From “Egg-Laying Beginning of the End for Dinosaurs” (ScienceDaily, Apr. 17, 2012), we learn,
Their reproductive strategy spelled the beginning of the end: The fact that dinosaurs laid eggs put them at a considerable disadvantage compared to viviparous mammals.
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. The staggering difference in size between newly hatched dinosaurs and their parents was down to the fact that there are limits to the size eggs can become: 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 an entirely 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-kilo animals to those that were occupied by the fully grown forms of over 30,000 kilograms.
The big problem, the authors believe, is that small dinosaurs that might have survived the extinction did not exist; they were crowded out by growing tyranno and bronto offspring.
It’s an interesting thesis, making use of both physics and ecology.
One difficulty is that so many life forms still do lay eggs and their offspring grow through the size ranges in their ecology without much extinction risk (none as big as Cretaceous dinosaurs, of course). Alligators and tortoises come to mind. In other words, it may be that general hugeness was a bigger problem for the Cretaceous dinosaur’s ecology than growing through a broad size range.
Interestingly, the discovery of possibly live-birth reptile embryos (mesosaur) is said to push back live birth by 60 million years, to 280 mya.
See also: Ancient seagoing reptiles (plesiosaurs) gave live birth?