Here we provide a survey of Triceratops localities and accompanying stratigraphic data from the Hell Creek Formation of northeastern Montana. The majority of the sites discussed here were relocated or discovered during the last 5 yr of the Hell Creek Project (1999–2010), a multi-institutional effort to record a large volume of faunal, floral, and geologic data on the Hell Creek Formation in order to test evolutionary, paleoecological, and geological hypotheses. Triceratops is the most abundant dinosaur in the Hell Creek Formation and one of the most common nonavian dinosaurs of the Upper Cretaceous. It is known from hundreds of specimens, which have been collected since it was first described in 1889. Although these specimens provide a wealth of morphological data on Triceratops , many lack detailed stratigraphic information and context. Detailed stratigraphic and contextual data for more than 70 specimens of Triceratops collected during the Hell Creek Project make this data set among the most comprehensive for any nonavian dinosaur.

Here, we describe a juvenile Triceratops sp. skull, UCMP 136306, from the Hell Creek Formation, McCone County, Montana. The relative completeness and superb preservation of this skull contribute to an improved understanding of the cranial ontogeny, morphology, and individual variation in Triceratops . Total skull length is 120 cm long (est.). UCMP 136306 is one of the most complete Triceratops skulls of this ontogenetic stage yet known. The cranial sutures are patent, and most are overlapping with minimal sinuosity, modest interdigitation, and overlapping flat sutural surfaces. The following cranial elements are preserved and described in this study: epinasal, rostral, quadrate, quadratojugal, jugal, pterygoid, dentary, surangular, postorbital horn, parietal, squamosal, epiparietal, episquamosal, occipital condyle, supraoccipital, and exoccipital. For decades following the initial description of Triceratops by O.C. Marsh in 1889, the typical collector attitude was “bigger is better.” Emerging scientific institutions and museums with newly constructed exhibit halls demanded the biggest and newest dinosaurs. We hypothesize that this historical practice, influenced by facies and taphonomic factors in the Upper Cretaceous Hell Creek Formation, Montana (and contemporaneous formations in neighboring states), resulted in the underrepresentation of nonadult Triceratops in museum collections. This practice contributed to the false notion that nonadult Triceratops specimens are rarely preserved in the fossil record, until now.

Ceratopsid dinosaurs are notable for their common occurrences in bonebeds; however, until recently, these have not been encountered for the chasmosaurine Triceratops . The aim of this investigation is to describe the taphonomy of Quittin' Time (Museum of the Rockies locality HC-430), a Triceratops bonebed in the Hell Creek Formation, Garfield County, Montana. Using site taphonomic descriptions with an evaluation of ontogeny, inferences regarding the paleobiology of this extinct taxon are possible. The locality is associated with abundant organic material, including woody debris, large seeds, and other fragments in isolated silty lenses, all incorporated within a siltstone matrix, indicating preservation within a floodplain environment. Based on the repetition and ontogenetic stages of cranial elements, the minimum number of individuals (MNI) is three. Evidence from the location and taphonomic condition of the bones preserved in close proximity within the same siltstone unit suggests that the individuals—one young adult, one subadult, and a juvenile—likely accumulated during distinct flooding events within a narrow region of the floodplain as a result of “bloat-and-float” transport. The relatively small scale of the bonebed, both in terms of total area and number of individuals, implies that future work on Triceratops sites requires careful scrutiny of cranial elements examined within an ontogenetic framework because they are potentially critical to establishing MNI. Preservation of multiple individuals within the same unit does not necessarily provide evidence of gregarious behavior in Triceratops but rather may be a reflection of site taphonomic history and accumulation processes.