Reidentification of avian embryonic remains from the cretaceous of mongolia

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) 100/2...

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Bibliographic Details
Published in:PloS one 2015-06, Vol.10 (6), p.e0128458-e0128458
Main Authors: Varricchio, David J, Balanoff, Amy M, Norell, Mark A
Format: Article
Language:English
Subjects:
Animals
Aves
Birds
Birds - anatomy & histology
Birds - embryology
Bones
Columns (structural)
Computed tomography
Cretaceous
Cretaceous period
Dinosauria
Dinosaurs
Egg shells
Eggs
Elbow
Elbow (anatomy)
Embryos
Femur
Fossils
Geology
Hatching
Homoplasy
Humerus
Identification
Immunoglobulin M
Juveniles
Knee
Microstructure
Models, Anatomic
Morphology
Museums
Ornithischia
Ossification
Paleontology
Posture
Taxonomy
Ulna
Vertebra
Vertebrae
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Summary: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.
ISSN:1932-6203
1932-6203
DOI:10.1371/journal.pone.0128458