Editing Triceratops (section)

====''Torosaurus'' as growth stage of ''Triceratops''====
{{Main|Torosaurus}}
''[[Torosaurus]]'' is a ceratopsid genus first identified from a pair of skulls in 1891, two years after the identification of ''Triceratops'' by Othneil Charles Marsh. The genus ''Torosaurus'' resembles ''Triceratops'' in geological age, distribution, anatomy, and size, so it has been recognised as a close relative.<ref name="Farke-2006">{{Cite book |title=Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs |last=Farke |first=A. A. |date=2006 |publisher=Indiana University Press |isbn=978-0-253-34817-3 |editor-last=Carpenter |editor-first=K. |location=Bloomington |pages=235–257 |chapter=Cranial osteology and phylogenetic relationships of the chasmosaurine ceratopsid ''Torosaurus latus''}}</ref> Its distinguishing features are an elongated skull and the presence of two ovular fenestrae in the frill. Paleontologists investigating dinosaur [[ontogeny]] in Montana's [[Hell Creek Formation]] have recently presented evidence that the two represent a single genus.

[[File:Torosaurus and Triceratops.tif|thumb|upright|A, ''Triceratops prorsus'' holotype YPM 1822 and B, ''Torosaurus latus'' ANSP 15192]]
John Scannella, in a paper presented in [[Bristol]] at the conference of the [[Society of Vertebrate Paleontology]] (September 25, 2009), reclassified ''Torosaurus'' as especially mature ''Triceratops'' individuals, perhaps representing a single sex. Horner, Scannella's mentor at Bozeman Campus, [[Montana State University]], noted that ceratopsian skulls consist of metaplastic bone. A characteristic of metaplastic bone is that it lengthens and shortens over time, extending and resorbing to form new shapes. Significant variety is seen even in those skulls already identified as ''Triceratops'', Horner said, "where the horn orientation is backwards in juveniles and forward in adults". Approximately 50% of all subadult ''Triceratops'' skulls have two thin areas in the frill that correspond with the placement of "holes" in ''Torosaurus'' skulls, suggesting that holes developed to offset the weight that would otherwise have been added as maturing ''Triceratops'' individuals grew longer frills.<ref name="growth09">{{cite web|url=https://www.sciencedaily.com/releases/2009/10/091031002314.htm|title=New Analyses Of Dinosaur Growth May Wipe Out One-third Of Species|date=October 31, 2009|work=Science News|publisher=ScienceDaily.com|access-date=November 3, 2009|archive-date=February 5, 2019|archive-url=https://web.archive.org/web/20190205024226/https://www.sciencedaily.com/releases/2009/10/091031002314.htm|url-status=live}}</ref> A paper describing these findings in detail was published in July 2010 by Scannella and Horner. It formally argues that ''Torosaurus'' and the similar contemporary ''[[Nedoceratops]]'' are synonymous with ''Triceratops''.<ref name="ScanHorn2010"/>

The assertion has since ignited much debate. Andrew Farke had, in 2006, stressed that no systematic differences could be found between ''Torosaurus'' and ''Triceratops'', apart from the frill.<ref name="Farke-2006"/> He nevertheless disputed Scannella's conclusion by arguing in 2011 that the proposed morphological changes required to "age" a ''Triceratops'' into a ''Torosaurus'' would be without precedent among ceratopsids. Such changes would include the growth of additional [[epoccipital]]s, reversion of bone texture from an adult to immature type and back to adult again, and growth of frill holes at a later stage than usual.<ref name="AF2011"/> A study by Nicholas Longrich and Daniel Field analyzed 35 specimens of both ''Triceratops'' and ''Torosaurus''. The authors concluded that ''Triceratops'' individuals too old to be considered immature forms are represented in the fossil record, as are ''Torosaurus'' individuals too young to be considered fully mature adults. The synonymy of ''Triceratops'' and ''Torosaurus'' cannot be supported, they said, without more convincing intermediate forms than Scannella and Horner initially produced. Scannella's ''Triceratops'' specimen with a hole on its frill, they argued, could represent a diseased or malformed individual rather than a transitional stage between an immature ''Triceratops'' and mature ''Torosaurus'' form.<ref name="longrichfieldstudy">{{Cite journal |last1=Longrich |first1=Nicholas R. |last2=Field |first2=Daniel J. |date=February 29, 2012 |title=Torosaurus Is Not Triceratops: Ontogeny in Chasmosaurine Ceratopsids as a Case Study in Dinosaur Taxonomy |journal=PLOS ONE |volume=7 |issue=2 |pages=e32623 |doi=10.1371/journal.pone.0032623 |pmid=22393425 |pmc=3290593 |issn=1932-6203|bibcode=2012PLoSO...732623L |doi-access=free }}</ref><ref name="bbcTriNotToro">{{Cite news |url=https://www.bbc.co.uk/news/science-environment-17192624 |title=Triceratops and Torosaurus dinosaurs 'two species, not one' |last=Bowdler |first=Neil |date=March 1, 2012 |work=BBC News |access-date=July 29, 2013 |language=en-GB |archive-date=March 15, 2013 |archive-url=https://web.archive.org/web/20130315215330/http://www.bbc.co.uk/news/science-environment-17192624 |url-status=live }}</ref>