Achelousaurus
Achelousaurus | |
---|---|
Skull of holotype specimen MOR 485 (with reconstructed parts in blue-gray), at the Museum of the Rockies, Montana
| |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | †Ornithischia |
Clade: | †Ceratopsia |
Family: | †Ceratopsidae |
Subfamily: | †Centrosaurinae |
Tribe: | †Pachyrhinosaurini |
Clade: | † Pachyrostra
|
Genus: | †Achelousaurus Sampson , 1994
|
Species: | †A. horneri
|
Binomial name | |
†Achelousaurus horneri Sampson, 1994
|
Achelousaurus (
A large centrosaurine, Achelousaurus supposedly was about 6 m (20 ft) long, with a weight of about 3 t (3.3 short tons). As a ceratopsian, it walked on all fours, had a short tail and a large head with a hooked beak. It had a bony neck-frill at the rear of the skull, which sported a pair of long spikes, which curved towards the outside. Adult Achelousaurus had rough bosses (roundish protuberances) above the eyes and on the snout where other centrosaurines often had horns in the same positions. These bosses were covered by a thick layer of keratin, but their exact shape in life is uncertain. Some researchers hypothesize that the bosses were used in fights, with the animals butting each other's heads, as well as for display.
Within the
History of discovery
Horner's expeditions to Landslide Butte
All known Achelousaurus specimens were recovered from the
During the field season of 1987 (early July), volunteer Sidney M. Hostetter located another horned dinosaur skull near the Canyon Bone Bed, specimen MOR 485.
Interpretation of the collected fossils
It was initially assumed that all the horned dinosaur material recovered by the expeditions could be assigned to a single "styracosaur" species distinct from
Horner, an expert on the
Meanwhile, Horner had come to a more complex view of the situation. He still thought that the fossil material had been part of a single population but concluded that this had developed over time as a
Sampson names Achelousaurus
Sampson had continued his studies of the material since 1989. In 1994, in a talk during the annual meeting of the
The holotype specimen MOR 485 was collected by Hostetter and Ray Rogers
Possible Achelousaurus finds
In addition to fossils that have been unequivocally assigned to Achelousaurus, some other material has been found of which the identity is uncertain. A
Description
General build
Achelousaurus is estimated to have been 6 m (20 ft) long with a weight of 3 t (3.3 short tons).
As a ceratopsid, Achelousaurus would have been a
Distinguishing traits
In 1995, when describing the species, Sampson gave a formal list of four traits that distinguish Achelousaurus from its centrosaurine relatives. Firstly, adult individuals have nasal bones with a boss on top that is relatively small and thin, and heavily covered with pits; secondly, adult individuals do not have true horns above the eye sockets but relatively large bosses with high ridges; thirdly, not yet fully grown individuals, or subadults, have true horncores (the bony part of the horns) above the eye sockets with the inward facing surface being concave; and fourthly, the parietal bones of the neck shield have a single pair of curved spikes sticking out from the rear margin to behind and to the outside.[22]
Besides these unique characteristics, Sampson pointed out additional differences with two very closely related forms. The frill spikes of Achelousaurus are more outwards oriented than the spikes of Einiosaurus, which are medially curved; the spikes of Achelousaurus are nevertheless less directed to the outside than the comparable spikes of Pachyrhinosaurus. Achelousaurus also differs from Pachyrhinosaurus in its smaller nasal boss that does not reach the frontal bones at its rear. Apart from the skull, no features of the skeleton are known that distinguish Achelousaurus from other members of the Centrosaurinae.[22]
Skull
Horned dinosaurs mainly differ from each other in their horns, which are located on the snout and above the eyes, and in the large skull frill, which covers the neck like a shield. Achelousaurus exhibited the build of
Adult Achelousaurus skulls had a rugose, heavily pitted boss on the snout or nasal region, where many other ceratopsids had a horn.
Adult skulls also possess large, rugose, and oval bosses on the supraorbital region above the eyes, instead of the horns of other ceratopsids. The supraorbital bosses extended from the
The skull roof of Achelousaurus had a midline cavity, with an opening at the top called the frontal
Like that of all other ceratopsids, the skull of Achelousaurus had a parietosquamosal frill or "neck shield", which was formed by the parietal bones at the rear and the squamosal bones at the sides. The parietal is one of the main bones used to distinguish centrosaurine taxa from each other and resolve relationships between them, whereas the squamosal is very similar across taxa.
