Sauropod nesting sites on mid-channel bars: Taphonomic evidence of environmental adaptation in the lower cretaceous Sihwa Formation, Korea

초록

The Gojeong-ri sauropod nesting area (Lower Cretaceous Sihwa Formation, Korea) captures sauropod reproduction within a high-energy braided-river system-contradicting the long-standing view that dinosaurs nested chiefly on placid flood plains. Detailed fieldwork documented 196 eggs on more than 25 horizons; four ootaxa are present, with faveoloolithids (n = 175) overwhelmingly dominant and attributable to sauropods. To unravel their burial history we applied an inside-outside matrix-contrast criterion, which simply compares the sediment that fills each egg with that of the enclosing egg-bearing deposit. This test distinguishes three taphofacies: (1) intact clutches in bar-top or overbank sandstone (Taphofacies I); (2) sand-filled eggs encased in conglomerates, indicating limited reworking (Taphofacies II); (3) gravel-filled, tightly packed eggs in scour-lag conglomerates-displaced, reworked clusters transported from upstream (Taphofacies III). Using this approach we find that 32 % of the eggs lie on-or were eroded from-elevated mid-channel bars, indicating a nesting strategy that sought sun-warmed, predator-deterring high ground encircled by water-filled channels acting as natural moats, while accepting the attendant flood risk. Similar risk-reward trade-offs exist among modern bar nesters, river-nesting birds and Amazon river turtles (Podocnemis expansa). Although bar-top strata are readily reworked and seldom preserved, their remnants in Taphofacies III record downstream egg transport and concentration. Gojeong-ri therefore expands the spectrum of dinosaur nesting behavior, showing that sauropods could exploit dynamic braided bars rather than defaulting to passive floodplain settings. The study further shows that the inside-outside matrix-contrast criterion is a robust tool for distinguishing in situ clutches from transported clusters and for tracing preservation pathways even within high-energy, architecturally complex fluvial deposits.

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