The Evolution of the Rhinoceros

Rhinos are some of the largest animals on Earth and also one of the most distinctive. There are currently five species of rhinos spread across Africa and Asia and are key players in their respective ecosystems. However, rhinos and their relatives were once more widespread, forming a group known as Rhinocerotoidea. They sported a wide array of body plants that diverged from what we see in rhinoceros species today. The history of the rhinoceros is an epic that spans over 50 million years of evolution during the Cenozoic. From their humble beginnings as dog-sized creatures, rhinos grew and diversified to become some of the largest and most fascinating mammals to have ever lived. Rhinos belong to an order known as Perissodactyla. These animals are distinguished by having their weight primarily distributed on their middle digits. Some other members of this group include horses, zebras, as well as tapirs. This group first evolved in the late Paleocene, around 55 million years ago, with horses splitting off first. There's been some recent controversy over what animal represents the earliest rhino ancestor, with the genus Hyracus of the early Eocene of North American Europe being long purported to be the earliest known rhinocerotoid. This 1.5 meter long animal bore striking resemblances to other perissodactyls. Hyracus' distinctive upper molars, which had crests in the shape of the greek letter pi and lower molars with L-shaped crests, set it apart from other members of the order. That dentition seemed to be shared among future rhino relatives. That being said, recent studies posit Hyracus not to be a direct rhinocerotoid ancestor, but rather a basal tapir morph, part of a group of animals in Perissodactyla that existed prior to the tapir-rhino split. Currently, the earliest known rhinocerotoid genus is Papyceris from the early Eocene of Asia. Papyceris, however, was still not seen as the progenitor of Rhinocerotoidea, given that it is closely aligned with one of the three lineages of the family that radiated during this epoch. The three families are Aminodontidae, Hyracodontidae, and Rhinocerotidae. These families were distinguished by the shape of the crest of their last upper molars, with Papyceris grouped together with Hyracodontidae. These three groups evolved at roughly the same time and were morphologically and behaviorally distinct from one another. Aminodontids first evolved during the middle Eocene. This group is characterized by having a large crest extended from the back of their upper molars, as well as having the upper and lower incisors flare out like tusks, similar to the teeth in hippos. They also had similar body plans to those of hippos, with thick burly bodies and stout legs. As a result, these animals have been nicknamed hippo-like rhinos, and according to the density of many of their fossilized bones, they likely had a semi-aquatic lifestyle not unlike those of hippos. Aminodon was among the earliest members of this group and had a depression in front of its eye socket that would house muscles to support a large upper lip. Aminodon today can be further divided into two tribes, the Metaminodontini and the Catercodontini. The Metaminodontini was home to the especially hippo-like amphibious rhino relatives. Metaminodon was one of the most iconic genera within this tribe, sporting an uncanny resemblance to hippopotamuses, with a wider jaw and large nostrils that were on top of its snout. It also sported several other aquatic adaptations, such as a barrel-shaped rib cage and powerful legs for movement in the water. The other branch, Catercodontini, was home to the much more tapir-like Aminodontids, with genera such as Catercodon likely possessing a tapir-like snout. Aminodontidae was widespread throughout North America and Eurasia up until the early Miocene, where the group likely went extinct thanks to the subsequent climatic changes. Hieracodontidae were perhaps the least rhino-like of all rhinocerotoids, at least in terms of appearance. They had no horns and generally thinner bodies with long, slender legs. They shared a very close resemblance to horses, converging on their body plans, and many members also had very fast travel speeds. Hieracodonts first showed up in the fossil record in Asia during the early Eocene and shortly after spread to North America. Along with Papyceris, Triplopus was one of the first genera of Hieracodonts. This North American rhinocerotoid highly resembled Hieracus, except that it was notably larger. In addition, it lost the backward-extending crest on its upper molars, and its front incisors were shaped like spatulas, dentition common among members of Hieracodontidae. Hieracodon represented the climax of the North American genera of Hieracodontidae. This creature was the size of a large dog and had longer legs compared to traditional rhinos. Due to the simple nature of its teeth, Hieracodon was likely a browser feeding on leafy soft vegetation. This animal was widespread throughout Eocene and Oligocene North America. Hieracodon had long legs and toes that helped for fast travel in its environment. It was quite similar in this regard to other ungulates of North America during this era, such as horses and camels. Despite having a similar morphology to those animals, however, this animal bore several key differences. These included a larger skull and thicker neck, key rhinocerotoid features not present in the more lightly built horses and camels of the big badlands the genus lived in. During the Middle Eocene, Hieracodontidae began to diverge into