The frill of Achelousaurus had two conspicuous large spikes that were directed backwards and curved to the sides away from each other. During the 1990s, it was increasingly understood that such spikes on the parietals were not random growths but specific traits that could be used to determine the evolution of horned dinosaurs, if only it could be analyzed how they corresponded among species. Sampson, in the paper describing Achelousaurus in 1995, therefore introduced a generalized numbering system for such parietal processes, counting them from the midline to the side of the frill.[22] This was applied to the Centrosaurinae as a whole in 1997.[48] The large spikes of Achelousaurus correspond to "Process 3" spikes of other centrosaurines and were similar to those of Einiosaurus, though curved more to the sides, similar to Pachyrhinosaurus.[22] They were shorter and thinner than the corresponding spikes of Styracosaurus.[49] Between these spikes, on both sides of the central frill notch, were two small tab-like processes ("Process 2") that were directed towards the midline.[22] Innermost "Process 1" spikes, as present in Centrosaurus, are lacking with Achelousaurus.[49] The frill had two large paired openings, the parietal fenestrae, with a midline parietal bar between them. A linear row of rounded swellings ran along the top of the parietal bar, which may be homologous to the spikes and horns in the same area of some Pachyrhinosaurus specimens. A row of relatively small processes ran along the parietal shield margin from the "Process 3" spikes outwards,[22] for a total per side of seven. They were largely equal in size, causing the P4 process to be reduced in comparison to the P3.[50] These lower processes appear to have been capped by epoccipitals, bones that lined the frills of ceratopsids.[22] In Achelousaurus these epoccipitals, which start as separate skin ossifications or osteoderms, fuse with the underlying frill bone to form spikes,[51] at least in the third position.[52] In 2020, it was denied that these processes were separate ossifications. In the most mature individuals, the front-most P6 and P7 processes would be less imbricated relative to each other, rotated around their longitudinal axes.[50]
Keratin sheaths
The bosses on the skull of Achelousaurus may have been covered in a
Evolution
Horner's hypothesis of anagenesis
In 1992, the study by Horner
The horned dinosaurs discovered by Horner exemplified this phenomenon. In the lowest layers of the Two Medicine Formation, 60 m (200 ft) below the overlaying Bearpaw Formation, "Transitional Taxon A" was present. It seemed to be identical to Styracosaurus albertensis, differing from it only in the possession of just a single pair of parietal spikes. The middle layers, 45 m (150 ft) below the Bearpaw, contained "Transitional Taxon B" that also had a single spike pair but differed in the form of its nasal horn that curved to the front over the anterior branches of the nasal bones. In the upper strata, 20 m (65 ft) below the Bearpaw, "Transitional Taxon C" had been excavated. It too had a spike pair but now the nasal horn was fused with the front branches. The upper surface of the horn was elevated and very rough. The orbital horns showed coarse ridges. Subsequently, "Taxon A" was named Stellasaurus,[9][50] "Taxon B" became Einiosaurus, while "Taxon C" became Achelousaurus.[23] In 1992, Horner et al. did not name these as species for the explicit reason that the entire evolutionary sequence was seen as representing a grade of transitional ceratopsians between Styracosaurus albertensis, known from the Judith River Formation, and the derived, hornless Pachyrhinosaurus from the Horseshoe Canyon Formation, which had the spike pair and bosses on the nose and above the eyes, as well as additional frill ornamentation.[23] In 1997, Horner referred to the three taxa as "centrosaurine 1.", "centrosaurine 2." and "centrosaurine 3.".[54]
Horner thought he had found the mechanism driving this evolution, elaborating on ideas he had developed even before he had investigated Landslide Butte.
