The Horned Dinosaur Leptoceratops (Ornithischia: Neoceratopsia) Raised its Young in Communal Nesting Burrows: Evidence from Three New Bonebeds in the Hell Creek Formation (Maastrichtian, Late Cretaceous), Montana
FOWLER, Denver W; Badlands Dinosaur Museum, Dickinson, ND. WILSON, John P; Bozeman, MT. FREEDMAN FOWLER, Elizabeth A; Dickinson State University, Dickinson, ND. HORNER, John R; Chapman University, CA.
In over 100 years of collecting in the Maastrichtian of North America, only 11 specimens of the rare neoceratopsian dinosaur Leptoceratops have been reported, of which only one skull, one postcranium, and two teeth have been reported from outside of Alberta.
Here we report the discovery of three new Leptoceratops bonebeds in the Hell Creek Formation of Montana. The first site records two partial skeletons of adult Leptoceratops preserved articulated in 3D, standing upright in a bentonitic mudstone. Further excavation led to recovery of 252 additional elements, 95% of which are referable to Leptoceratops. This includes 117 teeth, mostly juvenile sized with many bearing roots (indicating origination from dead individuals rather than being shed teeth), and a probable associated fragmentary skeleton ~1/3 adult length. Although these in situ remains were initially considered as a miring event, the site is now hypothesized to represent a communal nesting burrow, with the adult specimens buried during burrow collapses. The relatively uniform distribution of juvenile teeth and other remains suggests that the same location was used over many generations, with the remains of multiple broods being evenly reworked into the sediment.
A second, more areally limited site ~ 100m SE of Site1 and in the same bentonite horizon, also preserves juvenile and adult remains, suggesting the paleosurface was locally inhabited extensively by Leptoceratops, perhaps forming large communal burrow systems like extant prairie dogs. A third monodominant Leptoceratops bonebed ~ 100km NW, similarly exhibits many juvenile remains, including hatchling sized teeth and cranial bones, and three associated multituberculate teeth, suggesting that mammals might have contemporaneously burrowed into the same land surface, perhaps even sharing Leptoceratops burrows. Theropod shed teeth at Site1 and 3 suggest that the burrow horizons were frequented by predators.
The bentonite preserves root traces and insect burrows supporting interpretation as a paleosol. In parts of the bone-bearing layer, fine grained sandstone directly contacts the bentonite with rip-up fragments at its base, consistent with burrow infill reported at other dinosaur burrows.
All sites were located in the lower third of the Hell Creek Formation, where the environment is hypothesized to represent a low-accommodation, better drained setting, more accommodating for burrowing organisms than the more swampy conditions of the upper Hell Creek, from which Leptoceratops remains are not recorded.
Funding kindly provided by the American Prairie Foundation, Nathan Mhyrvold, and the Evolving Earth Foundation. Fieldwork permits granted by the Bureau of Land Management and the state of Montana.