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Torosaurus or Triceratops? A question of three horns.

By: tyrannoraptora

Ontogeny, the process in which an animal develops and grows, is a part of every day life in nature. It can also be a palaeontologists bane, sparking fierce debates on the validity of such genera as Nanotyrannus. Ontogeny has also has recently surrounded Torosaurus in a hailstorm of controversy. Torosaurus was a herbivorous dinosaur, and like Nanotyrannus, lived in the Late Cretaceous period, approximately 70 million years ago. Torosaurus was around 9m long, weighing in at up to 6 tonnes. Torosaurus’ skull is one of the largest skulls of any known animal (almost a third as long as its entire body). The original type specimen of Torosaurus (meaning ‘perforated lizard’, after the two fenestrae, holes, in its frill) was discovered in 1891, and up until relatively recently, there was no controversy surrounding the genus. 

 That was until relatively recently, in 2009 when John Scannella (under the provision of his mentor, the almost legendary palaeontologist, Jack Horner) suggested that Torosaurus was not a valid genus, but rather a mature (almost elderly) form of Triceratops.

Scannella noted that around half of all known juvenile Triceratops possess two circular bone depressions on the frill, which corresponds to the two holes (fenestrae) that make Torosaurus recognisable form Triceratops. Scannella is adamant that this, along with the knowledge that to date, no juvenile specimens of Torosaurus have been found, allows for Torosaurus to be classed as a mature specimen of the genus Triceratops. In 2011, Scannella and Horner produced another paper, with yet more evidence that seemed to seal Torosaurus’ fate as an invalid genus. The paper dealt with a separate ceratopsid, Nedoceratops hatcheri, which Scannella argued was an intermediate growth stage between Triceratops and Torosaurus (i.e., Nedoceratops was an invalid genus also, to be reclassified as a species in the genus Triceratops).

 Later in 2011, Scannella’s idea was challenged by Andrew Farke, a world-renowned ceratopsid expert, who, after re-examining the only fossil (a skull) of N. hatcheri to date, validated (albeit tentatively) that there is sufficient morphological differences that are unique to N. hatcheri to make it, and Torosaurus both distinct and valid genera.

 In late February 2012, a seemingly conclusive paper by Nicholas Longrich and Daniel Field, titled ‘Torosaurus Is Not Triceratops’, refuelled the debate. In this study, the results gained show that not all specimens of Torosaurus are more mature than specimens of Triceratops, with some Torosaurus specimens being classed a juveniles. If this were the case, it would certainly disprove Scannella’s theory that Torosaurus is an elderly Triceratops. The paper provides more tantalising evidence that could disprove Scannella’s work. Longrich & Field noted that the areas of bone depression seen on Triceratops differ in shape and position to the fenestrae of Torosaurus, which in their minds does not show any sort of intermediate relationship between the two. Much like with Nanotyrannus, this debate is far from being solved, and just one new discovery could blow the debate wide open all over again.

At time of writing (April 2012), I can’t call this argument, there’s simply not enough evidence. In my heart of hearts, however, I believe Torosaurus to be a valid genus, there seems to be a lot of morphological change to undergo for a Trike to become a Torosaurus at old age. Whilst seeming unlikely, it’s far from being in the realm of impossibility. Who knows what the future holds for Torosaurus, but one things for sure, this debates are going to get better with age. Like a fine, rather dinosaur-like, wine.

References:

  •  Scannella, J. B. & Horner, J. R. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): Synonymy Through Ontogeny. Journal of Vertebrate Paleontology 30, p.1157-1168 (2010).
  •  Scannella, J. B. & Horner, J. R. Nedoceratops: An Example of Transitional Morphology. PLoS ONE 6, e28705 (2011).
  • Longrich N. R. & Field D. J. Torosaurus is not Triceratops: Ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy”. PLoS ONE 7, e32623 (2012).