Altirhinus

Altirhinus was an ornithischian dinosaur that existed on the earth in the early Cretaceous Period. Order Ornithischia includes dinosaurs with pelvic bones similar to present day birds. The most prominent feature of the Altirhinus was the shape of its nasal bone. Such cranial and facial adaptations were very common in ornithischians belonging to infraorder Iguanodontia; the Altirhinus is a part of this taxon.

The fossils of this reptile were found in Mongolia. Mongolia lies on the border of Europe and Asia, being flanked by Russia anteriorly and China posteriorly. Thus, the Altirhinus could inhabited both these continents. The time period of its existence was between 120 and 105 million years ago. This lies in the Aptian and Albian ages of the Cretaceous.

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).

Nasal Arch

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.