RETURN TO EGG MOUNTAIN: TAPHONOMY OF A LATE CRETACEOUS DINOSAUR NESTING LOCALITY FROM MONTANA, USA

Discovered by Jack Horner in 1978, the Egg Mountain locality represents a small outcrop of the Upper Cretaceous Two Medicine Formation in Teton County, Montana. Radiometric dates place the site at just under 75.5 ± 0.40 Ma. Early excavations of its calcareous mudstones and carbonates produced the first dinosaur eggs and nesting grounds from North America, as well as dinosaur skeletons, varanoid lizards, jaws of metatherian mammals, and the most complete Late Cretaceous multituberculate mammal from the continent. Original interpretations argued that the thick, calcareous sequence represents deposition within or along the margins of a shallow, low-diversity alkaline lake. Recent excavation (2010-2013) of a large (10 x 7 x 1 m) quarry by the jackhammer removal of successive, 10 cm-thick, bedding-parallel intervals from the site has exposed specimens in plan view and led to revised understanding of the locality. New taphonomic, sedimentologic, and isotopic data favors deposition of floodplain overbank fines heavily modified by bioturbation, pedogenesis under subaerial conditions, and subsequent diagenetic overprinting, although some lacustrine influence cannot yet be completely ruled out. The diversity and condition of body fossils further supports largely subaerial conditions. Nearly 70 m³ of quarrying has not produced a single gar, turtle or crocodile element, some of the most common skeletal material in Upper Cretaceous formations. Instead, the assemblage consists only of terrestrial taxa: dinosaurs, mammals, and lizards, as well as planispiral terrestrial gastropods. Nearly ubiquitous trace fossil evidence in the form of abundant insect pupae cases, dinosaur and non-dinosaurian egg clutches, coprolites, and emetolites (regurgitates) also indicate in situ terrestrial biotic activity. Palynological data from the area further suggests a relatively dry paleoclimate. Mammal and squamate remains occur in one of four modes: (i) isolated elements, (ii) closely associated to articulated skeletons, (iii) multi-individual aggregations of predominantly cranial elements, and (iv) concentrations of highly fragmented and small bony debris indicating a range of taphonomic histories. Overall, data show that Egg Mountain represents an exceptional record of Late Cretaceous terrestrial paleoecology.