In egg-laying amniotes (reptiles, birds and monotremes), the developing embryo
is tethered to a number of the extraembryonic membranes that deliver oxygen
and nutrients and remove metabolic waste products throughout embryonic
development. Prior to, or soon after hatching, these membranes detach from the
animal leaving a temporary or permanent umbilical scar equivalent to the navel
or ‘belly button’ in some placental mammals, including humans. Now,
paleontologists report the oldest preserved umbilical scar in a fossil amniote
from Psittacosaurus, a species of ceratopsian dinosaur that lived some 130
million years ago.
“Amniotes are characterized by the presence of a number of extraembryonic
membranes (allantois, amnion, chorion and yolk sac) and a semipermeable
eggshell that provide a stable environment and nourishment for the
developing embryo,” said University of New England paleontologist Phil Bell
and his colleagues.
“Two of these membranes in particular — the allantois and yolk sac — are
intimately connected to the embryo via vitelline and allantoic blood vessels
that penetrate the abdominal wall and, in placental mammals, are enclosed
within a long umbilical cord.”
“Immediately prior to or soon after hatching, this communication is severed
and the yolk sac is internalized, although the opening in the abdominal wall
may take several days to weeks to fully close, leaving an umbilical scar or
umbilicus.”
“Detachment of the placental mammal umbilical cord after birth results in
the characteristic navel or ‘belly button,’ which is the topographic and
developmental equivalent of the umbilicus in reptiles and birds.”
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| Laser-stimulated fluorescence (LSF) image of the whole Psittacosaurus specimen showing the location of the umbilical scar. Insets show the umbilical scar close-up, including the distinctive scales surrounding it (highlighted in blue in the line drawing). Credit: Bell et al. 2022 |
In the study, the paleontologists examined the 125 million-year-old
fossilized skin of Psittacosaurus , a 2-m-long, two-legged plant eater from
the Jehol Group of Liaoning Province, China.
“While it’s common for land animals to have umbilical scars for at least
part of their life, until now, no evidence of a belly button had been found
on any dinosaur,” Dr. Bell said.
“The specimen is a superbly preserved skeleton that was found lying on its
back, entirely covered in fossilized skin.”
“The same specimen made news in 2021, when paleontologists revealed the
appearance of its cloaca — the common opening for the genitals and digestive
tract, which it shares with birds and reptiles.”
“Using the laser-stimulated fluorescence technique, we identified
distinctive scales that surrounded a long umbilical scar in the
Psittacosaurus specimen, similar to certain living lizards and crocodiles,”
said Dr. Michael Pittman, a researcher at the Chinese University of Hong
Kong.
“We call this kind of scar a belly button, and it is smaller in humans. This
specimen is the first dinosaur fossil to preserve a belly button, which is
due to its exceptional state of preservation.”
The findings appear in the journal BMC Biology.
Reference:
Phil R. Bell et al, Oldest preserved umbilical scar reveals dinosaurs had
'belly buttons', BMC Biology (2022).
DOI: 10.1186/s12915-022-01329-9