Horner did not perform an exact
Basing himself on revised data, Sampson in 1995 estimated that the layers investigated represented a longer period of time than the initially assumed 500,000 years: after the deposition of Gilmore's Brachyceratops quarry, 860,000 years would have passed, and after the Einiosaurus beds 640,000 years, until the maximal extent of the Bearpaw transgression. He did not adopt Horner's hypothesis of anagenesis but assumed speciation took place, with the populations splitting. These time intervals were still short enough to indicate that the rate of speciation must have been high, which might have been true of all centrosaurines of the late Campanian.[22]
In 1996, Dodson raised two objections to Horner's hypothesis. Firstly, the possession of just one pair of main spikes seemed more basal than the presence of three pairs, as with Styracosaurus albertensis. This suggested to him that the Einiosaurus–Achelousaurus lineage was a separate branch within the Centrosaurinae. Secondly, he was concerned that Einiosaurus and Achelousaurus were a case of sexual dimorphism, one type being the males, the other the females. This would be suggested by the short geological time interval between the layers their fossils had been found in, which was estimated by him at about 250,000 years. But if the hypothesis were true, it would be perhaps the best example of fast evolution in the Dinosauria.[15]
In 2010, Horner admitted that specimen TMP 2002.76.1 seemed to indicate that Achelousaurus was not descended from Einiosaurus, as it preceded both in age, and yet had a nasal boss. But he stressed that even if the lineages split off, its ancestor might have resembled Einiosaurus. Furthermore, it might still be possible that Einiosaurus was a direct descendant of Rubeosaurus. Also, the process of rapid displacements and extinctions of species could in his opinion still be elegantly explained by a westward expansion of the Bearpaw Sea.[8]
The process of anagenesis was affirmed by John Wilson and Jack Scannella in 2016, who studied the ontogenetic changes in horned dinosaurs. They compared a small Einiosaurus specimen, MOR 456 8-8-87-1, with Achelousaurus specimen MOR 591. Both proved to be quite similar, with the main differences being a longer face in MOR 456 8-8-87-1, and a sharper supraorbital horncore in MOR 591. They concluded that Achelousaurus was likely the direct descendant of Einiosaurus. The more adult Einiosaurus individuals approached the Achelousaurus morphology. The differences between the two taxa would have been caused by heterochrony – differential changes in the speed the various traits developed during the lifetime of an individual.[31] Since Wilson and colleagues found in 2020 that Stellasaurus (Horner's "Taxon A") was intermediate between Styracosaurus and Einiosaurus in morphology and stratigraphy, they could not discount that it was a transitional taxon within an anagenetic lineage.[50]
Classification
In 1995, Sampson formally placed Achelousaurus in the
Phylogeny
Sampson felt, in 1995, that there was not enough evidence to conclude that Achelousaurus was a direct descendant of Einiosaurus. Unlike Horner, he decided to perform a cladistic analysis to establish a
Subsequent studies have sought to determine the precise relationships within this part of the evolutionary tree, with conflicting results regarding the question whether Styracosaurus albertensis or Einiosaurus might have been in the direct line of ascent to Achelousaurus. In 2005, an analysis by Michael Ryan and Anthony Russell found Styracosaurus more closely related to Achelousaurus than to Centrosaurus.[64] This was confirmed by analyses by Ryan in 2007,[65] Nicholas Longrich in 2010,[66] and Xu et al. in 2010.[67] The same year Horner and Andrew T. McDonald moved Styracosaurus ovatus to its own genus, Rubeosaurus, finding it a sister species of Einiosaurus, while Styracosaurus albertensis was again located on the Centrosaurus branch. They also assigned specimen MOR 492, the basis of "Taxon A", to Rubeosaurus.[8] In 2011, a subsequent study by Andrew T. McDonald in this respect replicated the outcome of his previous one,[61] as did a publication by Andre Farke et al.[62] In 2017, J.P. Wilson and Ryan further complicated the issue, concluding that MOR 492 ("Taxon A") was not referable to Rubeosaurus and announcing that yet another genus would be named for it.[9] Wilson and colleagues moved MOR 492 to the new genus Stellasaurus in 2020, which therefore corresponds to "Taxon A". Their study found Rubeosaurus ovatus to be the sister species of Styracosaurus albertensis, and concluded Rubeosaurus to be synonymous with Styracosaurus.[50]
Before Achelousaurus was described, Pachyrhinosaurus canadensis had been considered a solitary
