Josh and the Burpee crew found 130 bones and bone fragments belonging to Triceratops in the Hell Creek Formation during exploration between 2005 - 2007. The remains, found in a massive mudstone, indicated a minimum number of three juvenile individuals, based on the presence of three left nasals. The site was dominated by cranial and appendicular elements, with analysis showing that the remains were sorted by currents, washing away smaller elements. This site is the first published occurrence of a Triceratops bonebed.
Mathews, J. C., Brusatte, S. L., Williams, S. A. and Henderson, M. D. 2009. The first Triceratops bonebed and its implications for gregarious behavior. Journal of Vertebrate Paleontology 29(1):286-290
"Ceratopsid dinosaurs are some of the most common fossils in Upper Cretaceous terrestrial strata of western North America. They are often found in bonebeds, which are accumulations of vertebrate fossils from more than one individual that are concentrated along a bedding plane or throughout a single bed (Eberth and Getty, 2005). For example, 20 bonebeds have been reported from Dinosaur Provincial Park in Alberta, Canada, which contain Centrosaurus and Styracosaurus (Visser,1986; Ryan et al., 2001; Eberth and Getty, 2005). Additional bonebeds from localities across western North America have yielded remains of Agujaceratops (Lehman, 1982, 1989, Lucas et al., 2006), Anchiceratops (Dodson, 1996, Ryan et al., 2001), Einiosaurus (Rogers, 1990, Sampson, 1995), Pachyrhinosaurus (Langston, 1975; Tanke, 1988; Ryan et al., 2001), Styracosaurus (Dodson et al., 2004), Torosaurus (Hunt, 2005), and Zuniceratops (Wolfe and Kirkland, 1998). Some of these bonebeds contain the remains of hundreds to possibly thousands of individuals and represent catastrophic mass death assemblages that strongly indicate herding behavior (Eberth, 1996). Others contain significantly fewer individuals, with some preserving fewer than five.
Although Triceratops is the most common dinosaur in the terminal Cretaceous Hell Creek Formation (White et al., 1998), known from over 50 singleton specimens collected since the late nineteenth century, no bonebeds or associations of multiple individuals have previously been reported. A new locality in the latest Maastrichtian Hell Creek Formation of southeastern Montana, discovered in the summer of 2005 by a field crew from the Burpee Museum of Natural History (BMR) in Rockford, Illinois, contains the remains of three juvenile-sized Triceratops. This is the first occurrence of multiple individuals of Triceratops in the same quarry and raises potentially interesting questions regarding Triceratops paleobiology."
Homer the Triceratops at the Burpee Museum. See more pictures here.
Mathews, J. C., Brusatte, S. L., Williams, S. A., & Henderson, M. D. (2009). The first Triceratops bonebed and its implications for gregarious behavior Journal of Vertebrate Paleontology, 29 (1), 286-290