The coal beasts that may have been the ancestors of the modern hippopotamus

Nature · Published 3 October 2026 Swipe more like this →

In short: Anthracotheres were once thought to be simple, semi-aquatic creatures, but new research into their fossilized brains reveals a more complex evolutionary story. By reconstructing the first-ever endocasts of these extinct artiodactyls, scientists are finally uncovering the hidden anatomy of the potential stem group of the modern hippopotamus.

Anthracotheriidae, a family of artiodactyls whose name translates to coal beast, roamed the Earth from the middle Eocene to the late Pliocene. While early interpretations suggested they were primarily semi-aquatic, recent findings indicate they occupied diverse habitats. These animals were widespread, inhabiting Eurasia, Africa, and North and Central America. Their evolutionary position remains a subject of intense scientific debate, with many researchers identifying the subfamily Bothriodontinae as the stem group of hippopotamids, a theory bolstered by the discovery of Epirigenys lokonensis in Kenya.

The study of anthracothere brain evolution relies on endocasts, which serve as reliable proxies for the shape of the brain. Researchers recently described the virtual endocast of Microbunodon minimum, a small, slender species from the late Oligocene of France, and the natural endocast of Merycopotamus medioximus, a larger, gregarious semi-aquatic species from the late Miocene of Pakistan. These reconstructions provide the first direct look at the internal cranial features of the family, including the relative proportions of the neocortex, cerebellum, and olfactory bulbs.

Analyzing these endocasts allows scientists to investigate behaviors and phylogenetic relationships. For instance, the backward shift of the pituitary in M. minimum and the potential presence of a K lobe in M. medioximus offer new data points for comparative studies. While some researchers argue that anthracotheres are the ancestors of hippos, others suggest alternative lineages, such as the palaeochoerids. By examining these endocranial characters, experts hope to resolve long-standing questions about the evolutionary history of the Hippopotamoidea and the broader development of the mammalian brain.

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