Kritosaurus

Kritosaurus (meaning "separated lizard"; sometimes misinterpreted as "noble lizard", in reference to the presumed "Roman nose"; the nasal region was fragmented, disarticulated, and originally restored flat) is an incompletely known but historically important genus of extinct hadrosaurid (duckbilled) dinosaur. It lived about 73 million years ago, during the Late Cretaceous of North and possibly South America. Its taxonomic history is convoluted, also incorporating Gryposaurus, Anasazisaurus, and Naashoibitosaurus; this tangle will remain unresolved until better remains of Kritosaurus are described. Despite the dearth of material, this herbivore appeared in dinosaur books until the 1990s, although what was usually represented was the much more completely known Gryposaurus, then thought to be a synonym.

Description
Kritosaurus is only definitely determined from a partial skull and lower jaws, and associated undescribed postcranial remains. The greater portion of the muzzle and upper beak are missing, but additional reconstruction in the early 2000s using fragments from the skull that had not been placed before show part of a crest in front of the eyes; the form of the crest is unknown at this point. The length of the skull is estimated at 87 centimeters (34 in) from the tip of the upper beak to the base of the quadrate that articulates with the lower jaw at the back of the skull. Potential diagnostic characteristics of Kritosaurus include a predentary (lower beak) without tooth-like crenulations, a sharp downward bend to the lower jaws near the beak, and a heavy, somewhat rectangular maxilla (upper tooth-bearing bone). If it turns out to be the same as Anasazisaurus or Naashoibitosaurus, then the form of the complete crest is that of a tab or flange beginning in front of the eyes and rising between and above them, but not extended beyond them.

Classification
Kritosaurus was a hadrosaurine hadrosaurid, a flat-headed or solid-crested duckbill. Because it is poorly known, its closest relatives are not yet known. Naashoibitosaurus and "K." australis, both of which appear to be very similar, form a clade with Saurolophus in the most recent review of duckbill phylogeny. In the same work, Kritosaurus is confusingly considered both as distinct at the species level and as a dubious name. Location and time separate Kritosaurus and the slightly older, primarily Canadian Gryposaurus, along with some cranial details.

Discovery and history
In 1904, Barnum Brown discovered the type specimen (AMNH 5799) of Kritosaurus near Ojo Alamo, San Juan County, New Mexico, United States, while following up on a previous expedition. 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. When discovered, much of the front of the skull had either eroded or fragmented, and Brown reconstructed this portion after what is now called Anatotitan, leaving out many fragments. However, he had noticed that something was different about the fragments, but ascribed the differences to crushing. He initially wanted to name it Nectosaurus, but found out that this name was already in use; Jan Versluys, who'd visited Brown before the change, inadvertently leaked the previous choice. He kept the species name, though, leading to the combination K. navajovius.

The 1914 publication of the arch-snouted Canadian genus Gryposaurus 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. He also synonymized Gryposaurus with Kritosaurus, a move supported by Charles Gilmore. This synonymy was used through the 1920s (William Parks's designation of a Canadian species as Kritosaurus incurvimanus) and became standard after the publication of Richard Swann Lull and Nelda Wright's influential 1942 monograph on North American hadrosaurids. 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), known from a dentary from the Campanian-age Judith River Formation of Montana, was also assigned to Kritosaurus by Lull and Wright, but this is no longer accepted.

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". One well-known work, David B. Norman's The Illustrated Encyclopedia of Dinosaurs, uses Kritosaurus for the Canadian material (Gryposaurus), but confusingly identifies the mounted skeleton of K. incurvimanus as Hadrosaurus. One more species was added to Kritosaurus in these years. In 1984, Argentine paleontologist José Bonaparte and colleagues named K. australis for hadrosaur bones from the late Campanian-early Maastrichtian Los Alamitos Formation of Rio Negro, Patagonia, Argentina. This species has been problematic and may not belong in Kritosaurus, as suggested by the reviews in both editions of The Dinosauria.

The history of Kritosaurus took another turn in 1990, when Jack Horner and David B. Weishampel once again separated Gryposaurus, citing the uncertainty associated with the latter's partial skull. Horner in 1992 described two more skulls from New Mexico that he claimed belonged to Kritosaurus and showed that it was quite different from Gryposaurus, but the following year Adrian Hunt and Spencer G. Lucas put each skull in its own genus, creating Anasazisaurus and Naashoibitosaurus. Not all authors have agreed with this, Thomas E. Williamson in particular defending Horner's original interpretation. At least two recent publications have upheld the different genera, for now.

Finally, the geographic range of potential Kritosaurus remains in North America has expanded. Bones from the late Campanian-age Aguja Formation of Texas, including a skull, have been found. Additionally, newly-described remains from Coahuila, Mexico may represent a new species, one about 20% larger than K. navajovius (around 11 meters [36 ft] long) and with a distinctively curved ischium. This animal would be the largest known well-documented North American hadrosaurine. Unfortunately, the nasal bones are also incomplete in the skull remains from this material.

Paleoecology
Kritosaurus was discovered in the De-na-zin Member of the Kirtland Formation. This formation dates from the late Campanian stages of the Late Cretaceous Period (74 to 70 million years ago), and is also the source of several other dinosaurs, like Alamosaurus, a species of Parasaurolophus, Pentaceratops, Nodocephalosaurus, Saurornitholestes, and as-yet-unnamed tyrannosaurids. The Kirtland Formation is interpreted as river floodplains appearing after a retreat of the Western Interior Seaway. Conifers were the dominant plants, and chasmosaurine horned dinosaurs appear to have been more common than hadrosaurids.

Diet and feeding
As a hadrosaurid, Kritosaurus would have been a large bipedal/quadrupedal herbivore, eating plants with a sophisticated skull that permitted a grinding motion analogous to chewing. Its teeth were continually replacing and packed into dental batteries that contained hundreds of teeth, only a relative handful of which were in use at any time. Plant material would have been cropped by its broad beak, and held in the jaws by a cheek-like organ. Feeding would have been from the ground up to ~4 meters (13 ft) above. If it was a separate genus, how it would have partitioned resources with the similar and contemporaneous Naashoibitosaurus is unknown.

Nasal crest
The nasal crest of Kritosaurus, whatever its true form, may have been used for a variety of social functions, such as identification of sexes or species and social ranking. There may have been inflatable air sacs flanking it for both visual and auditory signaling.

In popular culture
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, and the adaptation of this for the public in The Illustrated Encyclopedia of Dinosaurs.