Centrosaurinae

The classification of centrosaurines and the relationships among the various species is complicated by a wide degree of variation between individuals and growth stages. Some features that have traditionally been used to classify these dinosaurs, like the number and arrangement of frill ornaments or spikes, have been discovered to be more variable than previously thought. For example, the cladogram presented below follows a 2016 phylogenetic analysis by Chiba et al. (2017).[6] These authors treated the species Rubeosaurus ovatus as distinct from Styracosaurus albertensis, and recovered several distinct clades within Centrosaurini, which together formed a sister group to the Pachyrhinosaurini:

However, subsequent studies have cast doubt on the usefulness of minor variations in frill spike arrangement for classifying centrosaurines. In particular, large sample sizes of the species Centrosaurus apertus and Styracosaurus albertensis have shown a higher than predicted amount of variation. In 2020, Holmes et al. explored what the effect of recognizing such diversity would have on centrosaur classification. They used the same data as Chiba et al.'s 2017 study, but treated Rubeosaurus as a synonym of Styracosaurus, dropping it from their taxon list. The resulting cladogram (below) found Centrosaurini as a polytomy, a grouping with no discernable sister group relationships within it. The authors concluded that this meant the variation present within these species made it difficult to find any real resolution among them, and may even provide support for the hypothesis that centrosaurines evolved primarily via anagenesis (a single lineage changing through time) rather than cladogenesis (multiple branching lineages with shared common ancestors).[7]

Skull of an unnamed nasutoceratopsin exhibited in the Museo del Mamuth in Chihuahua City, from the municipality of Aldama.

Centrosaurine fossils have mostly been found in Western North America (Alberta, Montana, and Alaska).[1] In the United States, two taxa, Diabloceratops and Machairoceratops, have been found as far south as Utah. Yehuecauhceratops, a nasutoceratopsin from Coahuila, Mexico, is the southernmost occurrence of a centrosaurine in North America.[1] No centrosaurine fossils had been uncovered outside Western North America until the 2010 discovery of Sinoceratops in the Shandong Province of China.[8] However, some authors question the placement of Sinoceratops within Centrosaurinae. All other Late Cretaceous dinosaur groups from North America have also been found in Asia, so the initial absence of Asian centrosaurines had been surprising.[8] The current evidence suggests that Centrosaurinae originated in Laramidia 90-80 million years ago,[1] with the discovery of the oldest known centrosaurine, Menefeeceratops further proving this.[9][10] This means Sinoceratops would have migrated to China from North America.[5] Some hypothesize that centrosaurines originated in southern Laramidia and later radiated north.[11]

Compared to their sister group, Chasmosaurinae, centrosaurines are relatively small. The primitive Sinoceratops is an exception, with an estimated skull length of 180 centimetres (71 in).[8] By contrast, the skull length of Albertoceratops was more typical for this group at only 67 centimetres (26 in).[3] In general, centrosaurines were about the size of a rhinoceros with body lengths ranging from 2.5–8 metres (8.2–26.2 ft).[12]

Hypothesised ontogenic development of Pachyrhinosaurus perotorum

Possible neonate sized centrosaurine fossils have been documented in the scientific literature.[13] Research indicates that centrosaurines did not achieve fully developed mating signals until nearly fully grown.[14][2] Scott D. Sampson found commonality between the slow growth of mating signals in centrosaurines and the extended adolescence of animals whose social structures are ranked hierarchies founded on age-related differences.[14] In these sorts of groups, young males are typically sexually mature for several years before actually beginning to breed, when their mating signals are most fully developed.[15] Females, by contrast, do not have such an extended adolescence.[15]

• Timeline of ceratopsian research

• Sampson, S. D. (1995b). "Two new horned dinosaurs from the Upper Cretaceous Two Medicine Formation of Montana; with a phylogenetic analysis of the Centrosaurinae (Ornithischia: Ceratopsidae)". Journal of Vertebrate Paleontology. 15 (4): 743–760. doi:10.1080/02724634.1995.10011259.
• Sampson, S. D., 2001, Speculations on the socioecology of Ceratopsid dinosaurs (Orinthischia: Neoceratopsia): In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 263–276.
• Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206–218. In: Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.