Kritosaurus is an incompletely known genus of hadrosaurid (duck-billed) dinosaur. It lived about 74.5-66 million years ago, in the Late Cretaceous of North America. The name means "separated lizard" (referring to the arrangement of the cheek bones in an incomplete type skull), but is often mistranslated as "noble lizard" in reference to the presumed "Roman nose" [1] (in the original specimen, the nasal region was fragmented and disarticulated, and was originally restored flat).
In 1904, Barnum Brown discovered the type specimen (AMNH 5799) of Kritosaurus near the Ojo Alamo Formation, San Juan County, New Mexico, United States, while following up on a previous expedition.[2] He initially could not definitely correlate the stratigraphy, but by 1916 was able to establish it as from what is now known as the late Campanian-age De-na-zin Member of the Kirtland Formation.[3][4] When discovered, much of the front of the skull had either eroded or fragmented, and Brown reconstructed this portion after what is now called Edmontosaurus, leaving out many fragments.[2] However, he had noticed that something was different about the fragments, but ascribed the differences to crushing.[5] He initially wanted to name it Nectosaurus, but found out that this name was already in use; Jan Versluys, who had visited Brown before the change, inadvertently leaked the previous choice.[6] He kept the specific name, though, leading to the combination K. navajovius.
The 1914 publication of the arch-snouted Canadian genus Gryposaurus[7] changed Brown's mind about the anatomy of his dinosaur's snout. Going back through the fragments, he revised the previous reconstruction and gave it a Gryposaurus-like arched nasal crest.[5] He also synonymized Gryposaurus with Kritosaurus,[8] a move supported by Charles Gilmore.[3] This synonymy was used through the 1920s (William Parks's designation of a Canadian species as Kritosaurus incurvimanus,[9] now considered a synonym of Gryposaurus notabilis[10]) and became standard after the publication of Richard Swann Lull and Nelda Wright's 1942 monograph on North American hadrosaurids.[11] From this time until 1990, Kritosaurus would be composed of at least the type species K. navajovius, K. incurvimanus, and K. notabilis, the former type species of Gryposaurus. The poorly known species Hadrosaurus breviceps (Marsh, 1889),[12] known from a dentary from the Campanian-age Judith River Formation of Montana, was also assigned to Kritosaurus by Lull and Wright,[11] but this is no longer accepted.[13][14]
By the late 1970s and early 1980s, Hadrosaurus had entered the discussion as a possible synonym of either Kritosaurus, Gryposaurus, or both, particularly in semi-technical "dinosaur dictionaries".[15][16] David B. Norman's The Illustrated Encyclopedia of Dinosaurs, uses Kritosaurus for the Canadian material (Gryposaurus), but identifies the mounted skeleton of K. incurvimanus as Hadrosaurus.[17]
The synonymization of Kritosaurus and Gryposaurus that lasted from the 1910s to 1990 led to a distorted picture of what the original Kritosaurus material represented. Because the Canadian material was much more complete, most representations and discussions of Kritosaurus from the 1920s to 1990 are actually more applicable to Gryposaurus. This includes, for example, James Hopson's discussion of hadrosaur cranial ornamentation,[18] and the adaptation of this for the public in The Illustrated Encyclopedia of Dinosaurs.[19]
In 1984, Argentine paleontologist José Bonaparte and colleagues named Kritosaurus australis for hadrosaur bones from the late Campanian-early Maastrichtian Los Alamitos Formation of Rio Negro, Patagonia, Argentina.[20] In 2010, this species was thought to be a synonym of Secernosaurus koerneri.[21] Further analysis proved the bones to belong to a new genus. Thus, Huallasaurus was named by Rozadilla et al. (2022).[22]