Researchers report that a putative early animal fossil that bears superficial resemblance to sponges in 2D cross-sections lacks characteristic sponge structures when examined in three dimensions. Skeleton-building animals are crucial to biological and geochemical cycles. Hence, researchers look to the oldest known putative fossilized skeletal animals to determine how and when such an evolutionary development occurred. One genus of such fossils from the Ediacaran period is Namapoikia, hypothesized to be a calcifying sponge. Using serial grinding and neural-network-based image processing, Akshay Mehra and colleagues built and examined 3D reconstructions of two Namapoikia fossils from Namibia. Voids in both samples were large and irregularly distributed, and both partitions and voids varied in thickness between the samples. In contrast, sponges and animals typically build regularly spaced structures. Whereas sponges typically construct canal systems to transport fluid and nutrients throughout the skeleton, no such structures were observed in Namapoikia. Instead, partitions were sheet-like and lacking in chambers. The results suggest that the organism would have resembled meandering and branching ridges emerging from infilling seafloor cement or sediment. This structure is reminiscent of thrombolites and other incrementally growing microbial mats, which, unlike animals, vary in structure and form in response to environmental conditions. According to the authors, the results challenge existing ideas about life before the rise of modern animals.
###
Article #20-09129: "Three-dimensional reconstructions of the putativemetazoan Namapoikia show that it was a microbial construction," by Akshay Mehra, Wesley Watters, John Grotzinger, and Adam Maloof.
MEDIA CONTACT: Akshay Mehra, Princeton University, NJ; e-mail: akmehra@alumni.princeton.edu
Proceedings of the National Academy of Sciences