Despite their abundance, North American iguanodonts from this period are rare in the fossil recordexcept in one Utah rock formation, which spans about 40 million years and contains fossils of many types of creatures, according to study leader Andrew McDonald, a graduate student at the University of Pennsylvania.

　　Theyre part of this really interesting window into the early Cretaceous thats been emerging in western North America during the past two decades, McDonald said.

　　Theyre filling in another chapter of what will eventually be a complete and intricate story.

　　Iguanodonts: Cows of the Cretaceous

　　Already the new species are causing some shifts in the iguanodont family tree, McDonald said.

　　His team discovered that North American iguanodonts are less closely related to duck-billed dinosaursthe most evolutionarily advanced members of this dinosaur groupthan some European and Asian iguanodonts.

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