Cladistic analyses develop gradually, reflecting new discoveries and insights. Their results can be shown in a cladogram, with the relationships found ordered in an evolutionary tree. The cladogram below shows the phylogenetic position of Achelousaurus in a cladogram from Wilson and colleagues, 2020.[50]
Paleobiology
Function of skull ornamentation
In 1995, Sampson noted that earlier studies had found that the horns and frills of ceratopsians most likely had a function in
Hieronymus, in 2009, concluded that the nasal and supraorbital bosses were used for butting or ramming the head or the flank of a rival. The bone structure indicates that the bosses were covered by cornified pads as in modern muskoxen, suggesting dominance fights similar to those of members of the
Previously it had been suggested that the fusion of the first three neck vertebrae, such as seen in the mature specimen MOR 571, might have been a paleopathology, an instance of the disease spondyloarthropathy, but in 1997 it was concluded that it was more likely a normal ontogenetic trait, the vertebrae growing together to form a so-called "syncervical" to support the heavy head.[71] All three main known specimens have syncervicals consisting of three fused neck vertebrae;[72][73] the trait could have been inherited from a smaller ancestor using a stiffer neck for burrowing or food acquisition.[74]
Social behavior
It has been claimed that ceratopsian dinosaurs were herding animals, due to the large number of known bone beds containing multiple members of the same ceratopsian species. In 2010, Hunt and Farke pointed out that this was mainly true for centrosaurine ceratopsians.[29] Horner assumed that the horned dinosaurs at Landslide Butte lived in herds which had been killed by drought or disease.[75] Dodson concluded that the fact that the Achelousaurus bone beds were monospecific (containing only one species) confirmed the existence of herds.[70]
Metabolism
There has long been debate about the
Paleoenvironment
Achelousaurus is known from the Two Medicine Formation, which preserves coastal sediments dating from the Campanian stage of the Late
The Two Medicine Formation is typified by a warm
More or less contemporary dinosaur genera of the area included
The exact composition of the fauna Achelousaurus was part of is uncertain, as its fossils have not been discovered in direct association with other taxa. Its intermediate anagenetic position suggests that Achelousaurus shared its habitat with forms roughly found in the middle or at the end of the time range of its formation. As with horned dinosaurs, Horner assumed he had found transitional taxa in other dinosaur groups of the Two Medicine Formation. One of these was a form in between
Other ceratopsians from the Two Medicine Formation include Einiosaurus and Stellasaurus. In addition, remains of other indeterminate and dubious centrosaurines, including Brachyceratops, are known from the formation and though they may represent younger stages of the three valid genera, this is not possible to demonstrate.
The indeterminate specimen TMP 2002.76.1 is from the Dinosaur Park Formation and, if it belongs to Achelousaurus, the genus would be the stratigraphically oldest known pachyrhinosaurine taxon.
See also
References
- ^ Creisler, B. (1999). "Beipiaosaurus and Caudipteryx pronunciation". Dinosaur Mailing List (Mailing list). Retrieved September 26, 2017.
This one has to be pronounced ak-e-LOH-uh-SAWR-us, NOT a-KEE-lo-SAWR-us, as I recall hearing. The name derives from Greek Akheloos (the two o's being pronounced separately), which Latin rules turn into Achelous (divided as a-che-lo-us, short e, long o and short u), again pronounced in four syllables with the accent on the next-to-last, that is the one with the long o. All sources I have checked indicate that ak-e-LOH-us is the accepted English pronunciation of the Latin name. Since the scientific name Achelousaurus was formed by arbitrarily combining Achelous and saurus instead of using the stem-form Achelo-, the "u" needs to pronounced.
- ^ Horner & Dobb 1997, pp. 53–75.
- ISBN 978-0-06-097314-8.
- ^ Horner & Dobb 1997, pp. 60–61.
- .
- ^ Horner & Dobb 1997, p. 64.
- ^ Horner & Dobb 1997, pp. 65–66.
- ^ a b c d e f g McDonald & Horner 2010, pp. 156–168.
- ^ a b c Wilson, J.P.; Ryan, M.P.; Evans, D.C. (August 23–26, 2017). "A new Centrosaurine Ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the Evolution of the 'Styracosaur' Dinosaurs". In Farke, A.; MacKenzie, A.; Miller-Camp, J. (eds.). Abstracts of Papers. Society of Vertebrate Paleontology: 77th Annual Meeting. Calgary, AB, Canada. p. 214. Archived from the original (PDF) on August 14, 2019. Retrieved October 2, 2017.
- ^ Horner & Dobb 1997, p. 82.
- ^ Horner & Dobb 1997, p. 84.
- ^ Horner & Dobb 1997, p. 96.
