A rule thought central to vertebrate evolution that suggests natural selection tends to make organisms bigger over time has been falsified for a selection of dinosaurs.
Researchers evaluated a range of body sizes of extinct land animals that lived between 180-270 million years ago. They examined more than 400 fossils of therapsids, some of the early ancestors of mammals, and archosauromorphs, which gave rise to crocodiles and birds.
They found there was no “bigger is better” trend, with many of the fossils showing that the overall difference between smallest and largest had increased but without a move towards selecting only the largest members of any species.
Cope’s rule, named after American paleontologist Edward Drinker Cope, states that population lineages tend towards an increase in body mass over time. The fossil record has produced many giants and Cope’s rule was thought to be key to the rise of land vertebrates. However, there have been suggestions that the rule draws a false conclusion from groups of animals that actually see an increase in the range of size, rather than an overall trend towards becoming bigger.
Dr Roger Benson, a post-doctoral researcher at the University of Cambridge, co-authored the research published in Proceedings of the Royal Society. He said: “Various paleontological studies have looked at relatively short time intervals or restricted groups of animals and most of those have found evidence that Cope’s rule is happening. That conflicts with a lot of data based on recent animals.
“We looked at a time interval on land from the late Permian to the middle of the Jurassic. We were interested in that time because it witnessed some of the biggest changes in the distribution of body size among animals that has been seen in the history of the planet.”
Dr Benson explained that, whilst certain lineages of dinosaurs and mammal-like reptiles got bigger through time, this was balanced out by other lineages getting smaller.
“We expect through time that these animals will become increasingly large-bodied and it’s tied in with this idea that there are various advantages to having large body size, especially for animals like terrestrial vertebrates.”
The sky’s the limit
Being big certainly does have its evolutionary advantages. Larger animals have to fight less for territory or mating rights, they often have more access to food and they are likely to have fewer predators. Their large size means they are often more intelligent and better able to regulate their temperature, making them better equipped to adapt to changing environments.
In contrast, staying small can be useful for animals wishing to avoid attention, as well as being an important consideration for those that harbour ambitions of taking to the skies. There is a tendency to imagine dinosaurs as giants but there were many diminutive species such as microceratops, which measured 50 cm in height, and the bambiraptor, which just topped one metre.
However, there is a limit to how small an animal can become, whereas the sky is almost the limit for larger creatures.
The research suggests that the gargantuan sizes achieved by our dinosaur ancestors were down to the development of more efficient cardiovascular systems, rapid growth rates and fast breeding rates, allowing for quicker adaptation.