Tiny, ornamented eggs and eggshell from the Upper Cretaceous of Utah represent a new ootaxon with theropod affinities

Sara E Oser,Karen Chin,Jeffrey Rifkin,David J Varricchio,Seung Choi,Joseph J W Sertich

doi:10.1038/s41598-021-89472-1

Abstract

A 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.

Highlights

Since its designation in 1996, Grand Staircase-Escalante National Monument (GSENM) in southern Utah has protected a wealth of archaeological, biological, geological, and paleontological resources
Eggs are small; eggshell displays dispersituberculate ornamentation consisting of irregularly spaced, flattened nodes which grade from circular to oval in plan view over the surface of the egg; eggshell thickness ranging from 0.31 to 0.40 mm (0.31–0.59 mm including ornamentation); two structural layers of calcite include the mammillary layer (ML) and continuous layer (CL) which are delineated by an abrupt, linear boundary; ML:CL thickness ratio of 1:1.1–1:1.5; pores are funnel-shaped through the mammillary and continuous layers, increasing in diameter from the interior of the eggshell towards the surface and branching in the nodes; pore openings at the surface are small (0.03–0.04 mm) and located on the nodes; nodes are crystallographically discontinuous from the CL
Eggshell has been traditionally organized into structural morphotypes; this system is being largely supplanted by the use of parataxonomic binomial nomenclature[5,20]

Summary

Introduction

Since its designation in 1996, Grand Staircase-Escalante National Monument (GSENM) in southern Utah has protected a wealth of archaeological, biological, geological, and paleontological resources. The Kaiparowits Formation preserves an abundant and diverse assemblage of fossil e ggshell[1,2,3,4] including two partial eggs representing a new ootaxon among the smallest Mesozoic eggs described. DMNH EPV.128286, partial egg; UCM 1139 (University of Colorado Museum of Natural History), eggshell fragments. Denver Museum of Nature and Science (DMNH) localities 4386, 4361 and University of Colorado Museum of Natural History (UCM) locality 2000089 in Garfield and Kane counties, Utah; Upper Cretaceous (Upper Campanian) Kaiparowits Formation. Stillatuberoolithus storrsi is represented by two partial eggs and 29 eggshell fragments from 12 localities. X-ray computed tomography (CT) scans of the partial egg specimen DMNH EPV.128286 allows for threedimensional visualization of pore spaces, revealing that the pore canals branch in a complex anastomosing system within each node (Fig. 5c). Many of the branches are difficult to trace as they approach the outer surface of the eggshell and appear to pinch out before they reach the surface

Methods

Conclusion