Plesiolophus

MEANING: Near crest

PERIOD: Late Cretaceous

CONTINENT: North America

Plesiolophus is a lambeosaurine hadrosaur that lived during the Late Cretaceous of what is now Alberta, Canada. Plesiolophus is the oldest parasaurolophin with a preserved skull roof, predating Parasaurolophus in Alberta's fossil record, and cannot be ruled out as its ancestor.

Abstract from paper: Lambeosaurines are abundant in the Dinosaur Park Formation of Alberta, but are poorly known from the underlying Oldman Formation (Upper Cretaceous: Campanian). A new partial skull roof and braincase from the Comrey Sandstone Zone (middle Oldman) at the Milk River Ridge Reservoir locality in southern Alberta represents the first diagnostic material of an adult lambeosaurine from this formation. Derived characters of the frontal platform and braincase (including the steeply inclined frontal platform, boomerang-shaped dorsal surface of the frontals, and pendent alar processes of the basisphenoid) support the referral of this specimen to Parasaurolophini, as the stratigraphically lowest specimen with the skull roof and braincase preserved. The recognition of a new taxon is supported by a unique combination of characters, some resembling the inferred plesiomorphic condition for corythosaurians, including a triangular plateau on the parietal separating the interfrontal process from the sagittal crest, and the anterolaterodorsal orientation of the postorbital process of the laterosphenoid. Despite the large size and fused interfrontal contact, the prefrontal–postorbital contact is not strongly elevated, and the posterior margin of the frontal platform does not overhang the parietal, resembling the condition described in an immature Parasaurolophus from Dinosaur Provincial Park, and providing support for heterochrony in the evolution of the parasaurolophin skull roof. As one of relatively few diagnostic Albertan hadrosaurids older than the Dinosaur Park Formation, the new taxon provides significant new data on the diversification of Corythosauria, and the evolution of hadrosaurid communities in Laramidia.

Plesiolophus is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known groups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene (K–Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K–Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Plesiolophus is a lambeosaurine hadrosaur. The hadrosauridae, often called "duck-billed dinosaurs," were some of the most successful herbivores of the Late Cretaceous. They belonged to the broader group of ornithopods within the ornithischia, and were especially notable for their complex rows of tightly packed teeth, or dental batteries, that allowed them to efficiently process tough plant material. Hadrosaurs came in a wide range of sizes and shapes, but many shared traits like broad beaks, strong hindlimbs, and often elaborate crests on their heads. They were widespread, thriving across North America, Asia, and Europe, and represent one of the final flourishing groups of non-avian dinosaurs before the end-Cretaceous extinction.

Lambeosaurinae is a major subgroup of hadrosaurs, best known for their distinctive hollow, elaborate cranial crests. These crests varied greatly in shape and size between species and are thought to have played roles in visual display, species recognition, and possibly even vocal communication through resonating sounds. Lambeosaurines generally had more lightly built bodies compared to their sister group, the saurolophinae, and tended to favor more forested or coastal environments. This showcases an incredible diversity of forms as they flourished across North America and Asia during the Late Cretaceous.