Two independent studies simultaneously came to shake our world view of the dinosaurs. The increase during geological time, of the maximum size of tetrapods, and therefore dinosaurs , would be related to a combination of environmental factors. Moreover, the oviparity of these large reptiles, forced to lay small eggs, would have caused their loss in the biological crisis of Cretaceous-Tertiary, which occurred approximately 65.5 million years ago, allowing mammals to thrive.
Two questions torment the aficionados and specialists Dinosaurs: why were they so big and sometimes more importantly, why did they disappear ? For mammals, however, have survived! Some species were in fact posing impressive sizes. The seismosaurus measured about 50 meters long, to cite just one example.
Increasing the size of dinosaurs may be related to some multiple environmental factors: an increase in oxygen concentration in the atmosphere (better oxygenation of the muscles) and temperature (metabolic activities faster), but also due to the presence of a large area of land mass, Gondwana. This assumption, however, suffers from the lack of evidence.
Larger mammals can have larger babies, but dinosaurs could not due to the physical limitations of laying eggs. Most dinosaurs were either large or small, but mammals can fill all body size niches in the ecosystem. When a catastrophic event wiped out larger species 65.5 million years ago, mammals were better able to recover. These hypotheses do not explain the survival of mammals. But two studies published in the journal Biology letter this month brings new information to meet, at least partially, to the questions.
Biological factors have limited the size of tetrapods
German-British team led by Roland Sookias tested the links between the increase in the maximum size of tetrapods over geological time and evolution of three environmental factors mentioned above. To do this, femur belonging to nearly 400 species of Permian archosauromorphes who lived during the Cretaceous were measured, the aim being to draw a growth curve characterizing different maximum sizes of representative taxa (archosaurs, pterosaurs, and all groups linked to them). Data were also collected from the literature on mammals that lived during the Cenozoic.
The team used the model to track Geocarbsulf curves characterizing the concentrations of oxygen and carbon dioxide (temperature dependent) in the atmosphere for the same periods. The surfaces of the land masses at different geological epochs were also found in the literature. A regression model of type GLS applied to time series was used to compare all data collected.
Whether for archosauromorpha or for mammals, no correlation was found during analyzes. Environmental factors mentioned above have not conditioned the evolution of the maximum size of tetrapods. It must now be sought in biological factors (growth rate, interspecific competition, etc..).
The dinosaurs were betrayed by their eggs
Daryl Codron and his Swiss colleagues were interested about the extraordinary postnatal growth of the dinosaurs (the release of the egg to adult) and the link combining it to the abundance of species of archosaurs during the Mesozoic period. Their results, obtained using a model taking account of interspecific competition, is surprising. Egg size, and therefore oviparity, could have penalized the dinosaurs, the point of causing their loss at the KT crisis.
Large dinosaurs could not lay eggs too large. Why? Because the thickness of the shell is directly proportional to the size of the egg and that beyond a certain threshold, the embryo is not oxygenated. Titanosaurus adult is 2,500 times heavier than his little as in the elephant Elephas maximus, this ratio is only 25. Large dinosaurs were all so similar dimensions at birth and had to regularly change their niches during growth. At each level, they had to compete, including access to food, with other animals small and medium size.
This has certainly caused over time a significant decrease in the abundance of dinosaur species of small to medium size (weighing between 1 and 1000 kg). Mammals, occasionally emerging with large sizes, and especially fed by their mother, have not experienced this problem. During the crisis the Cretaceous-Tertiary, large archosaurs have disappeared. Since few dinosaur species small or medium size were then present on Earth, they have been supplanted by mammals. Only small species having conquered new ecological niches such as the sky, survived, giving rise to the lineage of birds.