I recently run into a video that talks very vaguely about how Torosaurus and Triceratops may be the same dinosaur. I'm a little skeptical because the video was very click-baity. What do you know on the topic and what's your opinion on it?
Ohhhh boy. Toroceratops. Here we go. Long post; TL;DR at the bottom.
Youâll probably know that Jack Horner is a palaeontologist with many theories. Some of them are true. Some of them are ambiguous. Some are just plain wrong. Toroceratops falls into the second category.Â
(Modified from Horner & Scannella, 2010)
In 2010, Horner & Scannella published a paper highlighting numerous similarities between Triceratops (A) and Torosaurus latus (B). Youâll probably notice that they really donât look that similar. Torosaurus proportionally has a longer frill, and, although it isnât immediately apparent from this pair, longer squamosal (brow) horns than Triceratops too.
Horner disagrees. He and Scannella suggested that these differences are merely ontogenic - in other words, Torosaurus is essentially post-puberty Triceratops, and puberty entails longer frills and longer horns. The analysis was based on 29 Triceratops skulls and 9 Torosaurus skulls.Â
(Horner & Scannella, 2010)
They supported their argument with a plot showing a clear relation between squamosal horn length and age (reproduced above), horn cores from Triceratops that are porous in structure (indicating that the horns are still growing), an assertion that the frill is made of metaplastic bone (which is easily molded and reshaped during ontogeny), evidence of thin spots in the frills of Triceratops corresponding to fenestrae in Torosaurus, and the taxonomic proximity of the two taxa.
(Modified from Longrich & Field, 2012)
Then, for the hell of it, they synonymized Torosaurus with Triceratops.
Horner and Scannella also stated that high subadult mortality explained the rarity of Torosaurus (if many subadults survived, youâd expect a lot of Torosaurus in the fossil record), and, in a follow-up paper in 2011, that Nedoceratops, another ceratopsian, made up for the lack of transitional ontogenic stages between Torosaurus and Triceratops.
Naturally, people have various counterarguments.
Longrich & Field (2012)Â analyzed the growth stages of various ceratopsians and found that they generally followed the following pattern:
[âŠ] development of cranial ornament occurs in juveniles, followed by fusion of the skull roof in subadults, and finally, the epoccipitals, epijugals, and rostral fuse to the skull in adults.
(Both of these are Torosaurus. Longrich & Field, 2012)
Surprise, Triceratops, Nedoceratops, and Torosaurus donât fit that pattern. Both Torosaurus and Triceratops also exhibit ontogenic stages and irregularities of their own (shown above), some Triceratops have bone just as mature as Torosaurus, and the depressions in the frill of Triceratops supposedly corresponding to the fenestrae in Torosaurus had the wrong shape and position.Â
They did concede, though, that a large number of Triceratops are interestingly subadult. They proposed that Triceratops retained subadult morphology to an unusually old age among its relatives.
(Nedoceratops. Farke 2011)
Farke (2011) contested Nedoceratops as an ontogenic stage of Triceratops. The texture of the bone and the arrangement of the frill ossifications indicates that it is an old adult, its lack of a nose horn is decidedly unique, and for Nedoceratops to represent an ontogenic stage between Triceratops and Torosaurus it would have to undergo morphological changes that are completely unknown in other ceratopsids.
Farke & Maiorino (2013) conducted a morphospace analysis of Triceratops, Nedoceratops, and Torosaurus. While Triceratops horridus and Triceratops prorsus generally occupied the same morphospace, they found that Torosaurus occupied a different morphospace than Triceratops altogether, even when differences in frill shape are considered. There was also little evidence that Nedoceratops was a Triceratops-Torosaurus intermediate either.
(Two of Farke & Maiorinoâs morphospace analyses, respectively form skull and squamosal shape (src). Modified from Farke & Maiorino 2013)
Then thereâs the problem of the second species of Torosaurus. Horner only proposed that T. latus was a junior synonym of Triceratops, not the more southerly To. utahensis. Thatâs mainly because the material for To. utahensis is, to say the least, crap. It was referred to Torosaurus solely on the basis of its elongated squamosals, but the isolated and fragmentary nature of its remains makes it hard to determine if it fits the Toroceratops hypothesis in any way. It could be Torosaurus, it could be Triceratops, it could be something completely different. Nobody knows, and this was something that even Horner conceded. Farke and Maiorino were inconclusive too - notice in the above right plot that To. utahensis isnât well-separated from either Triceratops or Torosaurus.
TL;DR We really donât have a definite consensus on the Toroceratops hypothesis yet. Horner makes some sound arguments, the opposition makes lots of sound arguments, and everything is complicated by possible preservational bias and inter-individual variation.TL;DR TL;DRÂ ÂŻ\_(ă)_/ÂŻ
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