brand new lines of rhinocerotoids, with perhaps the most fascinating being the Indracothyrs. These animals, while not starting off especially large, grew to become some of the biggest land mammals to have ever lived. Forster Cooperia evolved during the Middle Eocene of Asia during the same time frame as Hieracodon. Unlike the North American Hieracodonts, which became more and more adapted for running, the lineage that came from this animal instead evolved to become larger over millions of years. Jucsia of late Eocene China and Mongolia represented the next step of this lineage's evolution. Although it was only about the size of a cow, it had some new body features that would become more present on future Hieracodonts, such as a retracted nasal cavity for proboscis. Following Jucsia came Artinotherium, one of the Indracothyrs that truly lived up to the gigantism famous of the group. But even Artinotherium was dwarfed by the largest member of this line. Paraceratherium of Oligocene Central Asia was the largest of all rhino relatives and one of the largest land mammals to have ever lived. This enormous beast clocked in at 20 tons, 4.8 meters at the shoulder, 7.9 meters in length, and had a skull 2 meters long. Despite this titanic size, the animal didn't abandon the and still had longer toes which would support its weight. Beyond its enormous weight however, Paraceratherium was also interesting in how many conversion morphologies it shared with the giraffe, from its long neck to the prehensile lip that was used in a similar manner to strip brows. Paraceratherium was widespread during its heyday, living in open woodlands under either humid or arid conditions. However, the animal went extinct at the dawn of the Miocene. The extinction of Paraceratherium could have been linked to the spread of the elephant-like mastodons to Central Asia that could have reshaped their habitat and feeding grounds, as well as bringing in predators which could have posed a threat to their young. True rhinos emerged 40 million years ago in the late Eocene of Eurasia and North America. Of the three groups, this one was notable for containing genera with keratinized horns, although they weren't present in all members. They were generally characterized and differentiated from other rhinocerotoids by tusk-like lower incisors that sharpened on smaller chisel-shaped upper incisors. There is also strong dimorphism within rhinocera today, with genders being distinguished by their front teeth and their lower tusk-like incisors. The earliest members of this family, such as the Middle Eocene Uintesseras, were small hornless creatures that bore strong resemblances to other contemporary parasidactyls such as the horse and tapir species that evolved alongside it. These genera were also physically similar to the hieracodont running rhinos, though the structure of its legs placed it within rhinocerotidae and was not likely as well-built for running. Rhinocerotidae can be further broken down into two smaller subfamilies, Elasmotherinae and Rhinocerotidae. Elasmotherinae split from other rhinocerotidae around 47 million years ago. It is made up of two subgroupings, the Diceratherinae and the Elasmotherinae. Diceratherinae was characterized by many of its members having either no horns or horns unlike the rhino horns we're familiar with today. This group included the North American Trigonius, a rhinoceros that possessed no horns. In addition, there were genera such as Diceratherium from North America and Eurasia, which possessed two horns that are placed side by side. This configuration is rare in rhinoceros today and converges on similar horn placements from other Cenozoic mammals like Arsinoatherium. The second subgrouping, Elasmotherinae, had its own set of interesting rhinoceroses. There were some such as Monoceros from North America, which looked relatively similar to Diceratherium, and the two are the only known members of Rhinocerotidae to share their horn placements. In Monoceros, horns were prominent in males, although they were absent in females, illustrating the sexual dimorphism that was characteristic of Rhinocerotidae. The most famous member of Elasmotherinae and one of the most famous prehistoric rhinos of all time was the Siberian unicorn, Elasmotheria. This animal is the latest genus of Elasmotherinae, living from the late Miocene to the late Pleistocene, having likely evolved from Cenoatheria. Its size was more in line with elephants rather than fellow rhinos, growing up to 6 meters in length, over 2 meters tall at the shoulder, and weighing about 5 tons. At that size, it was the largest among Pleistocene rhinos. Despite its weight, it had relatively long legs that would have helped it to be able to gallop. The most iconic feature of the rhino, however, was its enormous horn that was 1.5 meters long. The placement of its horn was pretty unique among other members of Rhinocerotidae, as it is on the center of its forehead. This massive horn was supported by powerful back muscles. Despite the size of the horn, its brain was noticeably less developed than in other rhinos. Another difference it had to other rhinos such as the woolly rhinoceros was that its remains were often found near river valleys as opposed to steppes. Elasmotherium was known to feed near bodies of water, and its hypsodontic tissue would help it eat the tougher plant matter it is thought to have fed on. It's unknown whether or not Elasmotherium had a hairy coat like other Ice Age mammals, some depictions include it while others have it far less hairy, looking more like the African