Iguanodont dinosaurs were among the first to be discovered by scientists. The genus Iguanodon itself was described by the English naturalist Gideon Mantell in 1825, although the way he initially envisioned it—as a gargantuan iguana—greatly differs from the dinosaur with a thumb-spike that we are familiar with today. Since the time of that first discovery, additional genera and species have been found. The latest two discoveries were found within the 124-million-year -old Cedar Mountain Formation of eastern Utah.
As described by Andrew McDonald, James Kirkland and their co-authors in PLoS One, the Cedar Mountain Formation contains a relatively rich collection of iguanodont dinosaurs. The dinosaurs Eolambia caroljonesa, Planicoxa venenica and Cedrorestes crichtoni have all been found in these rocks, and the two new dinosaurs from two separate sites add to this diversity. The larger of the two animals, Iguanacolossus, would have been about 30 feet long and is described as a "somewhat ponderous beast with robust limbs." Hippodraco, at a comparatively paltry 15 feet, was a much smaller animal, and the remains described in the paper may have even belonged to a juvenile. Although both new dinosaurs are known from only partial skeletons, the bones are distinctive enough in anatomy and in their geologic context to justify placing them in new species. (Paleontologists also found the fragmentary remains of other dinosaurs at each of the two sites, but not enough was preserved to positively identify what genera or species they might belong to.)
When compared with other iguanodonts, Iguanacolossus and Hippodraco fall in different parts of the group's evolutionary tree. Whereas Hippodraco was most closely related to the 112-million-year-old Theiophytalia from Colorado, Iguanacolossus was placed near Dakotadon from South Dakota and Cedrorestes from Utah. Frustratingly, though, many of the North American iguanodonts are known only from partial remains which do not overlap with one another, and as excavations continue it is likely that some species will be lumped together and some unique specimens will be taken to represent new species.
Determining the true diversity of these iguanodonts and their relationships to one another will require more time and additional fossils, but at present it appears that the Early Cretaceous iguanodonts in western North America were quite different from their cousins elsewhere. Compared with relatives that lived at the same time in other places, both Hippodraco and Iguanacolossus appear to be relatively archaic species, meaning that they were more similar to earlier varieties of iguanodonts than the more specialized species such as Iguanodon. Andrew McDonald has already begun sorting all of this out, but for now it is clear that the Early Cretaceous West was home to a unique and varied collection of iguanodonts which we are only just beginning to understand.
For more, see Andy Farke's post on these dinosaurs.
McDonald, A., Kirkland, J., DeBlieux, D., Madsen, S., Cavin, J., Milner, A., & Panzarin, L. (2010). New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs PLoS ONE, 5 (11) DOI: 10.1371/journal.pone.0014075