A recent study of ancient DNA has helped to fill the gaps in the rhino evolutionary family tree by analyzing genomes of all five living species, together with the genomes of three ancient and extinct species. Findings include that the rhino family tree is split by geographic regions, not by horn configuration, and that historically rhinos have always had low levels of biodiversity. 

I produced this illustration in collaboration with the study's authors showing three extinct rhinoceros species-in the foreground Elasmotherium sibiricum (sometimes called the Siberian unicorn), close behind are two Merck's Rhinoceros (Stephanorhinus kirchbergensis). In the far background is the silhouette of Woolly Rhinoceros (Coelodonta antiquitatis). The paper appeared in the journal Cell in august 2021, along with the first illustration below.

I revisited this illustration to adjust Elasmotherium to reflect new research suggesting that the huge single horn structure usually depicted may have instead been a hollow resonating chamber with a bony dome.