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Three-Dimensional Reconstructions of the Putative Metazoan Namapoikia

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Three-Dimensional Reconstructions of the Putative Metazoan Namapoikia 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 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 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). calcifying organisms of definitive poriferan affinity were present Today, biomineralizing organisms are responsible for building some of Earth’s largest organic constructions (e.g., the Great Barrier Reef), which is indicative of the outsize impact that Significance biomineralization has had on the planet’s sedimentological, biological, and geochemical makeup. Animals that build skeletons have an outsized impact on To understand when, where, and why animals began to Earth’s biological, geochemical, and sedimentological cycles. biomineralize, as well as to determine the environmental, eco- To determine when, where, and why metazoan biomineraliza- logical, and evolutionary ramifications associated with the first tion first emerged, it is necessary to study the earliest record biomineralizers, it is necessary to study the earliest skeletal of skeletal animals. This record is made up of four genera from metazoan fossil record. This record comprises four genera from the Ediacaran period: Namacalathus, Cloudina, Sinotubulites, Ediacaran shallow water settings: Namacalathus, Cloudina, Sino- tubulites, and Namapoikia (3). Although morphologically simple, and Namapoikia. Here, we measure three-dimensional recon- these organisms have proven to be enigmatic, and their growth structions of Namapoikia to test the hypothesis that it is a habits, biological affinities, and environmental impacts are the calcifying sponge. We find that Namapoikia lacks the physi- subject of ongoing debate. cal characteristics expected of a sponge, or, for that matter, an With respect to early biomineralization, modes of shell build- animal. ing appear to have varied among the Ediacaran putative biomin- eralizers. Exactly how, and to what degree, each organism made Author contributions: A.M., W.A.W., J.P.G., and A.C.M. designed research; A.M. per- formed research; A.M. and A.C.M. analyzed data; and A.M. and A.C.M. wrote the hard parts remains unresolved. Workers have suggested that paper.y Namacalathus, a flexible, goblet-shaped organism, produced a The authors declare no competing interest.y foliated calcitic ultrastructure (4). Conversely, Namacalathus also has been shown to have been lightly calcified (5, 6). Cloud- This article is a PNAS Direct Submission.y ina, a tubular organism made up of a “cup in cup” morphology, Published under the PNAS license.y was thought to have precipitated carbonate on an organic matrix Data deposition: The computational source code used in this paper is available in GitHub (5, 7). More recent work, however, has demonstrated that phos- at https://github.com/giriprinceton/namapoikia.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 www.pnas.org/cgi/doi/10.1073/pnas.2009129117 PNAS Latest Articles j 1 of 7 Downloaded by guest on September 25, 2021 on Earth, and some sponges, namely the Archaeocyathids, ramp in the northern Zaris subbasin coincident with conver- were even responsible for the world’s first framework reefs (2). gence along the Damara and Gariep orogens (20, 21). There It stands to reason that poriferans, having appeared during the are no direct radiometric dates from the Driedoornvlakte stratig- Cryogenian and eventually becoming the dominant engineers raphy. However, uranium–lead zircon ages from the Kuibis of the Early Cambrian, may have first evolved the ability to constrain the maximum depositional age to 548.8 ± 1 Ma build calcified skeletons during the Late Ediacaran. To test this (20). Additionally, a uranium–lead zircon date in the overlying idea, we seek to determine whether Namapoikia is, in fact, a Schwarzrand subgroup (22) provides a minimum deposition age sponge-grade organism. of 545.41 ± 1 Ma. Like many other Ediacaran fossils, specimens of Namapoikia The reef at Driedoornvlakte Farm, which is 500 m thick, 10 km lack soft tissue preservation and exhibit signs of diagenetic alter- long, and dips 25 to 40◦ to the southeast, sits on Precambrian ation (e.g., recrystallization). As a result, it is necessary to analyze quartzite. The carbonate ramp was created over the course of the gross morphological characteristics of Namapoikia speci- three distinct accommodation cycles; in the final stage, just prior mens, such as the size, shape, and distribution of structures, to drowning by shales of the Urikos Member, pinnacle buildups in order to describe growth habit, identify possible analogs, developed on the platform margins (21). These pinnacles com- and evaluate biological affinity. Since 2D measurements (e.g., prise microbial mounds made up of a combination of columnar made on polished slabs or on bedding planes in outcrop) or encrusting stromatolites and columnar or massive thrombo- are subject to misinterpretation and measurement error (11), lites. Both Cloudina and Namacalathus can be found in the three-dimensional (3D) data are required for accurate analysis. fill between microbial buildups and in clinoformal grainstones. Unfortunately, Namapoikia skeletons are preserved as carbonate Namapoikia is found encrusting the walls and tops of microbial minerals within carbonate rock, precluding isolation via acid dis- buildups in decimeter-wide neptunian dykes (Fig. 1 D and E), solution or imaging with traditional, density-sensitive techniques. which are shallow fractures in the reef that opened to the To address this problem, we utilize
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