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Dinosaur origin of bird reproduction (free pdf)

From: Ben Creisler

A recent paper in open access not yet mentioned on the DML in the new
journal National Science Review, an English language publication about
Chinese research:

Jingmai K. O'Connor, Xiaoting Zheng, Xiaoli Wang, Yan Wang, and
Zhonghe Zhou (2014)
Ovarian follicles shed new light on dinosaur reproduction during the
transition towards birds. National Science Review 1 (1): 15-17
doi: 10.1093/nsr/nwt012

The reproductive systems of living archosaurs, crocodilians and birds
are very different; the derived features present in birds are inferred
to have evolved within the derived archosaur group, Dinosauria, which
includes Aves (the common ancestor of Archaeopteryx + living birds) as
a member of the derived theropod clade, Maniraptora. Compared to Aves,
crocodilians have two ovaries, prolonged folliculogenesis (maturation
of ovarian follicles), large clutches and small eggs. Living birds, on
the other hand, nearly all have a single functional ovary and oviduct
(all other amniotes have two) and rapid folliculogenesis. Living birds
are the most diverse Linnean class of tetrapods on the planet and
within this diversity there exists a spectrum of clutch sizes and egg
sizes (relative to body size). However, egg size is greater than that
in crocodilians, and clutches are typically much smaller. The derived
reproductive system present in living birds was acquired gradually
during the evolution of dinosaurs and basal birds (Fig. 1), but the
timing of these changes is poorly known given the preservational
constraints of the fossil record. However, the recent report of the
exceptional preservation of ovarian follicles in one specimen of the
basal bird Jeholornis and two enantiornithines from the Early
Cretaceous Jehol Biota helps elucidate important stages in the
evolution of the modern avian reproductive system as it occurred among
derived paravian dinosaurs.

Press release