Oser, Sara E.Chin, KarenSertich, SertichVarricchio, David J.Choi, SeungRifkin, Jeffrey2022-09-082022-09-082021-05Oser, S.E., Chin, K., Sertich, J.J.W. et al. Tiny, ornamented eggs and eggshell from the Upper Cretaceous of Utah represent a new ootaxon with theropod affinities. Sci Rep 11, 10021 (2021). https://doi.org/10.1038/s41598-021-89472-12045-2322https://scholarworks.montana.edu/handle/1/17104A new Cretaceous ootaxon (eggshell type) from the Kaiparowits Formation of Grand Staircase-Escalante National Monument is among a growing number of very small eggs described from the Mesozoic. Analyses of two partial eggs (~ 17.7 mm in diameter) and 29 eggshell fragments reveal that this new ootaxon exhibits nodose ornamentation with distinctive branching pore canals that open atop the nodes. Its two-layered microstructure consists of a mammillary layer and a continuous layer with rugged grain boundaries between calcite grains. Although the exact identity of the egg producer is unknown, the eggshell microstructure and small size is consistent with a small-bodied avian or non-avian theropod. The specific combination of small egg size, branching pores, two-layered microstructure, and dispersituberculate ornamentation preserved in this new ootaxon is unique among theropod eggs. This underscores that both eggshell and skeletal fossils of Cretaceous theropods can display a mosaic of transitional morphological and behavioural features characteristic of both avian and non-avian taxa. As such, this new ootaxon increases the diversity of Cretaceous eggs and informs our understanding of the evolution of theropod eggshell microstructure and morphology.en-UScc-byhttps://creativecommons.org/licenses/by/4.0/ornamented eggs utah cretaceous theropodArticle