With an estimated 15,000 living species (1), the animal phylum Porifera (colloquially known as sponges) is not a biodiversity heavyweight as are arthropods, mollusks, and chordates. Unassuming in character, sponges barely move in their adult lifetime of up to several thousand years, and they passively strain food particles from water currents that they generate continuously around the clock. They would be one of the most boring pets to have. However, sponges contribute to the global ecosystem and to our knowledge about animal evolution in their own ways. Because sponges diverged near the base of the animal family tree (2), they hold a special place in understanding early animal evolution. Because sponges were among the first animals to build mineralized skeletons through biologically controlled processes (3), they provide key insights into the origins of animal biomineralization. Also, because many sponges are important reef constructors, they play an essential role in modern reef ecosystems and may take over the reins from corals as global warming exacerbates (4). Thus, when and how sponges evolved, acquired biomineralization, and started contributing to reef construction become critical questions in the early evolution of animals, biomineralization, and reefs. In this context, theca. 550-My-old fossilNamapoikia rietoogensis (Fig. 1A andB), originally reported by Wood et al. (5) from carbonate rocks of the Omkyk Member in southern Namibia, has been featured prominently because it is considered one of the oldest sponges that built biologically controlled aragonitic skeletons and contributed to the construction of the oldest animal reefs (6). Writing in PNAS, however, Mehra et al. (7) question the sponge interpretation ofN. rietoogensis. Instead, they interpretN. rietoogensis as a microbial construction analogous to stromatolites and thrombolites, which are microbial buildups particularly common in Precambrian and early Paleozoic carbonates (8). The implications are thatN. rietoogensis did not build its calcareous structures in a biologically controlled fashion as do modern sponges, and it may have lacked the structural integrity to support reef growth (7).

Why does it matter whetherN. rietoogensis is a sponge? We need to understand early sponge evolution in order to understand early animal evolution, because sponges are either a paraphyletic group at the base of the animal tree (9) or a monophyletic clade constituting a sister group of all other animals (2). Molecular fossils or biomarkers indicate that one of the modern sponge classes, the demosponges, diverged no later thanca. 650 Ma in the Cryogenian Period (ref.10, but see ref.11). Molecular clock estimates, including those independent of the aforementioned biomarkers as calibrations, place the divergence of sponge classes at 700 to 800 Ma in the Tonian and Cryogenian Periods (12). The abundant evidence of Ediacaran eumetazoans and even bilaterian animals (e.g., refs.13 and14), which diverged after the sponges, also dictates the presence of at least total-group sponges in the Ediacaran Period or earlier. However, the first unequivocal sponge fossils do not appear until in the Cambrian Period, with disarticulated sponge spicules aroundca. 535 Ma (15) and fully articulated sponge bodies shortly after (16). Thus, there is a prominent gap in the sponge fossil record, and any bona fide sponge fossils from the Ediacaran Period (635 to 539 Ma) would help to fill this gap.

According to Wood et al. (5),N. rietoogensis (Fig. 1A andB) is an Ediacaran sponge that encrusted fissure walls in microbial–metazoan reefs constructed by thrombolites andCloudina (ref.17, but see ref.18), the latter of which is also regarded as a biomineralizing animal (ref.19, but see ref.20). It consists of irregularly tessellated tubules that are separated by poorly defined partitioning walls. Thus, it looks like palisades in longitudinal section but appears spongy and labyrinthine in transverse section (Fig. 1B). It also has structures interpreted as tabulae and dissepiments, features that are present in some demosponges (1). Overall,N. rietoogensis shares some morphological similarities with extant demosponges such asVaceletia cryptica and asAcanthochaetetes wellsi, and it is phylogenetically placed within the total group Porifera (6).

Mehra et al. question the sponge interpretation ofN. rietoogensis. Instead, they interpretN. rietoogensis as a microbial construction analogous to stromatolites and thrombolites, which are microbial buildups particularly common in Precambrian and early Paleozoic carbonates.

