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Baby enantiornithines in American Museum Novitates

LUIS M. CHIAPPE, JI SHU'AN2, and JI QIANG. 2007. Juvenile Birds from the
Early Cretaceous of China: Implications for Enantiornithine Ontogeny. AM
Novitates 3594:1-46. 

Mesozoic remains of embryonic and early juvenile birds are rare. To date, a
handful of in ovo embryos and early juveniles of enantiornithines from the
Early Cretaceous of China and Spain and the Late Cretaceous of Mongolia and
Argentina have comprised the entire published record of perinatal
ontogenetic stages of Mesozoic birds. We report on the skeletal morphology
of three nearly complete early juvenile avians from the renowned Early
Cretaceous Yixian Formation of Liaoning Province in northeastern China.
Evidence of the immaturity of these specimens is expressed in the intense
grooving and pitting of the periosteal surfaces, the disproportionately
small size of the sterna, and the relative size of the skull and orbits.
Size notwithstanding, anatomical differences between these three specimens
are minimal, leaving no basis for discriminating them into separate taxa.
Numerous osteological synapomorphies indicate that they are
euenantiornithine birds, the most diverse clade of Enantiornithes, but their
identification as members of a particular euenantiornithine taxon remains
unclear. Their early ontogenetic stage, however, provides important
information about the postnatal development of this specious clade of
Cretaceous birds. The presence of pennaceous wing feathers suggests that
fledging occurred very early in ontogeny, thus supporting a precocial or
highly precocial strategy for enantiornithine hatchlings. The morphology of
these new early-stage juveniles is also significant in that they allow a
better understanding of the homologies of several avian compound bones
because the components of these skeletal compounds are preserved prior to
their coossification. The general morphology of the wrist and ankle of these
juveniles highlights once again the striking similarity between nonavian
theropods and early birds.

DOI: 10.1206/0003-0082(2007)3594[1:JBFTEC]2.0.CO;2

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA