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Re: "Ceratopsian" egg reidentified as avian

A blog commentary:


On Mon, Jun 1, 2015 at 11:09 AM, Ben Creisler <bcreisler@gmail.com> wrote:
> Ben Creisler
> bcreisler@gmail.com
> 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
> doi:10.1371/journal.pone.0128458
> http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0128458
> 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.