Lambeosaurus

Lambeosaurus (/ˌlæmbi.ɵˈsɔrəs/ LAM-bee-ə-SOR-əs; meaning "Lambe's lizard") is a genus of hadrosaurid dinosaur that lived about 76 to 75 million years ago, in the Late Cretaceous period (Campanian) of North America. This bipedal/quadrupedal, herbivorous dinosaur is known for its distinctive hollow cranial crest, which in the best-known species resembled a hatchet. Several possible species have been named, from Canada, the United States, and Mexico, but only the two Canadian species are currently recognized as valid.

Lambeosaurus was belatedly described in 1923 by William Parks, over twenty years after the first material was studied by Lawrence Lambe. The genus has had a complicated taxonomic history, in part because small-bodied crested hadrosaurids now recognized as juveniles were once thought to belong to their own genera and species. Currently, the various skulls assigned to the type species L. lambei are interpreted as showing age differences and sexual dimorphism. Lambeosaurus was closely related to the better known Corythosaurus, which is found in slightly older rocks, as well as the less well-known genera Hypacrosaurus and Olorotitan. All had unusual crests, which are now generally assumed to have served social functions like noisemaking and recognition.

Description
Lambeosaurus, best known through L. lambei,[1] was quite similar to the more famous Corythosaurus in everything but the form of the head adornment. Compared to Corythosaurus, the crest of Lambeosaurus was shifted forward, and the hollow nasal passages within were at the front of the crest and stacked vertically.[2] It also can be differentiated from Corythosaurus by its lack of forking nasal processes making up part of the sides of the crest, which is the only way to tell juveniles of the two genera apart: the crests took on their distinctive forms as the animals aged.[3]

In build, Lambeosaurus was like other hadrosaurids, and could move on both two legs and all fours, as shown by footprints of related animals. It had a long tail stiffened by ossified tendons that prevented it from drooping. The hands had four fingers, lacking the innermost finger of the generalized five-fingered tetrapod hand, while the second, third, and fourth fingers were bunched together and bore hooves, suggesting the animal could have used the hands for support. The fifth finger was free and could be used to manipulate objects. Each foot had only the three central toes.[4]

The most distinctive feature, the crest, was different in the two well-known species. In L. lambei, it had a hatchet-like shape when the dinosaur was full-grown, and was somewhat shorter and more rounded in specimens interpreted as females.[2] The "hatchet blade" projected in front of the eyes, and the "handle" was a solid bony rod that jutted out over the back of the skull. The "hatchet blade" had two sections: the uppermost portion was a thin bony "coxcomb" that grew out relatively late in life, when an individual neared adulthood; and the lower portion held hollow spaces that were continuations of the nasal passages.[2] In L. magnicristatus, the "handle" was greatly reduced, and the "blade" expanded,[5] forming a tall, exaggerated pompadour-like crest. This crest is damaged in the best overall specimen, and only the front half remains.[6]

The Canadian species of Lambeosaurus appear to have been similar in size to Corythosaurus, and thus around 9.4 m long (31 ft).[7] Impressions of the scales are known for several specimens; a specimen now assigned to L. lambei had a thin skin with uniform, polygonal scutes distributed in no particular order on the neck, torso, and tail.[8] Similar scalation is known from the neck, forelimb, and foot of a specimen of L. magnicristatus.

Classification
Lambeosaurus is the type genus of the Lambeosaurinae, the subfamily of hadrosaurids that had hollow skull crests. Among the lambeosaurines, it is closely related to similar dinosaurs such as Corythosaurus and Hypacrosaurus, with little separating them but crest form.[4] The relationships among these dinosaur genera are difficult to pick out. Some early classifications placed these genera in the tribe Corythosaurini, which was found by David Evans and Robert Reisz to include Lambeosaurus as the sister taxon to a clade made up of Corythosaurus, Hypacrosaurus, and the Russian genus Olorotitan; these lambeosaurines, with Nipponosaurus.[6] However, later researchers pointed out that due to the rules of priority set forth by the ICZN, any tribe containing Lambeosaurus is properly named Lambeosaurini, and that therefore the name "Corythosaurini" is a junior synonym.[9] The following cladogram illustrating the relationships of Lambeosaurus and its close relatives was recovered in a 2012 phylogenetic analysis by Albert Prieto-Márquez, Luis M. Chiappe and Shantanu H. Joshi.