DIFFERENTIAL LINEAGE SURVIVAL DROVE BODY SIZE INCREASE IN BRONTOTHERES

Because evolutionary trends (sustained and almost unidirectional net phenotypic change) are efficient at expanding phenotypic disparity of life propelling lineages into unexplored regions of the morphospace, it is not surprising that they have attracted the interest of paleontologists for centuries. One of the most notorious debates on evolutionary trends regards the apparent recurrent increment of body size in animal clades. The universality of this rule —implicitly formulated by Cope in his “law of the unspecialized” — and the mechanisms underpinning long-term size increase have been long disputed. Brontotheres is an extinct family of Eocene perissodactyls known from North America, Asia, and Eastern Europe. As one of the first lineages of terrestrial mammals to have acquired large sizes, brontotheres are ideal candidates to understand how large sizes were early acquired by different herbivore lineages. We analyze body mass from a total of 59 brontothere species spanning over the Eocene. A morphological character matrix was analyzed using a Bayesian tip-dating phylogenetic analysis. To gain perspective on how body size evolved along the Brontotheriidae tree, we tested continuous phenotypic data against different phylogenetic models of evolution. Early brontothere species were predominantly small-sized, whereas later ones attained larger body masses, roughly reaching the size of rhinos and elephants. The observed scenario shows body size shifting through evolutionary leaps without preferential direction interrupting periods dominated by neutral diffusion. Our results support higher risk of extinction amongst smaller species, with larger taxa being more resilient due to lower extinction rates. These findings contradict the idea of large mammals being universally more prone to extinction due to their higher morphological specialisation, behaving as some sort of macroevolutionary sink. Further, the pattern revealed gives macroevolutionary patterns (species sorting) a major role in long-term body size increases during the early Cenozoic, ruling out a scenario where such trends are rendered by sustained microevolution and a widespread phyletic increase in size operating on most of the lineages.