Torosaurus latus model, WIP.
Torosaurus latus was an herbivorous ceratopsid, with a massive, 2.77 metre long frill. In life, it is thought to have reached approximately 7.6 to 9 meters in length, and weighed in at approximately 4 to 6 tons (Holtz, 2011). Torosaurus had two large fenestrae in its frill, long, dorsally concave squamosals, and ten or more epiparietals (small horns on the edges of the frill). Additionally, the nasal horn was rather short (Longrich and Field, 2012). While there is a second species of Torosaurus, though this species is slightly older and is not from Hell Creek - as such, it will not be featured.
Recently, the validity of Torosaurus latus has been disputed. Some authors have argued that Torosaurus latus is actually synonymous with Triceratops, representing the mature growth stage of the latter. Currently, no Torosaurus juveniles have been found, whereas a considerable number of juveniles have been uncovered for Triceratops. Additionally, one distinguishing trait of Triceratops is its short squamosals - these are absent in adult forms of other Chasmosaurines, making this a case of paedomorphosis. Some have argued that this fact is best explained by Triceratops and Torosaurus representing growth stages in the same species (Scanella, 2009).
A followup analysis of 38 skull specimens from Hell Creek (29 Triceratops, 9 Torosaurus) supported this hypothesis (Scanella and Horner, 2010). The authors placed particular emphasis on the fact that Ceratopsian frills are composed of metaplastic bone, which can lengthen and shorten with time, lengthening and resorbing to form new shapes. They also noted that there is already known to be considerable ontogenetic changes in the skull and frill morphology of Triceratops, with horn orientation changing from backward to forward-facing with age. Furthermore, approximately half of the Triceratops skulls had two thin areas in the frill that corresponded to the placement of the fenestrae in Torosaurus, which were surrounded by mature granular bone. They asserted that this change would help to reduce the weight of the frill as it continued to grow ever larger.
However, this synonymy is not without its problems. In their paper suggesting synonymy, Scanella and Horner also acknowledged that there is data that is not easily explained by the synonymy of the two genera. One such issue is the rarity of Torosaurus remains - if it does indeed represent the mature form of Triceratops, it would be expected to be far more common. However, they also noted that this could be due to higher mortality in subadults, as well as potential preservation biases - it may have been that the older animals preferred to live at higher elevations, where fossilization would be far less common due to erosion. Additionally, some analysis did seem to show the existence of authentic subadult specimens of Torosaurus, though they believed this was actually indicative of individual variation. They also noted the apparent lack of transitional forms showing the formation of the large fenestrae. To address this, they cited the contentious holotype of Nedoceratops as such a transitional form, explaining the problematic traits of the genus as being due to its transitional state. They also cited the variability in position and number of episquamosals within Triceratops as possibly being indicative of an increase in number with age, explaining the higher count found in Torosaurus.
Since the 2010 publication, other paleontologists have expressed their doubts as well. A 2011 paper noted that the characters in Nedoceratops that were originally interpreted as being indicative of a transitional form are actually pathological in nature (Farke, 2011), also noting that while there is individual variation in the number of episquamosals in many Ceratopsids, there are no known species in which this number changes with age. Furthermore, it was noted that in other Ceratopsids that possess large fenestrae, these holes are present even in very young juveniles, suggesting that formation of fenestrae is not related to ontogeny. Further issues were also addressed, such as the possibility that the areas on the frill of Triceratops actually were indicative of fenestrae formation, but I will not get into the nitty-gritty details here. For those who are interested in learning more, I would suggest reading the original source material.
Several more papers have also raised issues with this idea. One of these used morphometric analysis to examine the various ages of Torosaurus and Triceratops specimens, and while it did find a general trend of Triceratops juveniles and Torosaurus adults, there were several notable exceptions. Two specimens of Torosaurus appeared to be quite young, somewhere approaching the age of some known Triceratops individuals. Conversely, it was also found that ten of the Triceratops skulls had reached a level of maturation equal to that of most aged specimens of Torosaurus (Longrich and Field, 2012). This paper also raised further objections, but due to time and space constraints I cannot get into all of them here.
A third paper published in 2013 used a statistical morphospace analysis (actually something I described in the Sci-Day discussing Species Concepts) to describe the variation of Torosaurus, both species of Triceratops, and Nedoceratops correlated with maturation. They found that Torosaurus specimens still retained a distinct anatomy from that of both Triceratops species, even when frill shape isn't included. While they admitted that the low number of Torosaurus specimens makes the analysis a bit less reliable than it otherwise might be, it still seems sufficient to refute the synonymy of the two genera (Maiorino et al., 2013).
In Dinosaur Battlegrounds, we may try to make it a player choice as to whether or not Torosaurus will be a distinct population of its own, or will simply represent an adult growth form of Triceratops. This would be something that would take a while to implement, as there is a lot of work that would have to go into the population ecology and whatnot, but it could help us understand the issue better.
Well, I hope you have enjoyed this somewhat controversial Creature Feature! We do not want to take sides here at Dinosaur Battlegrounds - our goal is to simply give our fans the evidence, and let you practice your critical thinking skills so you can make your own informed decisions!
Acknowledgements:
Holtz, Thomas R. Jr. 2011. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2010 Appendix.
Longrich, N. R.; Field, D. J. 2012. Torosaurus is not Triceratops: Ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy. PLoS ONE 7 (2): e32623.
Scanella, J. 2009. And then there was one: synonymy consequences of Triceratops cranial ontogeny. Journal of Vertebrate Paleontology 29: 177A
Scannella, J.; Horner, J.R. 2010. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny. Journal of Vertebrate Paleontology 30(4): 1157 - 1168.
Farke, A. A. 2011. Anatomy and taxonomic status of the chasmosaurine ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, U.S.A. PLoS ONE 6 (1): e16196.
Maiorino, L.; Farke, A. A.; Kotsakis, T.; Piras, P. 2013. Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs. PLoS ONE 8(11): e81608.
whoa that looks great! good work with the creature feature and model guys
ReplyDeleteFurther analysis and discovery of Juivinile Torosaurus, specifically from Texas and potentially New Mexico have indicated Torosaurus forms two distinct species, and are not an ontogenetic stage of triceratops. (Hunt-foster and Lehman 2008) (Lucas 2017)
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