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Three-Dimensional Reconstructions of the Putative Metazoan Namapoikia Show That It Was a Microbial Construction Akshay Mehraa,B,C,1 , Wesley A

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Three-Dimensional Reconstructions of the Putative Metazoan Namapoikia Show That It Was a Microbial Construction Akshay Mehraa,B,C,1 , Wesley A Three-dimensional reconstructions of the putative metazoan Namapoikia show that it was a microbial construction Akshay Mehraa,b,c,1 , Wesley A. Wattersd, John P. Grotzingere, and Adam C. Maloofa aDepartment of Geosciences, Princeton University, Princeton, NJ 08544; bNeukom Institute, Dartmouth College, Hanover, NH 03755; cDepartment of Earth Sciences, Dartmouth College, Hanover, NH 03755; dDepartment of Astronomy, Wellesley College, Wellesley, MA 02481; and eDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 Edited by Andrew H. Knoll, Harvard University, Cambridge, MA, and approved June 23, 2020 (received for review May 12, 2020) Strata from the Ediacaran Period (635 million to 538 million built wave-resistant frameworks (9, 10), revealed that aggregates years ago [Ma]) contain several examples of enigmatic, puta- comprise transported and deformed individuals (11), furthering tive shell-building metazoan fossils. These fossils may provide the idea that Cloudina produced weakly-to-non-biomineralized insight into the evolution and environmental impact of biomin- tubes. Sinotubulites, another tubular organism, also had plastic eralization on Earth, especially if their biological affinities and walls, but ones that were made up of a predominately organic modern analogs can be identified. Recently, apparent morpho- matrix (12). logical similarities with extant coralline demosponges have been Recently, researchers have proposed that Namapoikia, a used to assign a poriferan affinity to Namapoikia rietoogen- labyrinthine encrusting construction, produced skeletal mate- sis, a modular encrusting construction that is found growing rial by rapidly calcifying an organic scaffold (13). Studies of between (and on) microbial buildups in Namibia. Here, we polished, two-dimensional (2D) transverse and longitudinal sec- present three-dimensional reconstructions of Namapoikia that tions of Namapoikia suggest a complex interplay between the we use to assess the organism’s proposed affinity. Our mor- construction and surrounding microbial growths, with the two phological analyses, which comprise quantitative measurements life forms competing and, in some cases, repeatedly encrusting of thickness, spacing, and connectivity, reveal that Namapoikia over one another (13). Workers have proposed that Namapoikia produced approximately millimeter-thick meandering and branch- shares morphological characteristics with Chaetetid sponges and ing/merging sheets. We evaluate this reconstructed morphol- have inferred a biomineralizing pathway that is like that of the ogy in the context of poriferan biology and determine that extant demosponges Vaceletia and Acanthochaetetes (13, 14). On Namapoikia likely is not a sponge-grade organism. the basis of these similarities, Namapoikia has been assigned a poriferan affinity. 3D reconstruction j Ediacaran j early life Molecular clock and phylogenetic estimates (15) suggest that poriferans evolved during the Cryogenian Period (720 Ma to 635 Ma). Indeed, the Precambrian fossil record is replete with exam- n the Late Ediacaran (∼550 Ma), microbe-dominated reefs ples of purported sponge remains. Spicules, biomarkers, and Ibore witness to the arrival of putative biomineralizing meta- even full body fossils, all older than the onset of the Ediacaran zoans. By the Cambrian radiation [beginning 538.6 Ma to 538.8 Period, have been described—and debated—by researchers (for Ma (1)], a time period during which most modern animal phyla a complete review, see ref. 16; also see refs. 17 and 18 for more first emerged, skeletal reef dwellers were producing framework recent examples of debate). Additionally, by the early Cambrian, constructions and effectively engineering their surroundings (2). Today, biomineralizing organisms are responsible for building some of Earth’s largest organic constructions (e.g., the Great Significance Barrier Reef), which is indicative of the outsize impact that biomineralization has had on the planet’s sedimentological, Animals that build skeletons have an outsized impact on Earth’s biological, and geochemical makeup. biological, geochemical, and sedimentological cycles. To deter- To understand when, where, and why animals began to mine when, where, and why metazoan biomineralization first biomineralize, as well as to determine the environmental, eco- emerged, it is necessary to study the earliest record of skele- logical, and evolutionary ramifications associated with the first tal animals. This record is made up of four genera from the biomineralizers, it is necessary to study the earliest skeletal