When citizenry think back to the time of thedinosaurs , most people picture a giantTyrannosaurus rexor a gallant sauropod roaming through ourprehistoric satellite . Mammals on the other hand , though around at the time of the dinosaur , remained small and rarely weighed over 10 kilo ( 22 pounds ) . However , 20 million years after the Cretaceous - Tertiary extinction event that wipe out all non - avian dinosaurs , mammals weighing more than 1 tonne were abundant throughout the landscape . So how did they get to be so big ?
Several ideas about how Cenozoic mammalian evolved to have huge multi - tonne bodies have been explored . By using brontotheres – mammals that look like modern - dayrhinosbut are in reality more intimately related tohorses – a team of researchers has been exploring the likely different pathways that conduct to mammalian supersizing .
Brontotheres were chosen because they see one of the most uttermost size of it increase seen in these early mammals and are well preserved in the fossil record book . The first cognise brontotheres were thought to be around 18 kilograms ( 39 pounds ) in size , yet in their 22 - million - year history more than half of the 57 specie belonging to this chemical group had an estimated system of weights of over 1 t .
Three scenarios that might explain this rapid increase in size were quiz . The first was that firm size increases happened through the line of descent ; the 2nd suggested that these brontotheres develop a larger physical structure size as an adjustment to successive adaptive zone ; and the third melodic theme is that speciational evolution hap without preferential direction .
After do modeling tests on a dataset of 276 brontothere dodo , the team suggest that the model where body size of it modification mainly go on at speciation events without a discriminatory direction is four times more likely than the two other scenarios . However they paint a picture that just this idea alone does not fully account for the parachuting in brontotheres from being around 20 kilograms ( 44 pounds ) to more than 3 tonnes .
The team propose that the bionomic niches in which the herbivorous brontotheres lived became pure , explain the divers range of size within these ancient mammals . The smaller brontotheres that lived in these highly competitory niches evolve to be bigger to combat the gamey extinction risk they experienced .
The team intimate that further modeling inquiry into climate and niches would shed light on whether environmental modification conduce to the death of the brontotheres as their overall parentage survival of the fittest decreased in the second half of their story .
The study is published inScience .
