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Evidence that ''Torosaurus'' is grown up ''Triceratops horridus''

Posted by gspauldinodotcom on 18 Apr 2012 at 22:36 GMT

Longrich and Field compare the ontogenetic development of a specimen of Triceratops prorsus and Torosaurus latus and conclude they are distinct taxa. The result is expected in that there has been no proposal that T. prorsus grew up to be T. latus. Scannella and Horner [1] noted that T. prorsus and T. horridus are distinct species from different stratigraphic levels, that T. horridus and T. latus are always and only found at the same level, and that the latter appears to be the adult of the former. The Longrich and Field analysis therefore does not address or refute the specific hypothesis offered by Scannella and Horner.

Some items support the Scannella and Horner hypothesis. For example some skulls assigned to and possessing T. horridus attributes have strongly elongated frills that are intermediate to those of other T. horridus and T. latus (see left column in figure link below).

Also informative is an extremely large, isolated frill from the Frenchman Formation that otherwise produces only T. prorsus ceratopsid specimens. Frill EM P16.1 has been assigned to Torosaurus because it has large fenestrae [2], but its square profile could hardly be more different from that of Torosaurus (see right column in figure link below). It is possible if not probable that P16.1 is the frill of an old T. prorsus, in which case the species developed an open frill with maturity as proposed for T. horridus whose frill additionally elongated into the Torosaurus shape. Incipient osteological erosion of continuous Triceratops frills towards full openings has been noted [3]. Meanwhile the frill did not elongate in aging T. prorsus. It is also notable that P 16.1 appears to lack the centerline hornlet on the posterior rim of the frill that is common in Triceratops. It is an attribute like that of Torosaurus and suggests a similar ontogenetic change. I noted the ontogenetic and taxonomic implications of the Frenchman frill [4], but placed it in a geographic rather than the more probably correct stratigraphic/temporal context. The alternative is that an otherwise unknown gigantic chasmosaurine was present in the Frenchman formation.

A potential complication regarding efforts to sort out ontogeny and taxonomy that may not be receiving sufficient attention is significant to strong sexual dimorphism. If present then this means that for a given species with a lot of dimorphism it is not possible to construct a simple growth series even if a large set of specimens is on hand, and this may mislead observers to think they are seeing different species. Perhaps one sex of T. horridus, possibly the females, retained a short frill into old age, while only the males elongated the frill and the latter has been mistaken as the distinct taxon T. latus. Likewise one sex of Pachycephalosaurus, perhaps females, may not have developed large cranial horns, while the males did, leading to confusion vis-à-vis Dracorex and Stygimoloch.

The figure link is at –

Left column, skulls drawn to same main body skull length to facilitate comparison of increasing frill lengths; Triceratops horridus USNM 1201, T. horridus MNHN 1912.20 [after ref. 5], T. latus or horridus MOR 1122. Right column, frills drawn to same breadth; EM P16.1, MOR 1122. Not to scale, except that both MOR 1122 images are same scale.

1. Scannella J, Horner JR (2010) Torosaurus is Triceratops, synonymy through ontogeny. Journal of Vertebrate Paleontology 30: 1157-1168.
2. Tokaryk TT (1986) Ceratopsian dinosaurs from the Frenchman Formation of Saskatchewan. Canadian Field-Naturalist 100: 192-196.
3. Beach, A (2011) Triceratops and Torosaurus synonymy: an evaluation of two large specimens from Brigham Young University. Journal of Vertebrate Paleontology, 31: A68.
4. Paul GS (2010) The Princeton field guide to dinosaurs. Princeton: Princeton University Press.
5. Goussard F (2006) The skull of Triceratops in the paleontology gallery, Museum national d’Histoire naturelle, Paris. Geodiversitas 28: 467-476.

No competing interests declared.