Egg-Laying May Have Caused Dinosaur Extinction

An international team of researchers has found why and how egg-laying reproduction may have led to the extinction of dinosaurs.

Tyrannosaurus rex, one of the biggest dinosaurs (Nobu Tamura)

Weighing in at four tons, the mother animal was 2,500 times heavier than its newly hatched dinosaur baby. By way of comparison, a mother elephant, which is just as heavy, only weighs 22 times as much as its new-born calf. In other words, neonates are already big in large mammal species.

After all, larger eggs require a thicker shell and as the embryo also needs to be supplied with oxygen through this shell, eventually neither the shell nor the egg can grow any more. Consequently, newly hatched dinosaur babies cannot be larger in the same way as in larger species of mammal.

In addition, new-born mammals occupy the same ecological niche as their parents. As they are fed with milk directly by the mother, they do not take any niche away from smaller species.

With large dinosaurs, however, it was a different story. They did not only occupy the adults’ one niche during their lifetime, but also had many of their own to pass through – from niches for animals with a body size of a few kilos and those for ten, 100 and 1,000 kg animals to those that were occupied by the fully grown forms of over 30 tons.

“The consensus among researchers is that animals of particular body sizes occupy particular niches,” said study lead author Dr. Daryl Codron of the University of Zurich. “In the case of the dinosaurs, this would mean that a single species occupied the majority of the ecological niches while mammals occupied these through numerous species of different sizes.”

The study, published in the journal Biology Letters, reveals that dinosaurs of a small and medium body size were represented with far fewer individual species than was the case in mammals – because their niches were occupied by the young of larger species.

“An overview of the body sizes of all dinosaur species – including those of birds, which are also dinosaurs after all – reveals that few species existed with adults weighing between two and sixty kilograms,” Dr. Codron explained.

“Firstly, this absence of small and medium-sized species was due to the competition among the dinosaurs; in mammals, there was no such gap,” said Dr. Marcus Clauss of the University of Zurich, a study co-author. “Secondly, in the presence of large dinosaurs and the ubiquitous competition from their young, mammals did not develop large species themselves.”

The third insight that the computer simulation illustrates concerns small dinosaurs. They were in competition both among their own ranks and with small mammals. And this increased pressure brought the small dinosaurs either to the brink of extinction or forced them to conquer new niches. The latter enabled them to guarantee their survival up to the present day, as Dr. Codron concluded, since “back then, they had to take to the air as birds”.

The dinosaurs’ supremacy as the largest land animals remained intact for 150 million years. The mass extinction at the Cretaceous-Tertiary boundary, however, spelled trouble as the species gap in the medium size range turned out to be disastrous for them.

According to the current level of knowledge, all the larger animals with a body weight from approximately ten to 25 kg died out. Mammals had many species below this threshold, from which larger species were able to develop after the calamity and occupy the empty niches again. The dinosaurs, however, lacked the species that would have been able to reoccupy the vacant niches. That was their undoing.