Abstract:
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 individuals1. Data indicate that
many ‘avian’ traits were already present in Paraves—the clade that
includes birds and their close relatives—andthat the early evolution
of the modern avian form of reproduction was already well on its
way2,3. Like living neornithine birds, non-avian maniraptorans had
daily oviposition and asymmetrical eggs with complex shell microstructure,
and were knownto protect their clutches4–6.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 do1,6. 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.
