359 million years ago there was a mass extinction – one of the five great extinctions that have occurred on the planet – known as the Hangenberg event, which ended the Devonian period. At that time, the cooling of the climate made glaciers grow, reaching tropical latitudes. One of the consequences of this phenomenon was that the level of the sea was greatly reduced and large areas of water around the continents disappeared . With this, 96% of the vertebrate species disappeared.
Before this happened, the norm in nature was that large creatures dominated ecosystems, but after extinction, and for 40 million years, small fish – no bigger than today’s sardines – took over the control of the oceans. According to this finding revealed by a study by Lauren Sallan and Andrew Galimberti of the University of Pennsylvania, small fish would have an adaptive advantage over larger animals in difficult times that followed extinction.
The reasons for size changes in animals have always been debated in academia. According to one theory, known as Cope’s rule, the body size of a certain group of species tends to increase due to the evolutionary advantages of being larger , such as the greater ability to hunt and the lower probability of being hunted. Another theory holds that the increase in size occurs when the concentration of oxygen increases or when the climate is cold, since large bodies conserve heat better than small ones.
However, there is another theory, known as the Lilliput effect, which maintains that after a mass extinction there is a tendency to decrease in size, but until now there was not much evidence that gave consistency to this idea on a global scale , although it had been found examples of a few species that when living on islands reduced their size.
Sallan and Galimberti have collected data on 1,120 fossils from 419 to 323 million years ago, from museum specimens, scientific articles, reports and books. According to their conclusions, vertebrates were increasing in size during the Devonian period (419-359 million years) according to Cope’s rule. At the end of the Devonian there were fish, like some placoderms, the size of a bus. In fact, most fish were a meter or more in size at that time. But after the extinction, body size decreased and continued to do so for 40 million years. And the few species that grew after extinction ended up becoming extinct.
The researchers compared the results on body size with data on atmospheric oxygen concentration and mean temperature, and found no correlation. This implies that the observed trends are based exclusively on ecological factors: the mass extinction of the time caused a Lilliput effect that lasted a long time , since small organisms had an advantage, because in times of scarcity the best strategy is to reproduce as soon as possible, which causes the size to evolve to be reduced. Another advantage is that when a species reproduces quickly it adapts better to environmental changes. This can be seen today in plants.
After a forest fire, it is precisely the grasses that colonize the burned ground, followed by the bushes and then the trees. Although in this case it occurs only in a few decades, it mimics what happens in processes on a global scale that take place over millions of years.
