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

Abstract

ABSTRACTThe 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.