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The skulls of Late Cretaceous hadrosaurs from western North America. From Campione and Evans, 2011.

Hadrosaurs just can’t get any respect. In a new paper published in PLoS One, paleontologists Nicolás Campione and David Evans have proposed that the immense, Late Cretaceous hadrosaur Anatotitan was actually just the fully mature stage of the dinosaur Edmontosaurus. No one batted an eyelid: “Huh? Anato-what?” Compare the lack of reaction to the tizzy the public fell into last year when confused reporters mistakenly told readers that paleontologists were sinking the name Triceratops. As far as I know, no one has started up a “Save Anatotitan!” group to object to the conclusions of Campione and Evans.

The new hadrosaur paper is just the latest in a growing body of research on the changes Late Cretaceous dinosaurs underwent as they grew up. In 2009 Horner and co-author Mark Goodwin proposed that the dinosaurs Dracorex and Stygimoloch were juvenile and sub-adult stages of the dome-headed genus Pachycephalosaurus, and Horner and John Scannella proposed that the horned dinosaurs Nedoceratops and Torosaurus were more mature growth stages of Triceratops. (Regarding each case, the names Pachycephalosaurus and Triceratops would be preserved while the others would be sunk.) These papers have been very controversial among paleontologists. Have we really been naming too many dinosaurs, or are we now entering an age when we’re lumping too many together?

So far, the focus of the lumping/splitting debate has been on the Late Cretaceous dinosaurs of western North America. The work by Campione and Evans continues this trend with Edmontosaurus and closely related genera. Specifically, the paleontologists chose to investigate 23 edmontosaur skulls, ranging from Edmontosaurus regalis and Thespesius edmontoni from roughly 73-million-year-old deposits in Alberta, to the dinosaurs Edmontosaurus saskatchewanensis, Edmontosaurus annectens and Anatotitan copei from the time interval spanning roughly 70 to 65 million years ago. Just how many of these dinosaur genera and species are valid has been debated for some time, and the new research narrows down this list to just two species of Edmontosaurus.

Through comparisons of particular anatomical landmarks on each edmontosaur skull, Campione and Evans concluded that individual variation and anatomical changes due to growth had led other researchers to name too many hadrosaurs from the pocket of Late Cretaceous deposits they investigated. Hadrosaurs given the name Thespesius edmontoni simply appear to be small individuals of  Edmontosaurus regalis from the same deposits, while Edmontosaurus saskatchewanensis and Anatotitan copei seem to be younger and older growth stages, respectively, of Edmontosaurus annectens. Just like that, five different dinosaurs are reduced to two species of a single genus.

Further study and debate will test the hypothesis proposed by Campione and Evans. (For example, do changes in bone microstructure follow the proposed growth series for Edmontosaurus annectens?) Of one thing, though, there can be no doubt: just how many different dinosaurs existed in North America during the last ten million years of the Cretaceous has become a matter of major debate among paleontologists. How things shake out will undoubtedly influence our understanding of how and why dinosaurs became extinct on the continent. If some of the new studies are correct and the number of different dinosaurs in western North America at the end of the Cretaceous was lower then previously expected, then we are left with the question of why the drop in diversity occurred and whether the changes made dinosaurs more vulnerable to extinction. Then again, if genera like Torosaurus, Dracorex and Anatotitan are preserved, we must ask how so many similar dinosaurs evolved and co-existed alongside each other. Right now, it is too early to tell. We are only at the beginning of what may become an important and long-running debate about how dinosaurs grew up and why they disappeared.

References:

Campione, N., & Evans, D. (2011). Cranial Growth and Variation in Edmontosaurs (Dinosauria: Hadrosauridae): Implications for Latest Cretaceous Megaherbivore Diversity in North America PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0025186