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Jeholornis and enantiornithines had one functioning ovary like modern birds

From: Ben Creisler

A new online paper in Nature:

Xiaoting Zheng, Jingmai O’Connor, Fritz Huchzermeyer, Xiaoli Wang, Yan
Wang, Min Wang & Zhonghe Zhou (2013)
Preservation of ovarian follicles reveals early evolution of avian
reproductive behaviour.
Nature (advance online publication)

The two groups of archosaurs, crocodilians and birds, form an extant
phylogenetic bracket for understanding the reproductive behaviour of
dinosaurs. This behaviour is inferred from preserved nests and eggs,
and even gravid individuals. Data indicate that many ‘avian’ traits
were already present in Paraves—the clade that includes birds and
their close relatives—and that the early evolution of the modern avian
form of reproduction was already well on its way. Like living
neornithine birds, non-avian maniraptorans had daily oviposition and
asymmetrical eggs with complex shell microstructure, and were known to
protect their clutches. However, like crocodilians, non-avian
maniraptorans had two active oviducts (one present in living birds),
relatively smaller eggs, and may not have turned their eggs in the way
that living birds do. Here we report on the first discovery of
fossilized mature or nearly mature ovarian follicles, revealing a
previously undocumented stage in dinosaur reproduction: reproductively
active females near ovulation. Preserved in a specimen of the long
bony-tailed Jeholornis and two enantiornithine birds from the Early
Cretaceous period lacustrine Jehol Biota in northeastern China, these
discoveries indicate that basal birds only had one functional ovary,
but retained primitive morphologies as a result of their lower
metabolic rate relative to living birds. They also indicate that basal
birds reached sexual maturity before skeletal maturity, as in
crocodiles and paravian dinosaurs. Differences in follicular
morphology between Jeholornis and the enantiornithines are interpreted
as forming an evolutionary gradient from the reproductive condition in
paravian dinosaurs towards neornithine birds. Furthermore, differences
between the two enantiornithines indicate that this lineage might also
have evolved advanced reproductive traits in parallel to the
neornithine lineage.

News commentary by Brian Switek: