Named By: Matthew - 1902.
Classification: Chordata, Mammalia, Rodentia, Mylagaulidae, Mylagaulinae.
Species: C. rhinocerus (type), C. anecdotus, C. hatcheri, C. minor.
Size: About 30 centimetres long.
Known locations: USA, Kansas - Ogallala Formation, Nebraska.
Time period: Langhian of the Miocene through to the early Zanclean of the Pliocene.
Fossil representation: Multiple individuals are known.
was a relatively large rodent that lived in North America from about
15.97 to 4.9 million years ago. Ceratogaulus
is thought to
have been a fossorial animal, which means that it would have spent a
large portion of its time underground in burrows. The curious thing
about this genus though is that on top its head were two large horns
that pointed upwards. Before a review of some of the theories
regarding the presence of these horns and the purpose they fulfilled,
we need to consider two important facts about these horns. First,
the horns seem to have been on both male and female Ceratogaulus with
no noticeable difference between them, so sexual dimorphism can be
ruled out. Second, the horns of Ceratogaulus
that lived in the
later stages of the Miocene are situated further up the skull than
those of Ceratogaulus that lived earlier.
The most tempting theory to go with is that the horns were burrowing aids for digging. The obvious problem with this is that the horns are simply in the wrong place. Burrowing animals usually dig and push dirt out of the way with the snout, but the horns of Ceratogaulus point up, not forwards. If an attempt to use them was made, the snout of the animal would also get in the way of the excavation, and when you remember that horns of later generations became positioned further back towards the top of the skull, you can appreciate that the snout would have become more and more of a hindrance as time went on. It is far more likely that Ceratogaulus relied more upon their short but powerful forelimbs for earth moving.
Display and inter species combat are also usually ruled out, firstly because since both males and females had horns, there is no clear way for them to identify one another. This leads to the second proposal of males using them to fight one another over such things as mating rights to females, perhaps in a similar manner to deer. This in turn leads to comparison between Ceratogaulus with a modern genus called Aplodonta rufa, better known as the mountain beaver. The mountain beaver is noted for having poor eyesight, but the optic foramen (the opening in the skull for the eye) of Ceratogaulus is measured as being between half and two thirds the size of the optic foramen of a mountain beaver. It follows that by the same proportion the eyesight of Ceratogaulus was a third to half as bad again as the eyesight of a mountain beaver, which leads to the idea that females would not be able to identify winning males.
These theories that are counter to the idea of the horn being for display do actually have their own problems. Assuming that Ceratogaulus had eyesight so poor that they could not recognise individuals of their species, the horns might still have been enabled them to not confuse other similar sized animals for their own species. Even if they could recognise not much more than a silhouette, a silhouette with a pair of horns on its nose looks different to a silhouette without one. Also, if males were fighting for dominance, the loser may have been driven away from a specific harem of females, which means the females would not have to recognise any individual in particular other than the resident male, who in all likelihood would probably do the identifying for them. This behaviour however is only speculation, but it’s worthwhile to remember that animals can recognise each other in other way such as smell, and this can also count against the idea of the horns being just for display.
A better theory to explain the presence of the two horns is that of defence. By being on top of the snout, the horns would almost always be facing in the general direction of a predator. At thirty centimetres long, Ceratogaulus were not small rodents, but they would have been smaller and lower than most predators of the day. So, if a predator such as say a bear dog got curious about a Ceratogaulus burrow, an individual inside may have been able to thrust its horn upwards into its snout. Likewise the horns would have been an effective defensive weapons for a Ceratogaulus caught above ground, especially if it was aggressively standing its ground. One quick thrust of the horns into the face or the throat of a predator, and it might be surprised and disorientated just long enough for the Ceratogaulus to escape, especially if the predator was too young and inexperienced to known how to tackle such animals.
Another burrowing mammal known to have had horns on top of its head is the xenarthran Peltephilus.
- The evolution of fossoriality and the adaptive role of horns in the Mylagaulidae (Mammalia: Rodentia), Samantha S. B. Hopkins - 2005.