Targeting a dinosaur related to modern-day birds, the tiny predator Troodon was warm-blooded and incubated its eggs in communal nests, but surprisingly retained certain reptilian traits. Surprisingly, paleontologists were able to “read” all this information from his eggshells. They used a method to precisely determine the temperature at which the limestone structures formed. The study sheds further light on the evolutionary processes that led from dinosaurs to birds, the researchers say.
Actually, they are not really extinct, it is said: If you want to marvel at a “dinosaur”, you usually only have to look out of the window, because birds are considered to be the direct descendants of the rulers of the Jurassic and Cretaceous periods. It is now accepted that they evolved over millions of years from a group of dinosaurs known as theropods. The gradual transformation from primordial forms to birds is one of the most intriguing topics in paleontology. There are still unanswered questions about how the changes in reproductive systems, nesting strategies, and transition to the warm-blooded nature of today’s birds took place.
The current study focused on a dinosaur that was already known from earlier studies and that belonged to the group of theropods: Troodon was a carnivore about two meters long that walked on two legs about 75 million years ago through the landscapes of what is now North America. While unable to fly, it possessed some distinctly avian features, such as hollow and light bones and feathered wings. Troodon eggs and nests have also been found. The females were already laying eggs that resembled the asymmetrical versions of modern birds more than the round eggs of reptiles.
Characteristic signatures in fossil eggshells
As part of their study, the researchers led by Mattia Tagliavento from the Goethe University Frankfurt have now examined some well-preserved Troodon eggshells in a special way: they analyzed the calcium carbonate they contained. The researchers used a method previously developed at Goethe University: so-called “dual clumped isotope thermometry”. It is analyzed to what extent heavy element variants (isotopes) of oxygen and carbon are grouped together in the carbonate. As the researchers explain, the extent of this “isotope grouping” is temperature dependent. This made it possible to determine the temperature at which the carbonates in the egg shells crystallized.
As the team reports, the analysis results showed that the carbonates and thus the eggshells were formed at temperatures of 42 and 30 degrees Celsius. This thus underpinned the assumption that these animals were no longer cold-blooded like reptiles, but were already warm-blooded like today’s birds. “The isotopic composition of the eggshells shows that Troodon had a body temperature of 42 degrees Celsius and was able to lower it to around 30 degrees Celsius – similar to what is known from modern birds,” says Tagliavento.
Conclusions about breeding behavior
To further explore the potential of isotopic eggshell analysis, the researchers then conducted comparative studies. They recorded the isotopic composition of the eggshells of various reptile species and birds. As they report, two fundamentally characteristic isotopic signatures emerged: The shells of reptile eggs have an isotopic composition that is apparently typical of slow formation at ambient temperature. In birds, on the other hand, a pattern can be seen that is apparently related to their comparatively rapid eggshell formation. “We think this very high production rate is related to the fact that birds, unlike reptiles, only have one ovary. Because they can only produce one egg at a time, birds have to do it very quickly,” explains Tagliavento.
This turned the questioning gaze back to Troodon: Interestingly, the researchers were not able to determine the isotopic composition typical of birds in the eggshells of these dinosaurs. “This shows that Troodon still formed its eggs in a way more comparable to that of modern reptiles. His reproductive system probably also consisted of two ovaries,” explains Tagliavento. Using the known body and eggshell weights of Troodon, the paleontologists then calculated that these animals could only produce four to six eggs per reproductive cycle.
This result in turn provides an indication of a possible behavior of these animals: “This observation is particularly interesting because Troodon nests are usually large and contain up to 24 eggs,” explains Tagliavento. “We think this is a strong indication that Troodon females laid their eggs in communal nests. We also observe such behavior today in modern ostriches, for example,” says the researcher.
Source: Goethe University Frankfurt am Main, original publication: PNAS, doi: 10.1073/pnas.2213987120