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Here we report on a multigenerational assemblage of Triceratops from the Upper Maastrichtian Lance Formation near Newcastle, eastern Wyoming, USA. While fieldwork is still ongoing, in excess of eight hundred elements have already been uncovered, belonging to a minimal number of six individuals. Detailed geological fieldwork at the excavation site shows that the Triceratops skeletons are entombed within an organic-rich clay to siltstone unit characterized by abundant micro-and macro-floral and-vertebrate remains. The Triceratops bones are associated, but also show clear disarticulation. The fifth skeleton is located at the same site, but circa 4 meters above the main bonebed.

The site is remarkable since it yields both cranial and post-cranial remains of individuals of different sizes, representing several ontogenetic stages. This assemblage is one of only a handful known sites of the genus Triceratops that contain abundant post-cranial material4, 6. Ontogeny and especially the validity of taxa in Triceratops, and other Chasmosaurinae (eg Torosaurus), have been heavily debated subjects5, 9. However, studies have mostly been restricted to descriptions of cranial material due to the scarcity of post-cranial remains1, 2, 3, 9. Therefore, this assemblage helps to fill in a gap in our knowledge regarding the post-cranial development of these highly derived ceratopsids. This site offers a higher resolution in the ontogenetic development in particularly the late juvenile or early sub-adult to adult stages of Triceratops. By combining histology, allometric measurements, and analysis of morphological characters, the timing of attaining skeletal maturity along …