Egg Mountain represents a small outcrop of the Upper Cretaceous Two Medicine Formation in Teton County, Montana dating to approximately 75.5 million years ago. Early excavations (1979-1983) produced the first dinosaur eggs and nesting grounds from North America. More recent excavations produced abundant, exceptionally well-preserved and largely autochthonous fossils (relatively complete skulls and articulated skeletal remains) of mammals and lizards. Objectives of this research were to 1) continue excavation of the site in order to find more specimens and gather data on the local paleo environment, 2) describe the new fossils and infer their broader evolutionary relationships; and (3) examine their paleoecology by analysis of their tooth and skeletal morphology.

Interpretations of the site’s thick, calcareous sequence included either deposition within or along the margins of a shallow, alkaline lake or as the product of soil-forming processes. But our recent excavation (2014-2016) of a large (10 x 7 x 1.7 m) quarry by jackhammer removal of successive, 10 cm- thick, bedding-parallel intervals exposed specimens in plan view and has led to revised understanding of the locality. Additionally, our three field seasons generated 147 mappable vertebrate specimens including 5 possible coprolites, ~25 mammal/lizard specimens, and ~250-300 dinosaur teeth, with >1,000 invertebrate traces. Evidence from the sedimentary rocks, fossil preservation and stable isotope geochemistry data favors deposition of floodplain overbank fines heavily modified by bioturbation, soil-formation under subaerial conditions, and subsequent diagenetic overprinting. The diversity and condition of body fossils further supports subaerial conditions and a terrestrial fauna. Nearly 70 m3 of quarrying has produced only one very small turtle fragment and a frog bone. Otherwise, the assemblage consists of terrestrial taxa: dinosaurs, mammals, and iguanomorph lizards, as well as terrestrial gastropods. Individual elements of one large associated hadrosaur are in a state of disintegration, whereas tyrannosaur teeth are often preserved as splint-like shards. Nearly ubiquitous trace fossil evidence in the form of abundant insect pupae cases, dinosaur and non-dinosaurian egg assemblages representing minimally five egg types, 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 preservational histories. Overall, data show that Egg Mountain represents an exceptional record of Late Cretaceous terrestrial paleoecology. 

Important findings from the work include the following:1) The fossils that have been collected from the Egg Mountain locality are the most complete skeletons and skulls of mammals and lizards from the Late Cretaceous of North America. They will contribute significantly to our understanding of the anatomy, phylogeny, and ecology of mammals and lizards from this time period—a key interval in both mammalian and squamate evolution. 2) Trace fossils proved an important tool in the interpretation of the Egg Mountain locality, providing important information on the environmental conditions as well as sedimentation history. Whereas traces have often been used to monitor changing conditions thru a stratigraphic section in marine settings, this is one few instances were this approach has been applied in a continental setting. 3)Egg Mountain remains a truly unique site representing a locality dominated by the terrestrial activity of vertebrates and invertebrates.  As such it provides a unique window into a Late Cretaceous ecosystem. Traces indicate a variety of abundant insect pupation chambers as well as vertebrate nesters that include three non-avian dinosaurs plus lizards and birds, and at least three varieties of scat. Given the spectacular condition many of the lizard and mammal specimens, these taxa were likely residence of this locality back in the Cretaceous.  Continued morphologic work should reveal how their ecologies related to this terrestrial setting. 4) Currently, we are developing an interactive 3D model of the site based on the field data and using architectural software designed to permit interactive planning, design, construction and management of buildings. Taphonomic reconstruction represents rebuilding a 3D model of what no longer exists. We hope that this novel approach to reconstructing a site may prove particular useful in tracking previously excavated specimens and horizons into the future and provide a taphonomic model that is user friendly for other paleontologic localities. 5) Work continues with the scat from the site.  Three broad varieties include small pelletal masses, larger turd-like forms, and finally aggregations of mammal and squamate elements.  This last variety is quite interesting.  At least two examples primarily consist of cranial material suggests the possibility of regurgitates or emetolites.  Additional preparation and scanning should reveal the details of these specimens so that more detailed interpretations can be made.

Last Modified: 12/22/2017
Modified by: David Varricchio