Abstract
We address the identity of putative ovarian follicles in Early Cretaceous bird fossils from the Jehol Biota (China), whose identification has previously been challenged. For the first time, we present a link to the botanical fossil record, showing that the “follicles” of some enantiornithine fossils resemble plant propagules from the Jehol Biota, which belong to Carpolithes multiseminalis. The botanical affinities of this “form-taxon” are currently unresolved, but we note that C. multiseminalis propagules resemble propagules associated with cone-like organs described as Strobilites taxusoides, which in turn are possibly associated with sterile foliage allocated to Liaoningcladus. Laser-Stimulated Fluorescence imaging furthermore reveals different intensities of fluorescence of “follicles” associated with a skeleton of the confuciusornithid Eoconfuciusornis zhengi, with a non-fluorescent circular micro-pattern indicating carbonaceous (or originally carbonaceous) matter. This is inconsistent with the interpretation of these structures as ovarian follicles. We therefore reaffirm that the “follicles” represent ingested food items, and even though the exact nature of the Eoconfuciusornis stomach contents remains elusive, at least some enantiornithines ingested plant propagules.
Highlights
Over the past decades, the Jehol Biota in northeast China yielded an extraordinary diversity of fossils, which produced unprecedented insights into Early Cretaceous ecosystems
The authors acknowledge that the purported follicles vary in texture and morphology across nine referenced specimens and ‒ unlike the heterogeneity seen in modern bird follicle development ‒ are homogenous in size within each specimen
Microscopic structural data is at risk of subjective interpretation[13], and such considerations have often been invoked with respect to controversial Archaean ‘microfossils’[14] and dubious dinosaur e rythrocytes[15]
Summary
The Jehol Biota in northeast China yielded an extraordinary diversity of fossils, which produced unprecedented insights into Early Cretaceous ecosystems. Even though the specimens from these localities are known for their exquisite soft-tissue preservation, the discovery of putative ovarian follicles in some of the bird fossils stands out and is otherwise unmatched in the avian fossil record Aggregations of such ovoid structures were first reported in specimens of the Enantiornithes and Jeholornithidae[1]. The first two of these observations were countered by a reference to eggs in Jehol fish fossils that remain unpublished and by the proposal that a similar follicle size may have been due to similar dimensions of the pelvic canal in differently-sized Mesozoic b irds[3] It is noted, that it is not the mere preservation of ovarian follicles as such that was deemed unlikely, but their occurrence in fossils that do not show other traces of preservation of comparable soft tissue t ypes[2]. The presence of similar structures was noted in a fossil of the non-avian coelurosaurian theropod Compsognathus[2], in which they were interpreted as ovarian follicles[3,6] rather than being of taphonomic or diagenetic origin as previously a ssumed[11]
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