New research on fossilized dinosaur embryo teeth indicates that non-avian dinosaur eggs (the 'typical' ones) took a long time to hatch: between three and six months. The study, led by scientists from Florida State University, the American Museum of Natural History (both in the US) and the University of Calgary (Canada), was published this week in the journal PNAS.
Its authors have discovered that, contrary to what was thought, the incubation period of those extinct animals was more similar to that of typical reptiles than that of birds. In addition, the work points out that a long incubation could have affected the ability of dinosaurs to compete with populations of birds, other reptiles and mammals, which multiplied more rapidly after the mass extinction event that occurred 65 million years ago.
"We know very little about the embryology of dinosaurs, although that phase is related to many aspects of their development, life history and evolution," says co-author Mark Norell, a paleontologist at the American Museum of Natural History; "But with the help of advanced tools like computed tomography (CT) scanners and high-resolution microscopy, we are discovering advances that we could not even imagine 20 years ago."
As birds are considered living dinosaurs, scientists have assumed that the incubation time of those extinct animals would be similar to that of birds, whose eggs take between 11 and 85 days to hatch. The team tested this theory by looking at the fossilized teeth of two very well-preserved ornithischian dinosaur embryos at either end of the size spectrum.
On the one hand, a Protoceratops, a dinosaur the size of a pig found by Norell and his colleagues in the Gobi desert (Mongolia), with quite small eggs, about 194 grams; and on the other, a dinosaur of the genus Hypacrosaurus found in Alberta (Canada), much larger, with a 'duckbill' and an egg weight of more than 4 kilograms.
Growth lines in teeth like rings in trees
The researchers scanned the embryonic jaws of the two dinosaurs using the CT technique to visualize the forming dentitions. They then used an advanced microscope to analyze - for the first time in dinosaurs - the so-called von Ebner lines, growth lines present in the teeth of all animals, including humans.
"These lines are established as teeth develop," explains Gregory Erickson, lead author and professor at Florida State University. "They are like tree rings, but they are generated daily so we can literally count them to see how long each dinosaur has been developing."
Using this method, the scientists determined that the Protoceratops embryos were about three months old when they died, and the Hypacrosaurus ones about six months old. This places the incubation of dinosaurs more in line with that of their reptilian cousins, whose eggs take twice as long to hatch as that of birds, and can range from several weeks to many months.
The study suggests that the birds likely developed faster incubation rates after they were separated from the rest of the dinosaurs. Although the authors acknowledge that the results could be very different if they could analyze a more bird-like dinosaur, such as the Velociraptor. Unfortunately very few fossilized dinosaur embryos have been found.
"A lot is known about the growth of dinosaurs from youth to adulthood," says co-author Darla Zelenitsky of the University of Calgary, adding: "Time in the egg is a crucial part of development with important biological implications, but it is poorly understood because dinosaur embryos are rare. "
According to the researchers, the study also has implications related to the extinction of dinosaurs. Prolonged incubation exposed dinosaur eggs and the parents who tended to them to predators, starvation, and inclement weather like floods. Furthermore, this slower embryonic development could put them at a disadvantage compared to other animals that survived the great Cretaceous-Paleogene extinction event.
Gregory Erickson et al. "Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development". PNAS, January 2017.