Upper Cretaceous (Coniacian-Santonian) dinosaur nesting colony preserved in abandoned crevasse splay deposits, Wi Island, South Korea

초록

Dinosaur egg-bearing deposits of the Upper Cretaceous (Coniacian-Santonian) Wido Volcanics at Wi Island, South Korea, were studied from a taphonomic perspective to improve understanding of nesting behavior. Based on facies, bedset geometry, and other sedimentological characteristics, the section can be classified into three facies associations: (1) ephemeral fluvial channels (FA I), (2) crevasse splay lobes (FA II), and (3) floodplains (FA III). FA I is characterized by single- and multi-storey channel fills enclosed within FA III, while FA II consists of horizontally-stratified sandstone and calcareous mudstone reflecting weakly developed paleosol profiles. The ephemeral fluvial channels are locally developed at 160 m distance from the main egg locality, suggesting that the nesting site was relatively protected from levee failure and favorable for fossil preservation. Isolated eggs and reworked eggshell fragments of three different types (Propagoolithus widoensis, Reticuloolithus acicularis, and Aenigmaoolithus vesicularis) are frequently found together. The most common type, P. widoensis is preserved as clutches and characterized by numerous closely-spaced pore canals, indicating that the eggs were buried during oviposition, increasing preservation potential. Such in situ clutches occur in at least nine horizons and some occur closely spaced (<1 m apart), lateral to crevasse splay lobe successions (FA II). It suggests that the egg-layers of P. widoensis preferred this facies as a nesting ground for extended periods. FA II was deposited at a higher paleoelevation than FA III, which may have benefited nesting dinosaurs, allowing them to easily detect predators (FA III) while being less frequently exposed to flood hazards.

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