The dinosaur-bird transition is among the most fascinating events in evolutionary history, but several aspects such as changes in reproductive system, nesting strategy, and body temperature are still poorly understood. New research led by Goethe University Frankfurt scientists shows that eggshells of modern reptiles and birds differ in their isotopic compositions. Interestingly, analyses of eggshells of Troodon formosus — a small, bird-like dinosaur that lived in North America during the Late Cretaceous epoch, some 77 million years ago — reveal that it retained a slower, reptile-like mineralization to produce its eggs, despite having already evolved the capacity of changing its body temperature (heterothermic endothermy) like modern birds. The findings also suggest that Troodon formosus possessed two functional ovaries and that their nests were shared by multiple females.
An artist’s impression of two Troodon formosus near their nest. Image credit: Alex Boersma / PNAS.
“Troodon formosus was about 2 m (6.6 feet) long and populated the vast semi-arid landscapes of North America about 75 million years ago,” said lead author Dr. Mattia Tagliavento from Goethe University Frankfurt and colleagues.
“Like some of its dinosaur relatives, it presented some bird-like features like hollow and light bones.”
“Troodon formosus 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.”
“The 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 colored and have been found half buried into the ground, probably allowing the females to sit and brood them.”
In their study, the authors examined the calcium carbonate of several well-preserved Troodon formosus eggshells.
Using a novel method called dual clumped isotope thermometry, 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 the researchers to determine the temperature at which the carbonates crystallized.
They were able to determine that the Troodon formosus eggshells were produced at temperatures of 30 and 42 degrees Celsius (86 and 108 degrees Fahrenheit).
“The isotopic composition of the Troodon formosus eggshells provides evidence that these extinct animals had a body temperature of 42 degrees Celsius, and that they were able to reduce it to about 30 degrees Celsius, like modern birds,” Dr. Tagliavento said.
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 formosus was closer to either birds or reptiles.
They identified 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.
“We think this very high production rate is connected to the fact that birds, unlike reptiles, have a single ovary,” Dr. Tagliavento said.
“Since they can produce just one egg at the time, birds have to do it more rapidly.”
“We did not detect the isotopic composition which is typical for birds.”
“This demonstrates that Troodon formosus 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 authors also combined their results with existing information concerning body and eggshell weight, deducing that Troodon formosus produced only 4 to 6 eggs per reproductive phase.
“This observation is particularly interesting because Troodon formosus nests are usually large, containing up to 24 eggs,” Dr. Tagliavento said.
“We think this is a strong suggestion that females of Troodon formosus laid their eggs in communal nests, a behavior that we observe today among modern ostriches.”
“These are extremely exciting findings,” said Dr. Jens Fiebig, a researcher at Goethe University Frankfurt.
“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.”
The findings were published in the Proceedings of the National Academy of Sciences.
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Mattia Tagliavento et al. 2023. Evidence for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran theropod. PNAS 120 (15): e2213987120; doi: 10.1073/pnas.2213987120