rhinos. Elasmotherium was alive for a considerable portion of the Quaternary, but ended up being part of the same Pleistocene extinction which wiped out other large Ice Age animals such as mammoths, saber-toothed cats, and woolly rhinos. Rhinocerotidae is the only living group of rhinocerotoids alive today. It consists of several smaller tribes home to rhinos like the hornless Esseratherium or the similarly hornless Chelotherium of Eurasia, the latter genus being notable for having two large protruding lower incisors. In addition, there was also the one-horned North American genus Teleoceros. This rhino is interesting for being the last among the North American genera of rhinocerotoids. Many rhinos and their relatives could trace back their long history to this continent, but the end of the Miocene and the subsequent cooling the Earth went through brought an end to their dynasty. By the Pliocene, all American rhinos had gone extinct. At this point, I want to zone in even further to three of the most recent tribes, Rhinocerotone, Dicerorhinus, and Dicerotone. Rhinocerotone likely originated in South Asia. The Miocene genus Ganditherium was one of the first members of this group and was likely the progenitor of the two living members of this tribe, the Indian or greater one-horned rhinoceros, Rhinoceros unicornis, and the Javan or lesser one-horned rhino, Rhinoceros syndiacus. It shared some key features with these Asian members, such as the nasal bones being oriented in a way to support their one horn. Rhinocerotone also contained the genus Nessorhinus, which was a similar genus to the Indian and Javan rhinoceroses but notably smaller. This was likely due to the fact that it lived in what are now today the island nations of Taiwan and the Philippines, and experienced island dwarfism. Dicerorhini is a fascinating tribe within Rhinocerotone, containing two-horned rhinogenera. One such member of this was Merck's rhinoceros, the Phanorhinus. This is a widespread rhino genus that could be found all over Europe from Spain to Turkey to as far east as Korea. There is even evidence to suggest that it could have lived as far south as Iran or North Africa. Merck's rhino was similar in appearance to the African white rhino, but it was a browser like the black rhino. This animal is also part of a sister lineage to the famous woolly rhino. The woolly rhinoceros, Coelodonta antiquitatus, It was known from remains to have had a woolly coat, but it may not have been as well adapted to heavy snow given the relatively short length of its legs. The woolly rhino was known to have been a grazer, feeding on forbs and grasses. Despite being prominent throughout northern Eurasia, the woolly rhino didn't manage to make it as far west as North America like many of its other Ice Age contemporaries. This is due in part to the fact that the habitat surrounding Beringia was not suitable for them. The woolly rhino went extinct 14,000 years ago due to the habitat change and subsequent alterations in vegetation that they fed on. The change from open plateaus and firm ground to more forested areas led to their decline. The only living member of Dicerurini is the Sumatran rhinoceros, Dicerurina sumatransis, a critically endangered rhinoceros species that lives in Sumatra and Borneo. Diceratoni was a tribe of African two-horned rhinos that would spawn the Ceratotherium and Diceros genera. These would evolve into the white rhinoceros, Ceratotherium simon, as well as the black rhinoceros, Diceros bicornis. There's been a lot of debate over the interrelationships among the current and recently extinct rhino species, with many different hypotheses being put forward to explain how certain species can be grouped together. One such hypothesis deals with their morphology, grouping together the two-horned African rhinos alongside the similarly two-horned Sumatran rhinoceros, having the Indian and Javan rhinoceros as another group. Another hypothesis is based on geography, grouping the Sumatran rhinoceros with the one-horned rhinos of Asia and omitting it from the group containing the African rhinos. In order to try and delve further into this mystery, Shanlin Liu and colleagues recently conducted a whole genome alignment for the five exant rhino species, as well as three recently extinct rhinos, the woolly rhino, Elasmotherium, and Merckx rhino. Using the domestic horse as an outgroup, their results yielded a tree with the clade Dicerotii that contained the African rhinos being sister to a clade containing the Sumatran rhino, Merckx rhino, and the woolly rhino, with the one-horned South Asian rhinos forming a separate clade more distantly related. Interestingly enough, this tree seemed to take from both the morphological as well as the geographical models, with the two-horned rhinos and one-horned rhinos being grouped separately but further divided based on continent. Even with only five species alive today, rhinos continue to be animals that excite us and fill us with awe. Every one of these under threat, however, with the Javan and Sumatran rhinos being particularly endangered, with less than 70 individuals of both species left in the wild. I've provided donation links to help rhinos in the description below, but there are also smaller ways we can help these animals, like buying sustainable wood, paper, and palm oil. Hey everybody, thanks for watching. As usual, a big shout out to Cabaramus09 for providing many of the documents I used for research in this video. If you have any ideas for future videos, feel free to share them in the comments below. Until next time.