similar parts have been
described from Late Cretaceous sauropod embryos in Argentina, so these baby di-
nosaurscouldhavemadesuchtracesineggs.Havehatchlingtracefossilsbeeniden-
tified in dinosaur eggs? Yes, which is amazing when one considers how much more
fragile an egg would have become once abandoned. Indeed, telling the difference
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