First record of elongatoolithid theropod eggshell from North America: the Asian oogenus Macroelongatoolithus from the lower Cretaceous of Utah

ABSTRACT

Theropod eggshell is identified for the first time from the Lower Cretaceous of North America. Eggshell microstructure and variability in ornamentation and thickness of this material is comparable to that present in elongatoolithid theropod eggs, known previously only from Asia. This identification is based on re-examination of the holotype of the putative spheroolithid Boletuoolithus carlylensis and description of additional eggshell fragments from the Lower Cretaceous of Utah. This oospecies is characterized by the ornithoid-ratite morphotype (i.e., of theropod parentage) and not, as recently suggested, by the dinosauroid-prolatospherulitic morphotype (i.e., of ornithopod parentage). Additionally, Boletuoolithus carlylensis includes a much broader range of eggshell morphologies than previously described: eggshell thickness ranges from 1.38–3.04 mm, ornamentation is remarkably variable, and sometimes the typical elongatoolithid microstructure is altered by extreme development of both the ornamentation and pore system. The thickness, ornamentation, and elongatoolithid microstructure indicate that the holotype and referred materials assigned to Boletuoolithus carlylensis are indistinguishable from the Asian elongatoolithid Macroelongatoolithus xixiaensis. Accordingly, the oogenus Boletuoolithus is rendered a junior subjective synonym of Macroelongatoolithus. In turn, Macroelongatoolithus xixiaensis is regarded as a junior synonym of Macroelongatoolithus carlylei (new combination and emendation of specific epithet) by virtue of publication priority.