Microstructural overlap of Macroelongatoolithus eggs from Asia and North America expands the occurrence of colossal oviraptorosaurs

ABSTRACT

The first intact North American Macroelongatoolithus specimen was excavated from sediments in the Wayan Formation of southeastern Idaho and initially reported by Krumenacker et al. (2017, Historical Biology 29:170–186). Here, we present a description of the specimen, including a zonal analysis of microstructural variation and comparison with an egg pair from the Liangtoutang Formation of Zhejiang, China. We assign egg pair IMNH 2428\49608 from the Albian–Cenomanian Wayan Formation of Idaho and egg pair ZMNH M8705 from the Liangtoutang Formation of China to the oospecies Macroelongatoolithus carlylei. The eggs are 39.8–41.7 cm long and 10.8–14.3 cm wide, and the eggshell consists of two structural layers of calcite demarcated by a distinct, undulating boundary. Eggshell thickness is 1.01–2.17 mm, and the continuous layer to mammillary layer ratio (CL:ML) ranges from 2.05:1 to 7.68:1. Identification of intact Macroelongatoolithus specimens from North America supports synonymy of M. xixiaensis with M. carlylei, as proposed by Zelenitsky et al. (2002) on the basis of eggshell fragments. Due to the substantial microstructural variation of the specimens reported here, we also regard M. goseongensis and M. qiongensis as junior synonyms of M. carlylei. The degree of microstructural variation within the new specimens expands the expected range of variation typically used in diagnosing oospecies and has implications for the naming of new oospecies on the basis of fragments or egg portions alone. The discovery of Macroelongatoolithus eggs from the latest Early Cretaceous of Idaho confirms the presence of this extraordinary oogenus outside of Asia and provides nonskeletal evidence of a Gigantoraptor-sized oviraptorosaur in western North America.

ACKNOWLEDGMENTS

We thank F. Jackson for her substantial field work and study design contributions, without which this project would not have been feasible. We thank F. Jackson, B. Jackson, J. Stiegler, P. Ullmann, D. Strosnider, J. Fearon, and A. Poust for fossil collection and field work assistance, and L. Spichner, T. Imai, and M. Holland for assistance with specimen preparation. The Idaho Museum of Natural History and the Zhejiang Museum of Natural History provided access to specimens, and Z. Wenjie assisted with specimen access and provided location data. This work was supported by the National Science Foundation [grant 0847777 to DJV] and National Geographic [grant 8725-10 to DJV]. The Imaging and Chemical Analysis Laboratory at Montana State University, the Varricchio Family Paleontology Preparation Laboratory at Montana State University, and the Museum of the Rockies paleontology laboratory provided access to equipment for specimen imaging and analysis. Finally, we thank D. Barta, V. Arbour, and D. Evans for comments on early drafts, as well as M. Azanza and an anonymous reviewer for edits that greatly improved the manuscript.