“Evolutionary Trends in Triceratops from the Hell Creek Formation, Montana” by Scanella, J.B.., Folwer, D.W., Goodwin, M.B., and Horner, J.R.
The article named “Evolutionary trends in Triceratops from the Hell Creek Formation, Montana” written by Scanella, J.B.., Folwer, D.W., Goodwin, M.B., and Horner, J.R. in 2014 explains the mode of evolution of Cretaceous dinosaurs by analyzing skulls from two species of the most abundant genus in the formation: Triceratops horridus and Triceratops prorsus.
The authors were trying to crack how the evolutionary development of these two species was in a 1 – 2 million years interval of time and how it affected diversity. The two opposing options they had were either cladogenesis or anagenesis. The importance of using Triceratops fossils in these analysis is because small sample sizes and lack of details interfere with the comprehension, but Triceratops’ fulfill all the requirements for a detailed breakdown. They also wanted to know the evolutionary patterns in the fossils and how noticeable they are.
To find the results, the investigators divided the formation in three parts: Upper third (U3), middle third (M3) and lower third (L3), using The Basal sand, Jenrex sand and Apex sand as limit markers, and then they placed the different specimens in their respective section. They said that according to different authors, these species were recognized based on the length of the postorbital horn, shape of the rostrum, closure of the frontoparietal fontanelle and morphology of the epinasal. Merging the sections of the formation and the morphology changes, gave as a result that T. horridus was found in L3, T. prorsus in U3 and in M3 there were specimens showing a combination of all the features. Giving this a closer look, Triceratops from L3 possessed a small epinasal, the nasal horn (NPP) was narrow and very inclined. Skulls from the M3 have a bigger nasal-horn length with a subtriangular shape and they also show that the NPP is more vertically inclined. Fossils from U3 exhibit a more vertically, narrower nasal horn, producing a convex rostrum, which is a characteristic feature of T. prorsus. This is a purely anagenetic scenario where a species with new traits replace an older organism.
The authors also explain the changes over time: they said that the nasal horn shifted in a specific way through time and its size increased. Referring to what was found in the lower half of the formation, the specimens exhibited a protuberance posterior to the nasal horn, a horizontal like direction of the epinasal and they said that the length of the horn varied without any trend; all these are T. horridus characteristics. As they ascended through the formation and times changed, the horn length kept increasing and it leaded into a different direction. An example of this transition is the fossil UCMP 113697, found by The University of California Museum of Paleontology in upper M3. This specimen, together with MOR 3027 and MOR 3045 possessed a combination of characteristics from L3 and U3: large epinasals (from L3) and a NPP vertically …
