From form to flow: reconstructing the life posture and hydrodynamics of iconic Ediacaran taxa

Pérez-Pinedo, Daniel (2025) From form to flow: reconstructing the life posture and hydrodynamics of iconic Ediacaran taxa. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Ediacaran successions of Newfoundland include some of the oldest macroscopic candidate metazoans. These fossils are preserved as two-dimensional near-census assemblages on deep-marine volcaniclastic sediments beneath tuffites. The communities, dominated by the frondose morphogroups Rangeomorpha and Arboreomorpha, were traditionally interpreted as consisting primarily of erect taxa that had been felled and aligned by paleocurrents, alongside some randomly oriented reclining forms. However, in the absence of sedimentological evidence, paleocurrent direction has been inferred from fossil orientation, despite the unresolved posture of many of these organisms in life. This PhD focuses on the Melrose Surface near the Catalina Dome of the Discovery UNESCO Global Geopark, which provides sedimentological evidence for current direction in the form of current ripples and cross-lamination. This site offers a unique opportunity to reconstruct the life position and orientation of Ediacaran taxa relative to sedimentologically-determined paleocurrents. An integrative approach has been employed herein, combining different strategies detailed in three papers: 1. Taphonomic reinterpretation of the genus Charniodiscus: by examining the taphonomic clues of the type material C. concentricus, this taxon was reconstructed as a conical bifoliate erect/recumbent frond, distinguishing it from reclining arboreomorphs such as C. procerus (present in the surface), which shows consistently extraordinary stem preservation across stratigraphic units. 2. Clustering algorithms on Fractofusus misrai orientations: using novel statistics based on modified polythetic and monothetic clustering techniques adapted to circular variables, preferential orientation trends oblique to the paleocurrent were identified in the population of F. misrai from the Melrose Surface. 3. Computational Fluid Dynamics (CFD) over Fractofusus misrai: turbulent flows were modeled to understand the hydrodynamic phenomena associated with highly detailed reconstructions F. misrai. The preferential orientations found on the Melrose Surface result from a balance between maximizing aspect ratio for feeding efficiency and reducing current-induced drag. This work presents an integrative approach based on taphonomy, statistics, and CFD to reconstruct the position of Ediacaran taxa and the currents they lived in. The papers challenge the assumed erect position for all arboreomorphs and the random orientation for reclining rangeomorph taxa. This thesis advocates for interpreting fossils as reclining flat, responding rheotropically to paleocurrents, unless evidence supports otherwise.

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