Evolution: the riddle of body sizes

Primal horses and tapir

Living environment in the Geiseltal 47 million years ago: Propalaeotherium (left) and Tapir Lophiodon (center) (image: Marton Szabo)

The ancestors of today’s horses have changed their body size significantly in the course of evolution – overall, the original horses became larger, but there were also times when they temporarily shrank. But what was the motivation for this? A study by German researchers disproves one of the common explanations for this change in size. The most extensive comparative analysis of roughly 45 million year old primitive horse and tapir fossils from the Geiseltal site reveals that, contrary to popular belief, the climate was not the driving force. Because then the body sizes of both types should have developed similarly – but the opposite was the case.

The former lignite mining area in the Geiseltal west of Merseburg in Saxony-Anhalt is a treasure chest for paleontology. Because tens of thousands of fossils from 42 to 47 million years ago were discovered there between 1933 and 1993. At that time, the Middle Eocene, there was a swampy subtropical forest in its area, which included primitive horses, early tapirs, large land-based crocodiles, as well as giant tortoises, lizards and ground-living birds. The finds from the Geiseltal are so numerous and extensive that they give researchers a picture of evolutionary dynamics down to the level of animal populations with unprecedented level of detail. “The Geiseltal is just as important as a fossil site as the Messel pit near Darmstadt, which is a UNESCO World Heritage Site,” explains study director Marton Rabi from the University of Tübingen and the Martin Luther University in Halle-Wittenberg. However, while the finds from the Messel pit only provide a less than a million year long “snapshot” from the world at that time, the finds in the Geiseltal, which are distributed over five million years, also allow to investigate temporal developments.

Original horse and tapir in comparison

Rabi, first author Simon Ring from the University of Tübingen and her team have now carried out such a comparative analysis for two mammals that are particularly numerous in the Geiseltal: the original horse Propalaeotherium isselanum and the tapir Lophiodon remensis. “In the beginning, we were particularly interested in the evolution of primeval horses that were about the size of a Labrador. There are a particularly large number of them among the Geiseltal fossils, ”explains Rabi. The primeval tapirs were around 47 million years ago, with an average body weight of 124 kilograms, significantly larger and about three times as heavy as the original horses. For their study, the researchers examined whether and how the height of these two species changed over the next five million years and what role the climate could have played in this. For the latter, they analyzed the carbon and oxygen isotopes in fossil teeth from the found layers. According to current theory, the climate of their habitat influences the size of mammals – the Bergmann rule, for example, states that species in cold areas tend to be larger than related species in tropical regions.

The analyzes showed: Although the climate in the Middle Eocene gradually cooled globally, this does not seem to have been the case in the Geiseltal: “Our data indicate a humid tropical climate. However, we found no evidence of climate change in the Geiseltal during the period examined, ”reports co-author Hervé Bocherens from the University of Tübingen. If the climate was the decisive factor for the body size of the mammals at that time, primitive horses and tapirs should have changed little in size and weight. However, the comparison of fossils from different phases of the Middle Eocene painted a different picture: The primeval horses became significantly smaller in the period from 47 to 42 million years ago, as Rabi and his team found. Her average body weight dropped from initially 39.2 kg to just 26 kg. But it was even more astonishing that the tapirs showed an opposite development: their body mass increased from an average of 124 kg body weight to 223 kg, as the researchers report.

Avoiding competition instead of climate?

Contrary to expectations, not only did the body sizes of these two species change despite the constant climate – they also did so in the opposite direction. The tapirs got bigger, the primitive horses got smaller. But why? Rabi and his team believe that their data argue against a climatic effect for this development. Instead, ecological factors must have taken effect at that time. “The influence of ecology on the height of mammals is just as important as the climate and independent of it,” the researchers explain. For example, increased competition between two animal species can significantly change their way of life and their shape over time. “Current theory predicts that competitive pressure leads to a shift in characteristics in which both competitors minimize their similarities as much as possible,” says Rabi and his team.

In the case of primal horses and tapir, this means that the prehistoric horses, which are already smaller, became even smaller. Because smaller animals need less food and breed faster, this means an advantage over medium-sized animals. The tapirs, on the other hand, gained weight and size. This gave them the opportunity to look for their food over long distances and also protected them from some of their predators. “The tapirs and horses from the Geiseltal probably maximized the advantages of their respective life strategies, which resulted in an opposite evolution of body size,” the scientists concluded.

Source: Simon Ring (University of Tübingen) et al., Scientific Reports, doi: 10.1038 / s41598-020-60379-7

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