Altirhinus ("high snout") is a genus of iguanodontian ornithopod dinosaurs from the Early Cretaceous Period of Mongolia.
Altirhinus was herbivorous and bipedal when walking or running, but probably became quadrupedal when feeding from the ground. According to the original description, the entire body probably extended 8 m from snout to tail tip. In 2010, Gregory S. Paul estimated the length at 6.5 metres (21 ft), the weight at 1.1 tonnes. The skull alone is about 760 mm long, with a wide mouth and a distinctive tall arch on top of its snout, from which this dinosaur derives its name.
Altirhinus is definitely an advanced iguanodontian, just basal to the family Hadrosauridae, but there is little agreement on the arrangement of genera and species in this area of the ornithopod family tree.
In the original description, it was included with Iguanodon and Ouranosaurus in a family Iguanodontidae (Horman, 1998). More recent analyses all find Altirhinus more derived than either of those two genera, but less than Protohadros, Probactrosaurus, and hadrosaurids (Head, 2001; Kobayashi & Azuma, 2003; Norman, 2004). The former two studies also place Eolambia between Altirhinus and hadrosaurids, while Norman's analysis finds that the two genera share a clade.
Fukuisaurus is just basal to Altirhinus according to the only analysis in which the former has been included (Kobayashi & Azuma, 2003).
Many aspects of Altirhinus' anatomy allow speculation on its behavior.
As its forelimbs were roughly half the length of its hindlimbs, Altirhinus appears to have been primarily bipedal. However, its carpals (wrist bones) were thick and blocky, and the three middle fingers of its hand were wide, hyperextendable, and ended in hoof-like bones. This indicates that the forelimbs were also capable of supporting weight. Like many ornithopods, Altirhinus may have spent a significant amount of time in a quadrupedal position, perhaps while feeding.
While the three middle digits of each forelimb (digits II, III, & IV) were very thick and probably weight bearing, the outside fingers (digits I & V) were modified in different ways. The first digit was a simple sharp spike, as seen in Iguanodon. Aside from defense, the thumb spike could possibly have also been used for breaking the shells of seeds or fruit. The fifth digit was somewhat opposable to the rest of the hand and may have been useful for grasping food.
There is a large diastema, or gap, between the keratinous beak on the front of the mouth and the main chewing teeth in the side of the mouth, which would allow the two sections to work independently, so Altirhinus could crop with its beak while simultaneously chewing with its teeth. Many herbivorous mammals show a similar adaptation and can crop with their incisors without disturbing their chewing molars.
Altirhinus was one of a number of advanced iguanodontians with snouts expanded outwards towards the end. This is quite possibly an example of convergent evolution with hadrosaurids, famous for their wide "duckbill" snouts. These adaptations are also paralleled in many living mammalian herbivores of different lineages. Modern cows, horses, and white rhinoceros all exhibit wide muzzles and all are grazing animals. Grazing most often occurs at ground level, and if the expanded muzzles of Altirhinus and other related species were an adaptation to grazing, this may also explain corresponding weight-bearing adaptations of the forelimbs in derived iguanodontians, in order to get the head closer to the ground.
The characteristic arched snout of Altirhinus was formed primarily by the nasal bones, and a similar structure is seen on the snout of the Australian Muttaburrasaurus. Many different functions have been proposed for the nasal arch. It may have housed tissues to cool the blood, conserve water, or enhance the sense of smell. Alternatively, it may have facilitated communication through vocalization or visual display. As only two skulls have been located, it is entirely possible that the arched snout is only found in one gender, in which case it may have been used for sexual display, like in modern-day elephant seals.
- Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 291