Two clades (groups consisting of all taxa that share a common ancestor) of ceratopsid dinosaurs—Centrosaurinae and Chasmosaurinae—are recognized based mainly on the elaborate ornamentation of their skull roofs.
In 2010, Sampson and colleagues placed Kosmoceratops in the latter group due to the premaxilla having a narial strut and a triangular process, as well as the presence of an elongated squamosal bone.
Their phylogenetic analysis found Kosmoceratops to be the sister taxon to Vagaceratops, in a clade grouped with derived chasmosaurines from the latest Campanian and Maastrichtian stages of the Late Cretaceous epoch including Triceratops, but not closely related to the more basal (or "primitive") Chasmosaurus.
In 2011, paleontologist Jordan C. Mallon and colleagues also found support for the clade containing Kosmoceratops and Vagaceratops, as did paleontologists Steven L. Wick and Thomas M. Lehman in 2013.

In 2011 and 2014, Longrich found the sole species of Vagaceratops to be retained in Chasmosaurus, where it had been placed originally (as C. irvinensis), while Kosmoceratops did not cluster closely with other taxa.
In his 2015 article that failed peer review, Longrich argued that the similarity between the neck frills of the two was due to convergent evolution, stating that the ten epiossifications on the back of the frill of C. irvinensis (or Vagaceratops, which he by then conceded could be a distinct genus) were located on the parietal bones but were located on both the parietal and squamosal bones in Kosmoceratops (Sampson and colleagues stated that they were located on the parietal and squamosals in both).
In a 2014 Master's thesis, Campbell stated that Sampson and colleagues had initially identified all ten epiossifications on the hind margin of the frill of the adult Kosmoceratops as epiparietals, before realizing that only six of them were epiparietals after examining the subadult specimen, where the sutures between the parietal and squamosal bones were more evident (sutures fuse and become less apparent with age).
Sampson and colleagues had therefore also reinterpreted the pattern of epiossifications in Vagaceratops similarly, but Campbell suggested that it had four or five epiparietals instead, therefore finding it to be the sister taxon to Chasmosaurus rather than Kosmoceratops.
Campbell and colleagues also found Vagaceratops to be more closely related to Chasmosaurus than Kosmoceratops in 2016, and in 2019, they considered V. irvinensis to be a species of Chasmosaurus.

In a 2011 conference abstract, paleontologist Denver W. Fowler and colleagues suggested that many ceratopsid genera were merely ontogenetic morphs (or growth stages) of other genera and that Kosmoceratops was an immature stage of C. irvinensis (with the prominent skull ornamentation reducing with age).
In 2015, paleontologists Caleb M. Brown and Donald M. Henderson tested a new scheme for homology between epiossifications in different chasmosaurines (which of these that corresponded to each other between taxa), and found the clade containing Kosmoceratops and Vagaceratops to be the sister taxon of all other chasmosaurines, contrary to earlier studies.
In a 2016 popular book, paleontologist Gregory S. Paul suggested that Kosmoceratops and Vagaceratops were not distinct enough from Chasmosaurus to warrant their own genus and that their species should be placed in it (or alternatively that the two should be united under Kosmoceratops), while also synonymizing several other chasmosaurine genera with Chasmosaurus.
Most ceratopsid genera are generally considered monotypic (containing a single species).

In 2016, Mallon and colleagues found Kosmoceratops and Vagaceratops to form a clade, with the new genus Spiclypeus as sister taxon.
While they acknowledged that some researchers had grouped Vagaceratops with Chasmosaurus instead, they found the forward-curled epiossifications at the back of the frill very distinctive, suggesting that Spiclypeus provided clues to explain the evolution of this feature.
They suggested that the curving of these epiossifi
Two clades (groups consisting of all taxa that share a common ancestor) of ceratopsid dinosaurs—Centrosaurinae and Chasmosaurinae—are recognized based mainly on the elaborate ornamentation of their skull roofs. In 2010, Sampson and colleagues placed Kosmoceratops in the latter group due to the premaxilla having a narial strut and a triangular process, as well as the presence of an elongated squamosal bone. Their phylogenetic analysis found Kosmoceratops to be the sister taxon to Vagaceratops, in a clade grouped with derived chasmosaurines from the latest Campanian and Maastrichtian stages of the Late Cretaceous epoch including Triceratops, but not closely related to the more basal (or "primitive") Chasmosaurus. In 2011, paleontologist Jordan C. Mallon and colleagues also found support for the clade containing Kosmoceratops and Vagaceratops, as did paleontologists Steven L. Wick and Thomas M. Lehman in 2013.
In 2011 and 2014, Longrich found the sole species of Vagaceratops to be retained in Chasmosaurus, where it had been placed originally (as C. irvinensis), while Kosmoceratops did not cluster closely with other taxa. In his 2015 article that failed peer review, Longrich argued that the similarity between the neck frills of the two was due to convergent evolution, stating that the ten epiossifications on the back of the frill of C. irvinensis (or Vagaceratops, which he by then conceded could be a distinct genus) were located on the parietal bones but were located on both the parietal and squamosal bones in Kosmoceratops (Sampson and colleagues stated that they were located on the parietal and squamosals in both). In a 2014 Master's thesis, Campbell stated that Sampson and colleagues had initially identified all ten epiossifications on the hind margin of the frill of the adult Kosmoceratops as epiparietals, before realizing that only six of them were epiparietals after examining the subadult specimen, where the sutures between the parietal and squamosal bones were more evident (sutures fuse and become less apparent with age). Sampson and colleagues had therefore also reinterpreted the pattern of epiossifications in Vagaceratops similarly, but Campbell suggested that it had four or five epiparietals instead, therefore finding it to be the sister taxon to Chasmosaurus rather than Kosmoceratops. Campbell and colleagues also found Vagaceratops to be more closely related to Chasmosaurus than Kosmoceratops in 2016, and in 2019, they considered V. irvinensis to be a species of Chasmosaurus.
In a 2011 conference abstract, paleontologist Denver W. Fowler and colleagues suggested that many ceratopsid genera were merely ontogenetic morphs (or growth stages) of other genera and that Kosmoceratops was an immature stage of C. irvinensis (with the prominent skull ornamentation reducing with age). In 2015, paleontologists Caleb M. Brown and Donald M. Henderson tested a new scheme for homology between epiossifications in different chasmosaurines (which of these that corresponded to each other between taxa), and found the clade containing Kosmoceratops and Vagaceratops to be the sister taxon of all other chasmosaurines, contrary to earlier studies. In a 2016 popular book, paleontologist Gregory S. Paul suggested that Kosmoceratops and Vagaceratops were not distinct enough from Chasmosaurus to warrant their own genus and that their species should be placed in it (or alternatively that the two should be united under Kosmoceratops), while also synonymizing several other chasmosaurine genera with Chasmosaurus. Most ceratopsid genera are generally considered monotypic (containing a single species).
In 2016, Mallon and colleagues found Kosmoceratops and Vagaceratops to form a clade, with the new genus Spiclypeus as sister taxon. While they acknowledged that some researchers had grouped Vagaceratops with Chasmosaurus instead, they found the forward-curled epiossifications at the back of the frill very distinctive, suggesting that Spiclypeus provided clues to explain the evolution of this feature. They suggested that the curving of these epiossifi
Kosmoceratops