Ediacaran period: Namacalathus, Cloudina, Sinotubulites, and metazoan fossil record. This record comprises four genera from Namapoikia. Here, we measure three-dimensional reconstruc- Ediacaran shallow water settings: Namacalathus, Cloudina, Sino- tions of Namapoikia to test the hypothesis that it is a cal- tubulites, and Namapoikia (3). Although morphologically simple, Namapoikia these organisms have proven to be enigmatic, and their growth cifying sponge. We find that lacks the physical habits, biological affinities, and environmental impacts are the characteristics expected of a sponge, or, for that matter, an subject of ongoing debate. animal. With respect to early biomineralization, modes of shell build- Author contributions: A.M., W.A.W., J.P.G., and A.C.M. designed research; A.M. per- ing appear to have varied among the Ediacaran putative biomin- formed research; A.M. and A.C.M. analyzed data; and A.M. and A.C.M. wrote the eralizers. Exactly how, and to what degree, each organism made paper.y hard parts remains unresolved. Workers have suggested that The authors declare no competing interest.y Namacalathus, a flexible, goblet-shaped organism, produced a This article is a PNAS Direct Submission.y foliated calcitic ultrastructure (4). Conversely, Namacalathus also has been shown to have been lightly calcified (5, 6). Cloud- Published under the PNAS license.y ina, a tubular organism made up of a “cup in cup” morphology, Data deposition: The computational source code used in this paper is available in GitHub was thought to have precipitated carbonate on an organic matrix at https://github.com/giriprinceton/namapoikia.y (5, 7). More recent work, however, has demonstrated that phos- See online for related content such as Commentaries.y phatized Cloudina share a nanoparticulate fabric with extant 1 To whom correspondence may be addressed. Email: [email protected] biomineralizers, suggesting that the organism formed skeletons This article contains supporting information online at https://www.pnas.org/lookup/suppl/ in the same way as modern animals (8). That said, reconstruc- doi:10.1073/pnas.2009129117/-/DCSupplemental.y tions of Cloudina, made to test the assertion that the organism First published August 3, 2020. 19760–19766 j PNAS j August 18, 2020 j vol. 117 j no. 33 www.pnas.org/cgi/doi/10.1073/pnas.2009129117 Downloaded at California Institute of Technology on August 19, 2020 calcifying organisms of definitive poriferan affinity were present Neoproterozoic Nama Group. They formed on a carbonate ramp on Earth, and some sponges, namely the Archaeocyathids, in the northern Zaris subbasin coincident with convergence along were even responsible for the world’s first framework reefs (2). the Damara and Gariep orogens (20, 21). There are no direct It stands to reason that poriferans, having appeared during the radiometric dates from the Driedoornvlakte stratigraphy. How- Cryogenian and eventually becoming the dominant engineers ever, uranium–lead zircon ages from the Kuibis constrain the of the Early Cambrian, may have first evolved the ability to maximum depositional age to 548.8 ± 1 Ma (20). Additionally, a build calcified skeletons during the Late Ediacaran. To test this uranium–lead zircon date in the overlying Schwarzrand subgroup idea, we seek to determine whether Namapoikia is, in fact, a (22) provides a minimum deposition age of 545.41 ± 1 Ma. sponge-grade organism. The reef at Driedoornvlakte Farm, which is 500 m thick, 10 km Like many other Ediacaran fossils, specimens of Namapoikia long, and dips 25 to 40◦ to the southeast, sits on Precambrian lack soft tissue preservation and exhibit signs of diagenetic alter- quartzite. The carbonate ramp was created over the course of ation (e.g., recrystallization). As a result, it is necessary to analyze three distinct accommodation cycles; in the final stage, just prior the gross morphological characteristics of Namapoikia speci- to drowning by shales of the Urikos Member, pinnacle buildups mens, such as the size, shape, and distribution of structures, developed on the platform margins (21). These pinnacles com- in order to describe growth habit, identify possible analogs, prise microbial mounds made up of a combination of columnar and evaluate biological affinity. Since 2D measurements (e.g., or encrusting stromatolites and columnar or massive thrombo- made on polished slabs or on bedding planes in outcrop) lites. Both Cloudina and Namacalathus can be found in the are subject to misinterpretation and measurement error (11), fill between microbial buildups and in clinoformal grainstones. three-dimensional (3D) data are required for accurate analysis. Namapoikia is found encrusting the walls and tops of microbial Unfortunately, Namapoikia skeletons are preserved as carbonate buildups in decimeter-wide neptunian dykes (Fig. 1 D and E), minerals within carbonate rock, precluding isolation via acid dis- which are shallow
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