National History Museum

Dinosaurs' Small Eggs Their Achilles Heel

A number of changes at the transition from the Cretaceous to the Tertiary led to an enormous mass extinction that terminated the era of the dinosaurs – even if dinosaurs survived as birds to our times. Why did mammals recover so successfully from this event, whereas non-avian dinosaurs such as Tyrannosaurus or Apatosaurus did not? Dr Daryl Codron, recently appointed Honorary Research Associate of the National Museum and hosted at the Florisbad Quaternary Research Department, in collaboration with scientists at the University of Zürich, Switzerland, and the Zoological Society of London, showedthat the different reproductive strategies of dinosaurs and mammals may be responsible – a fundamental difference in constraints to oviparous (egg-laying) and viviparous (giving birth to live young) animals.


The explanation, published in Biology Letters, is based on the concept that animals of a certain body size occupy a certain niche. The main difference between dinosaurs and mammals is that whereas newborn mammals are relatively large in large species, and indirectly via milk use the same niche as their mothers, dinosaur hatchlings cannot increase in size in proportion to the size of their parents. Eggs cannot increase in size, because larger eggs need thicker shells, and shell thickness is constrained by the need of the embryo for oxygen that has to diffuse through that shell. Therefore, the size difference between dinosaur hatchlings and their parents is enormous – a 4 ton elephant mother is ‘only’ about 22 times the mass of her newborn, whereas a 4 ton dinosaur female was about 2500 times the mass of her hatchling!


Therefore, large dinosaur species did not only occupy one niche during their lifetimes, but had to go through many different niches – for 1 kg, 10 kg, 100 kg, 1000 kg up to 30000 kg and more. One single dinosaur species would thus have occupied a series of niches that would be filled by many different mammal species. The researchers argue that therefore much fewer small and intermediate-sized dinosaur species could have co-existed as compared to mammal species. Actually, when collating body size data from fossil assemblages of avian and non-avian dinosaurs, there is a body size gap at about 2-60 kg that contains fewer species than expected. The researchers used a series of computer simulations to show that competition from larger species leads to this gap in dinosaurs, whereas no such effect is found in mammals. The simulations also show that in the presence of dinosaurs, mammals were held to small body sizes because of competition from juveniles of large dinosaur species. In addition to this, the simulations showed that competition amongst the smallest (<2 kg) dinosaurs, coupled with competition from small mammals, could have forced many small dinosaur species to extinction, or forced them to occupy an entirely new niche space (the aerial niches of birds).


The body size gap did not pose a problem for the terrestrial predominance of dinosaurs for 150 million years. It was only the catastrophe at the end of the Cretaceous, during which animals above a body size threshold of about 10-25 kg went extinct, that led to the dinosaurs’ demise. Mammals had many species below that threshold from which new species could evolve to occupy the now wide-open niches for larger animals. But because of the body size gap, dinosaurs simply had too few species with which to enter this new phase of the competition race.


Image: Jeanne Peter, University of Zurich,Switzerland


Further Reading:

Codron D, Carbone C, Müller DWH, Clauss M (2012) Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates. Biology Letters DOI:10.1098/rsbl.2012.0240