Beyond the stony veil: Reconstructing the Earth’s earliest large animal traces via computed tomography X-ray imaging

Abstract

a b s t r a c tTrace fossils are superb lines of evidence for examining the ancient biologic world because they offer an opportunity to infer behavioral ecology of organisms. However, traces can be difficult to parse from their matrix, which leads to the loss of important morphological and behavioral data. This is especially true for the earliest marine animal traces from the Ediacaran Period (635-541 Ma), which are usually small (<5 mm in diameter) and simple (mostly small horizontal trails and burrows), and are sometimes difficult to be distinguished from co-existing tubular body fossils. There is also evidence that the prevalence of microbial substrates in Ediacaran oceans may have influenced emerging trace makers in nonactualistic ways from a late Phanerozoic perspective (e.g., microbial mats may have facilitated a strong geochemical gradient across the sediment-water interface). Therefore, the discovery of the relatively large traces of Lamonte trevallis from the Ediacaran Shibantan Member of the Denying Formation ($551-541 Ma) in the Yangtze Gorges area of South China provides a unique opportunity to study early bioturbators. These trace fossils are large enough and have sufficient compositional contrast (relative to the matrix) for in situ analysis via X-ray computed tomography (CT) and microcomputed tomography (microCT). Each analytical method has its own advantages and disadvantages. CT scans can image larger specimens, but cannot adequately resolve small features of interest. MicroCT scans can achieve higher resolution, but can only be used with small samples and may involve more post-processing than CT scans. As demonstrated in this study, X-ray CT and microCT in combination with other 3D imaging techniques and resources have the potential to resolve the 3D morphology of Ediacaran trace fossils. A new Volumetric Bioturbation Intensity (VBI) is also proposed, which quantifies whole rock bioturbation using 3D analysis of subsurface traces. Combined with the ability to examine trace fossils in situ, the VBI can enhance our view of ancient ecologies and life's enduring relationship with sediments.

Late Ediacaran trackways produced by bilaterian animals with paired appendages

“…541-Ma Shibantan Member deposited in a subtidal environment between fair-weather and storm wave bases (15,18). Like other trace fossils from the same locality and horizon, they exclusively occur in beds characterized by crinkled microlaminae, which are interpreted as microbial mats, suggesting intimate ecological interactions between the trace makers and the mats (13,15,19). These microbial mats comprised a few millimeters of sediment and may have also helped trace fossil preservation (20,21).…”

“…This is consistent with the preservational style of undermat burrows in the Shibantan Member, where both microbial mats above and below the burrows can serve as parting surfaces [for example, figure 2E in the study of Meyer et al . ( 19 )], resulting in positive epirelief or positive hyporelief, respectively. Consistent with other undermat burrows in the Shibantan Member (fig.…”

“…Consistent with other undermat burrows in the Shibantan Member (fig. S1) ( 13 , 15 , 19 ), UB1 to UB3 are filled with carbonate sediments with a significant amount of calcite cements. This is best seen in UB3 ( Fig.…”