Torosaurus NOT Triceratops

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com


A new paper in PLoS ONE:


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.
doi:10.1371/journal.pone.0032623
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0032623


Background

In horned dinosaurs, taxonomy is complicated by the fact that the
cranial ornament that distinguishes species changes with age. Based on
this observation, it has been proposed that the genera Triceratops and
Torosaurus are in fact synonymous, with specimens identified as
Torosaurus representing the adult form of Triceratops. The hypothesis
of synonymy makes three testable predictions: 1) the species in
question should have similar geographic and stratigraphic
distributions, 2) specimens assigned to Torosaurus should be more
mature than those assigned to Triceratops, and 3) intermediates should
exist that combine features of Triceratops and Torosaurus. The first
condition appears to be met, but it remains unclear whether the other
predictions are borne out by the fossil evidence.

Methodology/Principal Findings

We assessed the relative maturity of Torosaurus and Triceratops
specimens by coding skulls for characters that vary with maturity, and
then using a clustering analysis to arrange them into a growth series.
We found that a well-defined sequence of changes exists in horned
dinosaurs: 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.
Using this scheme, we identified mature and immature individuals of
both Torosaurus and Triceratops. Furthermore, we describe the ventral
depressions on the frill of Triceratops, and show that they differ in
shape and position from the parietal fenestrae of Torosaurus. Thus, we
conclude that these structures are not intermediates between the solid
frill of Triceratops and the fenestrated frill of Torosaurus.

Conclusions/Significance

Torosaurus is a distinct genus of horned dinosaur, not the adult of
Triceratops. Our method provides a framework for assessing the
hypothesis of synonymy through ontogeny in the fossil record.