Mehra et al.’s (7) objection to a sponge interpretation forN. rietoogensis is based on three-dimensional morphological reconstructions of two specimens using a serial grinding technique. They found that the tubules and partitions ofN. rietoogensis are much larger and morphologically more variable than corresponding features in extant and extinct demosponges. Mehra et al. (7) did not observe tabulae and dissepiments in the two specimens they analyzed, and they argue thatN. rietoogensis lacks sponge synapomorphies (or characters uniquely evolved in sponges) such as ostia and oscula, although this could be related to the small number of specimens analyzed or the limited resolution of fossil preservation (ostia, for example, are too small to be preserved in Omkyk carbonates). It is also inherently difficult to rule out a stem-group sponge interpretation forN. rietoogensis even if it does lack such sponge synapomorphies as ostia and oscula. Mehra et al. (7) favor a microbial interpretation, although they also note that no known microbial constructions provide a perfect analog forN. rietoogensis. Nonetheless, Mehra et al. (7) provide a three-dimensional reconstruction ofN. rietoogensis, and this is an important step forward toward a phylogenetic resolution of this controversial fossil.

N. rietoogensis is not the only purported sponge fossil from the Ediacaran Period. There are dozens of putative sponges from the Ediacaran Period, but their sponge affinity has been disputed (15,21). Certainly, none of the purported Ediacaran sponges have unambiguous spicules (21,22), in sharp contrast to Cambrian sponges that are mostly identified on the basis of their biomineralized spicules. In one scenario, assuming sponge spicules have a single origin and a good preservation potential (21), the last common ancestor of extant sponges must have had spicules, and the lack of Precambrian spicules means the divergence of crown-group sponges at the Ediacaran–Cambrian boundary (21); hence, any Precambrian sponges must be stem lineages and aspiculate (Fig. 1C). Alternatively, considering the documented cases of multiple origins of biomineralization in animals (3) and the possibility of independent evolution of spicules among sponge classes (23), it is conceivable that modern sponge classes may have diverged in the Precambrian but independently evolved biomineralized spicules at the Ediacaran–Cambrian boundary (24) (Fig. 1D). Both scenarios are consistent with the absence of Precambrian spicules, but the latter scenario is also consistent with the molecular clocks and biomarkers and predicts the existence of aspiculate crown-group sponges in the Ediacaran Period. In this context, it is worth noting Muscente et al.’s (22) report of Ediacaran organic filaments with a rectangular prismatic shape that are interpreted as possible precursors of axial filaments later recruited to template spicule formation, as well as Tang et al.’s (24) documentation that early Cambrian hexactine sponge spicules are only weakly biomineralized, with a large axial filament but proportionally less biomineral when compared with younger sponge spicules. These new data lend some support to the hypothesis that sponge classes diverged in the Precambrian but independently evolved spiculogenesis in the Cambrian. This hypothesis implies that the search for Precambrian sponges should be shifted away from the search for spicules (24), and further adds to the remarkable number of animal lineages that independently acquired biomineralization in the early Cambrian (3).

It should be pointed out that whetherN. rietoogensis is a sponge, whether it had biomineralized skeletons, and whether it contributed to reef construction are three separate questions. Even ifN. rietoogensis is not a sponge, it seems to have constructed biomineralized skeletons with some regularity and it may have used an organic template for biomineralization (6). Regardless of the degree of biological control in mineralization,N. rietoogensis was definitely part of the Ediacaran reef community, because it dwelled in cryptic crevices and fissures in thromobolitic reefs (5). Its ability to encrust thromobites and to cement cryptic space via biomineralization (6) means that it contributed to the overall structural integrity of the reefs.

The debate onN. rietoogensis and other putative Ediacaran sponges will likely continue. Questions about their phylogenetic affinity, their ability to biomineralize, and their paleoecology are difficult to answer but worth asking, because they inform us about the early evolution of animals and sponges’ impact on the global ecosystem and environment (25). Sponges are not as dull as they first seem to be.