3D images of a turtle embryo

Turtle eggs containing embryos are exceedingly rare in the fossil record.
Here, we provide the first description and taxonomic identification, to
our knowledge, of a fossilized embryonic turtle preserved in an egg, a
fossil recovered from the Upper Cretaceous Xiaguan Formation of Henan
Province, China. Through Computed Tomography and Three-Dimensional
reconstruction, many bones can be clearly displayed, including the
maxillae, mandibles, ribs, plastral plates, scapula, forelimbs, and hind
limbs. The specimen is attributed to the Nanhsiungchelyidae
(Pan-Trionychia), an extinct group of large terrestrial turtles (possibly
the species Yuchelys nanyangensis). The egg is rigid, spherical, and is
one of the largest and thickest shelled Mesozoic turtle eggs known.
Importantly, this specimen allowed identification of other
nanhsiungchelyid egg clutches and comparison to those of Adocidae, as
Nanhsiungchelyidae and Adocidae form the basal extinct clade Adocusia of
the Pan-Trionychia (includes living soft-shelled turtles). Despite the
differences in habitat adaptations, nanhsiungchelyids (terrestrial) and
adocids (aquatic) shared several reproductive traits, including relatively
thick eggshells, medium size clutches and relatively large eggs, which may
be primitive for trionychoids (including Adocusia and Carrettochelyidae).
The unusually thick calcareous eggshell of nanhsiungchelyids compared to
those of all other turtles (including adocids) may be related to a nesting
style adaptation to an extremely harsh environment.