This specimen was originally described as a neoceratopsian dinosaur egg by Balanoff et al., 2008. However, further examination of the high-resolution X-ray CT data has resulted in a reidentification of the specimen as an enantiornithine bird in a paper entitled Reidentification of Avian Embryonic Remains from the Cretaceous of Mongolia, by D.J. Varricchio, A.M. Balanoff and M.A. Norell (2015, PLoS ONE 10(6): e0128458). The abstract is as follows:

Embryonic remains within a small (4.75 by 2.23 cm) egg from the Late Cretaceous, Mongolia are here re-described. High-resolution X-ray computed tomography (HRCT) was used to digitally prepare and describe the enclosed embryonic bones. The egg, IGM (Mongolian Institute for Geology, Ulaanbaatar) 100/2010, with a three-part shell microstructure, was originally assigned to Neoceratopsia implying extensive homoplasy among eggshell characters across Dinosauria. Re-examination finds the forelimb significantly longer than the hindlimbs, proportions suggesting an avian identification. Additional, postcranial apomorphies (strut-like coracoid, cranially located humeral condyles, olecranon fossa, slender radius relative to the ulna, trochanteric crest on the femur, and ulna longer than the humerus) identify the embryo as avian. Presence of a dorsal coracoid fossa and a craniocaudally compressed distal humerus with a strongly angled distal margin support a diagnosis of IGM 100/2010 as an enantiornithine. Re-identification eliminates the implied homoplasy of this tri-laminate eggshell structure, and instead associates enantiornithine birds with eggshell microstructure composed of a mammillary, squamatic, and external zones. Posture of the embryo follows that of other theropods with fore- and hindlimbs folded parallel to the vertebral column and the elbow pointing caudally just dorsal to the knees. The size of the egg and embryo of IGM 100/2010 is similar to the two other Mongolian enantiornithine eggs. Well-ossified skeletons, as in this specimen, characterize all known enantiornithine embryos suggesting precocial hatchlings, comparing closely to late stage embryos of modern precocial birds that are both flight- and run-capable upon hatching. Extensive ossification in enantiornithine embryos may contribute to their relatively abundant representation in the fossil record. Neoceratopsian eggs remain unrecognized in the fossil record.

This specimen was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Matthew Lewin of the UCSF Medical Center. Funding was provided by Dr. Lewin.

Egg and scan

This specimen was scanned by Matthew Colbert on 3 February 2004 along the 'coronal' (long) axis for a total of 1296 slices. Each 1024 x 1024 pixel slice is 0.036 mm thick, with an interslice spacing of 0.036 mm and a field of reconstruction of 32 mm.

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