An exquisitely preserved in-ovo theropod dinosaur embryo sheds light
on avian-like prehatching postures.

Highlights

A Late Cretaceous oviraptorid theropod dinosaur embryo is preserved
in-ovo

Its head lies ventral to the body, and the back curled along the egg's
blunt pole

Its posture is similar to that of a late-stage modern bird embryo

Avian tucking behavior possibly originated among non-avian theropods

Summary

Despite the discovery of many dinosaur eggs and nests over the past
100 years, articulated in-ovo embryos are remarkably rare. Here we
report an exceptionally preserved, articulated oviraptorid embryo
inside an elongatoolithid egg, from the Late Cretaceous Hekou
Formation of southern China. The head lies ventral to the body, with
the feet on either side, and the back curled along the blunt pole of
the egg, in a posture previously unrecognized in a non-avian dinosaur,
but reminiscent of a late-stage modern bird embryo. Comparison to
other late-stage oviraptorid embryos suggests that prehatch
oviraptorids developed avian-like postures late in incubation, which
in modern birds are related to coordinated embryonic movements
associated with tucking — a behavior controlled by the central nervous
system, critical for hatching success. We propose that such
pre-hatching behavior, previously considered unique to birds, may have
originated among non-avian theropods, which can be further
investigated with additional discoveries of embryo fossils.

Open access:
<https://www.cell.com/iscience/fulltext/S2589-0042(21)01487-5>
https://doi.org/10.1016/j.isci.2021.103516