|A) Triceratops skull B) Torosaurus skull|
Photo credit: Nicholas R. Longrich and Daniel J. Field
In recent years, paleontologists have proposed that the well-known and beloved dinosaur Triceratops was actually a juvenile form of a different dinosaur, Torosaurus. The three horned dinosaurs, also known as Ceratopsidae, have been extremely hard to identify and catagorize because their skull and frill tend to morph with maturity. This may have been to distinguish adult dinosaurs (those ready to reproduce) from immature juveniles. There are so many morphological differences in Ceratopsidae fossils that these dinosaurs were originally catagorized into 10 different genera. Today, there are only two, Triceratops and Torosaurus. Now, some paleontologists are calling for a merge between these two genera, while others maintain that Triceratops and Torosaurus are two distinctly different dinosaurs.
Traditionally, Triceratops is classified by its short, solid frill. Torosaurus has a longer frill that contains two large holes. One hypothesis is that as the dinosaur matured, changes in a Triceratops' short frill caused it to elongate and become the open skull that is normally associated with Torosaurus. To support this hypothesis both types of dinosaur fossils must be found in overlapping geographic locations. Dinosaurs with longer, open frills should also be more mature than those with a short, solid frill. Lastly, there should be intermediate fossils that would depict the maturation of the dinosaur overtime.Paleontologists agree that the first condition is supported. Fossils from both types of three horned dinosaurs have been discovered across modern day Western United States and Canada. These fossils all seem to be from the same time period as well (the late Maastrichtian period).
The evidence for the second condition is more controversial. Researchers analyzed these dinosaurs’ development by looking at bone remodeling. More mature animals would have more bone remodeling because their skeleton (specifically their skull) would have morphed as they aged. However, there are very few fossils that are intact enough to accurately make this analysis.
The third condition is just as difficult to support for the same reasons. There are few fossils that are complete enough to determine if the physiological characteristics are intermediates between Triceratops and Torosaurus. One fossil of a Triceratops seemed to have indentations in its frill, which could have been the beginnings of the holes that are found in Torosaurus frills. However, some paleontologists argue that the indentations are not in the expected location to develop into a Torosaurus frill.
Though unsolved, this controversy has definitively affected the way paleontologists identify dinosaur genera and species. They must take into account the way that the animal’s morphology developed and changed over its lifetime. Currently, new systematic approaches are being developed to explain and characterize fossils, helping to clarify dinosaur diversity. Overall, more caution is being taken when identifying new species (or eliminating old ones).