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"Ceratopsian" egg reidentified as avian

Ben Creisler

New in PLoS ONE:

David J. Varricchio , Amy M. Balanoff & Mark A. Norell (2015)
Reidentification of Avian Embryonic Remains from the Cretaceous of Mongolia.
PLoS ONE 10(6): e0128458

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.


Still no fossils eggs confirmed for Protoceratops or Psittacosaurus...

The re-identification of IGM 100/2010 highlights that no eggs or
eggshell directly association with neonate or adult individuals exists
for the neoceratopsian clade. The absence of any egg remains stands in
marked contrast to the several specimens of very small juveniles,
possibly neonates for the neoceratopsians such as Psittacosaurus,
Bagaceratops and Protoceratops, including assemblages within possible
nesting traces.