Though the area of Laramidia was only 20 percent that of modern North America, it saw a major evolutionary radiation of dinosaurs, including the common hadrosaurs and ceratopsians.
It has been postulated that there was a latitudinal array of dinosaur "provinces" or biomes on Laramidia during the Campanian and Maastrichtian ages of the Late Cretaceous, the boundary lying around modern northern Utah and Colorado; the same major clades are known from the north and south but are distinct from each other at the genus and species levels.
This hypothesis has been challenged; one argument claims that northern and southern dinosaur assemblages during this time were not coeval but reflect a taxonomic distribution over time, which gives the illusion of geographically isolated provinces, and that the distinct assemblages may be an artifact of sampling bias between geological formations.
Due to a lack of well-dated fossils from southern Laramidia, this idea had been difficult to test, but discoveries in the Kaiparowits Formation have increased knowledge of fossil vertebrates from the region during the Late Cretaceous.
The evolutionary radiation of ceratopsids appears to have been restricted both in time and geographically (the turnover of species was high, and each existed for less than a million years), most taxa being known from latest Cretaceous sediments in the Western Interior Basin, therefore appearing to have originated and diversified on Laramidia.

The discovery of Kosmoceratops and Utahceratops greatly increased the number of known chasmosaurines from the Western Interior Basin.
Sampson and colleagues viewed this as the strongest support for the idea of dinosaur "provinces" in their 2010 description.
They pointed out that in contrast to the Maastrichtian, the preceding Campanian stage had a better sampled, diverse, and far-ranging dinosaur assemblage, as well as more precise geographical and stratigraphical data.
The stratigraphic ranges of Kosmoceratops and Utahceratops show that they lived at the same time and likely in the same ecosystems, which was rare among ceratopsids.
According to Sampson and colleagues, geochronologic dating indicates that the brief geological interval preserved within the Kaiparowits Formation was contemporaneous with the Dinosaur Park Formation in Alberta and followed other formations in the north (the upper Judith River and Two Medicine formations) and southeast (the Fruitland and Aguja formations), with the brief duration, overlap in time, and longitudinal span permitting significant comparisons between them.
That Kosmoceratops and Utahceratops were not closely related to each other or to Chasmosaurus and Mojoceratops from the coeval Dinosaur Park Formation, and that Vagaceratops from Alberta overlapped with Pentaceratops from New Mexico in time, were cited by Sampson and colleagues as evidence against the claim that northern and southern dinosaur assemblages were not coeval during this time.

Sampson and colleagues stated that their study was the first time intracontinental endemism within dinosaurs was documented (with distinct chasmosaurine taxa co-occurring north and south on Laramidia).
They suggested a sequence of events in the evolution of chasmosaurines that they found consistent with the phylogenetic, stratigraphic, and biogeographic evidence.
According to them, chasmosaurines originated in Laramidia around 80–90 million years ago and dispersed throughout much of this landmass by 77 million years ago.
By this time, a barrier had appeared in the area of Utah and Colorado, preventing animals dispersing between north and south Laramidia and leading to northern and southern chasmosaurines (and probably other animal clades) evolving independently through vicariance within separate, latitudinally arrayed centers of endemism 77.0–75.8 million years ago.
After the barrier dissolved around 75.7 million years ago, the Kosmoceratops lineage (represented by Vagaceratops) that had been restricted to southern Laramidia dispersed to the north, giving rise to all later chasmosaurines, such as Anchicerat
Though the area of Laramidia was only 20 percent that of modern North America, it saw a major evolutionary radiation of dinosaurs, including the common hadrosaurs and ceratopsians. It has been postulated that there was a latitudinal array of dinosaur "provinces" or biomes on Laramidia during the Campanian and Maastrichtian ages of the Late Cretaceous, the boundary lying around modern northern Utah and Colorado; the same major clades are known from the north and south but are distinct from each other at the genus and species levels. This hypothesis has been challenged; one argument claims that northern and southern dinosaur assemblages during this time were not coeval but reflect a taxonomic distribution over time, which gives the illusion of geographically isolated provinces, and that the distinct assemblages may be an artifact of sampling bias between geological formations. Due to a lack of well-dated fossils from southern Laramidia, this idea had been difficult to test, but discoveries in the Kaiparowits Formation have increased knowledge of fossil vertebrates from the region during the Late Cretaceous. The evolutionary radiation of ceratopsids appears to have been restricted both in time and geographically (the turnover of species was high, and each existed for less than a million years), most taxa being known from latest Cretaceous sediments in the Western Interior Basin, therefore appearing to have originated and diversified on Laramidia.
The discovery of Kosmoceratops and Utahceratops greatly increased the number of known chasmosaurines from the Western Interior Basin. Sampson and colleagues viewed this as the strongest support for the idea of dinosaur "provinces" in their 2010 description. They pointed out that in contrast to the Maastrichtian, the preceding Campanian stage had a better sampled, diverse, and far-ranging dinosaur assemblage, as well as more precise geographical and stratigraphical data. The stratigraphic ranges of Kosmoceratops and Utahceratops show that they lived at the same time and likely in the same ecosystems, which was rare among ceratopsids. According to Sampson and colleagues, geochronologic dating indicates that the brief geological interval preserved within the Kaiparowits Formation was contemporaneous with the Dinosaur Park Formation in Alberta and followed other formations in the north (the upper Judith River and Two Medicine formations) and southeast (the Fruitland and Aguja formations), with the brief duration, overlap in time, and longitudinal span permitting significant comparisons between them. That Kosmoceratops and Utahceratops were not closely related to each other or to Chasmosaurus and Mojoceratops from the coeval Dinosaur Park Formation, and that Vagaceratops from Alberta overlapped with Pentaceratops from New Mexico in time, were cited by Sampson and colleagues as evidence against the claim that northern and southern dinosaur assemblages were not coeval during this time.
Sampson and colleagues stated that their study was the first time intracontinental endemism within dinosaurs was documented (with distinct chasmosaurine taxa co-occurring north and south on Laramidia). They suggested a sequence of events in the evolution of chasmosaurines that they found consistent with the phylogenetic, stratigraphic, and biogeographic evidence. According to them, chasmosaurines originated in Laramidia around 80–90 million years ago and dispersed throughout much of this landmass by 77 million years ago. By this time, a barrier had appeared in the area of Utah and Colorado, preventing animals dispersing between north and south Laramidia and leading to northern and southern chasmosaurines (and probably other animal clades) evolving independently through vicariance within separate, latitudinally arrayed centers of endemism 77.0–75.8 million years ago. After the barrier dissolved around 75.7 million years ago, the Kosmoceratops lineage (represented by Vagaceratops) that had been restricted to southern Laramidia dispersed to the north, giving rise to all later chasmosaurines, such as Anchicerat
Kosmoceratops