Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia, Argentina

At the Auca Mahuevo locality in the Upper Cretaceous Anacleto Formation in Patagonia, Argentina pedogenic processes associated with Vertisol development affected changes in both individual titanosaur egg morphology and three-dimensional egg distribution. These changes resulted primarily from vertical and lateral movement within fluvial overbank sediments due to clay mineral expansion and contraction in alternating wet and dry seasonal conditions. At the scale of individual sauropod eggs, pedogenic sediment movement produced egg shearing, eggshell fracture and displacement, mechanical abrasion of egg ornamentation, and alteration of egg size and shape. Movement of either individual eggs or subsets of eggs along slickensided surfaces (1) modified the number and relative position of eggs within individual clutches, (2) combined eggs of one of more clutches produced by different females, and (3) combined eggs from one or more nesting horizons, producing a time-averaged fossil assemblage. These mechanisms of egg rearrangement suggest that accurate interpretation of dinosaur reproductive behavior using fossil egg assemblages preserved in fine-grained fluvial overbank deposits require careful assessment of pedogenic processes.