Some dinosaur eggs took 3 to 6 months to hatch

How long does it take to hatch the eggs of dinosaurs? The question seems to be unnecessary considering that an answer is already in the title. But it’s interesting to think a bit about it. Despite what many books and articles keep repeating, it’s wrong to think of dinosaurs as they were extinct. They live in the form of birds and – as we all see very clearly – they are pretty successful. And we know that the eggs of birds usually hatch within the first weeks after they are laid.

Now, let’s have a look at what we traditionally mean when we speak of dinosaurs. We would probably expect that the Mesozoic bird relatives didn’t need a long time to break out of their eggshells either. In fact, that’s what the recent studies actually suggested.

In the beginning of August 2016, Scott A. Lee from the University of Toledo published an article in which he attempted to estimate how long might it take to hatch the eggs of non-bird dinosaurs. Lee based his research on what is known about the embryos of birds (the living dinosaurs) and crocodiles (dinosaur closest relatives). He concluded that “the incubation times vary from about 28 days for [the early paravian theropod] Archaeopteryx lithographica to about 76 days for [the large titanosaur sauropod] Alamosaurus sanjuanensis“.

However, a new study published by a team of scientists led by Gregory M. Erickson from the Florida State University suggests that the incubation was much slower.

Erickson and his colleagues studied the incremental lines of von Ebner in teeth of embryonic non-bird dinosaurs. In particular, the researchers studied the teeth belonging to the embryos of the ceratopsian Protoceratops andrewsi from the Upper Cretaceous of Mongolia and the hadrosaurid (duck-billed dinosaur) Hypacrosaurus stebingeri from the Upper Cretaceous of Alberta, Canada.

The lines of von Ebner are growth lines that are forming daily. In order to get the incubation time, we can count these lines in the teeth of near-term embryos. Naturally, it’s not as simple as it sounds. First, we need to have near-term embryos. And these are rare. Second, we need to know the timing when the embryos establish functional teeth.

Fortunately, the timing is well known. As the authors say, in crocodiles, the teeth appear “between 42% and 52% of the total incubation period”. In order to infer the incubation time for Protoceratops and Hypacrosaurus, Erickson and his colleagues followed a more conservative scenario.

If the hatchling teeth began formation at 42% of incubation time, it took the Protoceratops eggs about 83.16 days (almost 3 months) to hatch. The incubation time of the Hypacrosaurus embryos was even longer – about 171.47 days (almost 6 months).

We could speculate that the embryos of larger dinosaurs, such as titanosaur sauropods, needed an even longer time. Nevertheless, any far-reaching conclusions based on these results, such as the potential impact of the long incubation time on the extinction of non-bird dinosaurs at the end of the Cretaceous, would be certainly premature.

Featured image © Sinclair Stammers, Science Photo Library.
Picture of the skeleton of Protoceratops andrewsi by FunkMonk. CC BY 2.0.

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