- ^ Horner & Dobb 1997, p. 103.
- ^ Horner & Dobb 1997, p. 104.
- ^ ISBN 978-0-691-62895-0.
- ^ Rogers 1989, p. 49.
- ^ Dodson, Forster & Sampson 2004, p. 496.
- ^ Horner & Dobb 1997, pp. 80–81.
- ^ Ryan, Holmes & Russell 2007, p. 944.
- ISBN 978-0-7924-5606-3.
- ISBN 978-0-520-25408-4.
- ^ .
- ^ S2CID 4283438.
- .
- ISBN 978-1-4081-1906-8.
- ISBN 978-0-87842-445-0.
- ^ a b c Ford 2010, p. 53.
- ^ Maiorino 2015, p. 210.
- ^ a b Hunt & Farke 2010, pp. 447–455.
- ^ Maiorino 2015, p. 334.
- ^ a b Wilson, J.P.; Scannella, J.B. (2016). "Comparative cranial osteology of subadult centrosaurine dinosaurs from the Two Medicine Formation, Montana". Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2016: 252.
- ^ Maiorino 2015, p. 365.
- ^ a b c d e f g h i Ryan et al. 2010, pp. 141–155.
- ^ McDonald & Farke 2011, p. 1.
- ^ a b Sampson & Loewen 2010, pp. 405–427.
- ^ Ryan, M.J. (2006). "The status of the problematic taxon Monoclonius (Ornithischia: Ceratopsidae) and the recognition of adult-sized dinosaur taxa". Geological Society of America Abstracts with Programs. 38 (4): 62.
- ^ McDonald & Farke 2011, p. 11.
- ^ ISBN 978-0-691-13720-9.
- ^ Currie, Langston & Tanke 2008, p. 17.
- ^ Currie, Langston & Tanke 2008, p. 25.
- ^ Maiorino 2015, p. 139.
- PMID 23864598.
- ^ a b McDonald & Farke 2011, p. 3.
- ^ Currie, Langston & Tanke 2008, p. 29.
- ^ Currie, Langston & Tanke 2008, p. 36.
- ^ Currie, Langston & Tanke 2008, p. 41.
- ^ Currie, Langston & Tanke 2008, p. 50.
- .
- ^ a b Ryan, Holmes & Russell 2007, p. 951.
- ^ PMID 32431910.
- ISBN 978-0-12-226810-6.
- ^ McDonald & Farke 2011, p. 6.
- ^ PMID 19711467.
- ^ Horner & Dobb 1997, p. 195.
- ^ Horner, J.R. (1984). "Three ecologically distinct vertebrate faunal communities from the Two Medicine Formation of Montana, with discussion of evolutionary pressures induced by interior seaway fluctuations". Montana Geological Society 1984 Field Conference and Symposium Guidebook: 299–303.
- ^ Horner & Dobb 1997, p. 193.
- ^ a b Horner & Dobb 1997, p. 198.
- ^ Horner & Dobb 1997, p. 196.
- ^ Horner & Dobb 1997, pp. 196–197.
- ^ .
- ^ a b McDonald & Farke 2011, p. 9.
- ^ .
- .
- S2CID 128478038.
- S2CID 130607301.
- S2CID 232343758.
- S2CID 128972108.
- ^ Currie, Langston & Tanke 2008, p. 16.
- PMID 23864598.
- ^ ISBN 978-0-12-226810-6.
- ISBN 978-0-253-35701-4.
- ^ VanBuren 2013, p. 53.
- ^ VanBuren 2013, p. 100.
- ^ VanBuren 2013, p. 80–84.
- ^ Horner & Dobb 1997, p. 66.
- JSTOR 3515240.
- ^ Rogers 1989, p. 68–71.
- ^ Retallack, G.J. (1997). Wolberg, D.L. (ed.). "Dinosaurs and dirt". Dinofest International: Proceedings of a Symposium Sponsored by Arizona State University, Academy of Natural Sciences: 345–359.
- PMID 23557203.
- JSTOR 3514834.
- ISBN 978-0-521-56723-7.
- ^ Brink et al. 2015, pp. 245–267.
- ^ Carr et al. 2017, p. 3.
- ^ Carr et al. 2017, p. 7.
- ^ Henderson 2010, pp. 293–307.
- .
