Portal de Periódicos da CAPES

Fossil eggs and eggshell from the lowermost Two Medicine Formation of western Montana, Sevenmile Hill locality

2010; Taylor & Francis; Volume: 30; Issue: 4 Linguagem: Inglês

10.1080/02724634.2010.483537

1937-2809

Frankie D. Jackson, David J. Varricchio,

Evolution and Paleontology Studies

ABSTRACT In the last 30 years, the Two Medicine Formation of western Montana has provided a wealth of information about dinosaur reproductive biology. Here, we describe a fossil egg-bearing stratum that occurs approximately 105 m above the base of the formation. This site in the Sevenmile Hill outcrops south of Choteau, Montana, lies immediately above a volcanic tuff and bentonite, dated as 80.0 Ma. Spherulitic eggshells from a quarry at this locality are similar to Spheruprismatoolithus candensus CitationBray, 1999, which were assigned to the oofamily Prismatoolithidae. However, we refer this eggshell to the Spheroolithidae CitationZhao, 1979, as Spheroolithus choteauensis, oosp. nov. The quarry also produced Triprismatoolithus stephensi, oogen. et oosp. nov. These symmetrical 30 mm × 75 mm prismatic eggs exhibit three structural layers of calcite and round tubercles on the shell surface. Four additional ootaxa occurred at the First Find Microsite (OTM 99-19): Prismatoolithus hirschi, oosp. nov.; Tubercuoolithus tetonensis, oogen. et oosp. nov.; Continuoolithus canadensis CitationZelenitsky et al., 1996; and Krokolithes CitationHirsch, 1985. Spheroolithus choteauensis, T. stephensi, P. hirschi, T. tetonensis, and Krokolithes are unique to the lowermost Two Medicine Formation, whereas C. canadensis may occur elsewhere in the middle and upper strata of the Two Medicine and Oldman formations of Montana and Alberta, respectively. Although poorly represented by dinosaur osteological remains, the lower Two Medicine Formation locality yields egg types that suggest a significant difference in faunal composition compared to the middle and upper portions of the formation. ACKNOWLEDGMENTS We thank Robert E. 'Short' Stevens and the Old Trail Museum of Choteau, Montana, for access to land and specimens; R. Avci for use of the Image and Chemical Analysis Laboratory, Montana State University; and J. Horner for access to the Gabriel Laboratory for Molecular and Cellular Biology, Museum of the Rockies. We thank J. Schmitt and B. Jackson for discussions and editing; J. Hayward, J. Nestler, B. Hanna, T. Crowell, G. Dyke, and D. Strosnider for assistance with fieldwork; and particularly D. Frazier and J. Anderson for discovery of the Dave and Joel site. We thank D. L. Hayes of the Johnston Atoll National Wildlife Refuge for extant Frigatebird eggshell, and the Jurassic Foundation and the Carthage College Quality of Life Committee for grant support. Finally, we are indebted to K. Stanton for her insights and discussions of the Sevenmile Hill specimens and the use of some thin sections included in this study.