An international research team led by Goethe
University Frankfurt, Germany, determined that Troodon, a dinosaur very
close to modern birds, was a warm-blooded animal (an endotherm), but had a
reproductive system similar to that of modern reptiles. The scientists applied
a new method which allowed for accurate determination of the temperature when
the egg's carbonate shell was formed. Furthermore, the researchers showed that Troodon
laid 4 to 6 eggs per clutch. As nests with up to 24 Troodon eggs had been found, the scientists conclude that several Troodon females laid their eggs in
FRANKFURT. In millions of years and with a long sequence of
small changes, evolution has shaped a particular group of dinosaurs, the theropods,
into the birds we watch fly around the planet today. In fact, birds are the
only descendants of dinosaurs which survived the catastrophic extinction 66
million years ago that ended the Cretaceous period.
Troodon was such a theropod. The carnivorous
dinosaur was about two meters long and populated the vast semi-arid landscapes of
North America about 75 million years ago. Like some of its dinosaur relatives, Troodon
presented some bird-like features like hollow and light bones. Troodon walked
on two legs and had fully developed feathery wings, but its relatively large size
precluded it from flying. Instead, it probably ran quite fast and caught its
prey using its strong claws. Troodon
females laid eggs more similar to the asymmetric eggs of modern birds than to
round ones of reptiles, the oldest relatives of all dinosaurs. These eggs were coloured
and have been found half buried into the ground, probably allowing Troodon to sit and brood them.
An international team of scientists led by
Mattia Tagliavento and Jens Fiebig from Goethe University Frankfurt, Germany, has
now examined the calcium carbonate of some well-preserved Troodon eggshells.
The researchers used a method developed by Fiebig's group in 2019 called “dual
clumped isotope thermometry". By using this method, they could measure the
extent to which heavier varieties (isotopes) of oxygen and carbon clump
together in carbonate minerals. The prevalence of isotopic clumping, which is
temperature-dependent, made it possible for scientists to determine the
temperature at which the carbonates crystallized.
When analyzing Troodon eggshells, the
research team was able to determine that the eggshells were produced at temperatures
of 42 and 30 degrees Celsius. Mattia Tagliavento, leading author of the study,
explains: “The isotopic composition of Troodon
eggshells provides evidence that these extinct animals had a body temperature
of 42°C, and that they were able to reduce it to about 30°C, like modern
The scientists then compared isotopic
compositions of eggshells of reptiles (crocodile, alligator, and various
species of turtle) and modern birds (chicken, sparrow, wren, emu, kiwi,
cassowary and ostrich) to understand if Troodon was closer to either
birds or reptiles. They revealed two different isotopic patterns: reptile eggshells
have isotopic compositions matching the temperature of the surrounding
environment. This is in line with these animals being cold-blooded and forming
their eggs slowly. Birds, however, leave a recognizable so-called non-thermal
signature in the isotopic composition, which indicates that eggshell formation
happens very fast. Tagliavento: “We think this very high production rate is connected to the fact that
birds, unlike reptiles, have a single ovary. Since they can produce just one
egg at the time, birds have to do it more rapidly."
When comparing these results to Troodon
eggshells, the researchers did not detect the isotopic composition which is typical
for birds. Tagliavento is convinced: “This demonstrates that Troodon formed its eggs in a way more
comparable to modern reptiles, and it implies that its reproductive system was
still constituted of two ovaries."
The researchers finally combined their
results with existing information concerning body and eggshell weight, deducing
that Troodon produced only 4 to 6 eggs per reproductive phase. “This
observation is particularly interesting because Troodon nests are usually large, containing up to 24 eggs",
Tagliavento explains. “We think this is a strong suggestion that Troodon
females laid their eggs in communal nests, a behaviour that we observe today
among modern ostriches."
These are extremely exciting findings, Jens
Fiebig comments: “Originally, we developed the dual clumped isotope method to accurately
reconstruct Earth's surface temperatures of past geological eras. This study
demonstrates that our method is not limited to temperature reconstruction, it
also presents the opportunity to study how carbonate biomineralization evolved
throughout Earth's history."
Mattia Tagliavento, Amelia J. Davies, Miguel Bernecker,
Philip T. Staudigel, Robin R. Dawson, Martin Dietzel, Katja Goetschl, Weifu
Guo, Anne S. Schulp, François Therrien, Darla K. Zelenitsky, Axel Gerdes,
Wolfgang Müller, Jens Fiebig: Evidence
for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran
theropod. PNAS (2023) https://www.pnas.org/cgi/doi/10.1073/pnas.2213987120
for download: https://www.uni-frankfurt.de/134845598
Artist's impression of two Troodons with a
common nest. Illustration: Alex Boersma/PNAS
Institute of Geosciences, Goethe
University Frankfurt, Germany.
Frankfurt Isotope and Element Research Center (FIERCE), Goethe University
Institute of Applied Geosciences, Graz University of Technology, Austria.
Royal Tyrrell Museum of Palaeontology, Drumheller, Canada.
Department of Geoscience, University of Calgary, Canada.
Naturalis Biodiversity Center, Leiden, the Netherlands.
Deptartment of Earth Sciences, Universiteit Utrecht, the Netherlands
Department of Geosciences, University of Massachusetts, USA.
Morrill Science Center, Amherst, USA
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, USA
Thermometers for Earth history: "Dual
clumped isotope" method for carbonate analysis (2020).
Institute for Geosciences
Dr. Mattia Tagliavento
Phone. +49 176 64735849
Professor Jens Fiebig
Phone: +49 (0) 69 798 40182
and Utrecht University, Leiden/Utrecht, The Netherlands
Professor Anne S. Schulp (English, German,
Phone: +31 6 51229317
Twitter-Handles: @goetheuni @UUGeo @UMass
@UniGraz @WHOI @Naturalis_Sci @RoyalTyrrell @UCalgarySWC