Bibliography
- Brink, K. S.; Zelenitsky, D. K.; Evans, D. C.; Horner, J. R.; Therrien, F. (2015), "Cranial Morphology and Variation in Hypacrosaurus stebingeri (Ornithischia: Hadrosauridae)", in Eberth, D. A.; Evans, D. C. (eds.), Hadrosaurs, Indiana University Press, ISBN 978-0-253-01385-9
- Carr, Thomas D.; Varricchio, David J.; Sedlmayr, Jayc C.; Roberts, Eric M.; Moore, Jason R. (2017), "A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system", Scientific Reports, 7: 44942, PMID 28358353
- Currie, P. J.; Langston, W. Jr; Tanke, D. H. (2008), New Horned Dinosaur from an Upper Cretaceous Bone Bed in Alberta, Ottawa, Ontario: NRC Research Press, pp. 1–108, ISBN 978-0-660-19819-4
- Dodson, P.; Forster, C. A.; Sampson, S. D. (2004), "Ceratopsidae", in Weishampel, D. B.; Dodson, P.; Osmólska, H. (eds.), The Dinosauria (2 ed.), Berkeley: University of California Press, pp. 494–513, ISBN 978-0-520-25408-4
- Horner, J. R.; Dobb, E. (1997), Dinosaur Lives: Unearthing an Evolutionary Saga, San Diego, New York, London: Hartcourt Brace & Company
- Maiorino, L. (2015), Macroevolutionary pattern in Ceratopsian Dinosaurs (Dinosauria: Ornithischia) and biomechanics: an integrated approach by means of geometric morphometrics and finite element analysis (Doctoral Thesis), Università degli studi Roma Tre
- McDonald, A. T.; Farke, A. A. (2011), "A subadult specimen of Rubeosaurus ovatus (Dinosauria: Ceratopsidae), with observations on other Ceratopsids from the Two Medicine Formation", PLOS ONE, 6 (8): e22710, PMID 21853043
- Rogers, R. R. (1989), Taphonomy of three monospecific dinosaur bone beds in the Late Cretaceous Two Medicine Formation, northwestern Montana: Evidence for dinosaur mass mortality related to episodic drought (Master Thesis), University of Montana
- Ryan, M. J.; Holmes, R.; Russell, A. P. (2007), "A revision of the late campanian centrosaurine ceratopsid genus Styracosaurus from the Western Interior of North America", Journal of Vertebrate Paleontology, 27 (4): 944–962, S2CID 86218327
- Ryan, M. J.; Chinnery-Allgeier, B. J.; Eberth, D. A., eds. (2010), New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Indiana University Press, ISBN 978-0-253-35358-0
- Ford, T. L., "A Ceratopsian Compendium", in Ryan, Chinnery-Allgeier & Eberth (2010), Supplemental CD-ROM
- Henderson, D. M., "Skull shapes as indicators of niche partitioning by sympatric chasmosaurine and centrosaurine dinosaurs", in Ryan, Chinnery-Allgeier & Eberth (2010), pp. 293–307
- Hunt, R.; Farke, A., "Behavioral interpretations from ceratopsid bonebeds", in Ryan, Chinnery-Allgeier & Eberth (2010), pp. 447–455
- McDonald, A. T.; Horner, J. R., "New Material of "Styracosaurus" ovatus from the Two Medicine Formation of Montana", in Ryan, Chinnery-Allgeier & Eberth (2010), pp. 156–168
- Ryan, M. J.; Eberth, D. A.; Brinkman, D. B.; Currie, P. J.; Tanke, D. H., "A New Pachyrhinosaurus-Like Ceratopsid from the Upper Dinosaur Park Formation (Late Campanian) of Southern Alberta, Canada", in Ryan, Chinnery-Allgeier & Eberth (2010), pp. 141–155
- Sampson, S. D.; Loewen, M. A., "Unraveling a radiation: a review of the diversity, stratigraphic distribution, biogeography, and evolution of horned dinosaurs. (Ornithischia: Ceratopsidae)", in Ryan, Chinnery-Allgeier & Eberth (2010), pp. 405–427
- VanBuren, C. S. (2013), The Function and Evolution of the Syncervical in Ceratopsian Dinosaurs with a Review of Cervical Fusion in Tetrapods (Master Thesis), University of Toronto