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A Bridge Too Far? In Search of Precambrian Sponges

Photo credit: Tony Webster, via Flickr (cropped).

In Günter Bechly’s very balanced and scholarly series about the Ediacaran hopeful monster Kimberella, one subtext pops out: some evolutionists seem very eager to make more of a fossil than it is willing to offer. This is understandable, because ever since Darwin, the Cambrian explosion has been one of their biggest headaches. You get the feeling of two guys without enough rope to build a makeshift bridge across a canyon. One is pulling his rope from the south side to reach the north side. The other is pulling his rope on the north side to reach the south side. No matter how hard they pull, there’s not enough rope to meet in the middle. They remain convinced, nonetheless, that there is enough rope in the kit because the instructions assure them as much. There is even a picture of the bridge on the box top.

Darwinian paleontologists think they can alleviate the embarrassment of the Cambrian explosion by the same strategy. Some stretch the interpretation of Ediacaran organisms to make them look more Cambrian (like finding molluscan traits in Kimberella). At the other end, some keep looking for Cambrian transitional forms back in the Ediacaran. The fossils are not cooperating, but the molecular clocks, like the instructions, assure them that the gap must not be real.

A key in this exercise is how to interpret sponges, as readers will recall from Dr. Bechly’s article here back in May, “The Myth of Precambrian Sponges.” There is one fossil that has generated a lot of discussion. Before looking at the latest news about it, recall how serious the Cambrian explosion is for Darwinism by considering two newly discovered fossil specimens from other phyla.

Oh Danny Boy

Annelida. Nature reported the earliest known member of this phylum which includes the familiar earthworms. The paper by Chen et al.1 begins, 

The phylum of annelids is one of the most disparate animal phyla and encompasses ambush predators, suspension feeders and terrestrial earthworms. The early evolution of annelids remains obscure or controversial, partly owing to discordance between molecular phylogenies and fossils…. Here we describe a new fossil polychaete (bristle worm) from the early Cambrian Canglangpu formation that we name Dannychaeta tucolus, which is preserved within delicate, dwelling tubes that were originally organic. The head has a well-defined spade-shaped prostomium with elongated ventrolateral palps. The body has a wide, stout thorax and elongated abdomen with biramous parapodia with parapodial lamellae…. To our knowledge, Dannychaeta is the oldest polychaete that unambiguously belongs to crown annelids, providing a constraint on the tempo of annelid evolution and revealing unrecognized ecological and morphological diversity in ancient annelids. [Emphasis added.]

The problem is that this is a complex crown annelid when what they need is a stem annelid (i.e., a primitive one). Already in the early Cambrian a complex (derived, crown) annelid is evident.

Tiny Shrimp with Jumbo Knives

In the phylum Arthropoda, a new swimming scavenger has been found in China’s famous Chengjiang fossil beds. Live Science describes the shrimp-like Xiaocaris luoi as no slouch when it comes to dinner utensils:

About 518 million years ago, a fierce shrimp-like creature didn’t brandish just one “knife,” it flashed more than 800 of them; each of its 54 legs had up to 15 dagger-like spines on it, a new study finds.

The name of this newly discovered Cambrian period scavenger, Xiaocaris luoi, literally means “Luo’s small shrimp” — and it was small, just 0.8 inches (2 centimeters) long — but its vicious weapons likely meant that its mealtimes were filled with frenzied cutting, the researchers said. 

“This is a tiny animal that is nevertheless pretty well equipped for scavenging,” study co-lead researcher Javier Ortega-Hernández, assistant professor of organismic and evolutionary biology at Harvard University, told Live Science.

This specimen doesn’t look like anything alive today, and it differs from other Cambrian arthropods, such as Anomalocaris. Yet there it is, at the base of the Cambrian record, fully formed and operational. It thrived around the same time that the crown annelid Dannychaeta lived. So, with the following look at a questionable sponge, remember that there are many other phyla with complex body plans appearing abruptly in the early Cambrian without any fossil ancestors.

Sponge Bobbing

Against this challenging backdrop, can Darwinians pull a bridge across the gap using sponges? Other than Kimberella, sponges seem to be their best hope for finding some continuity across the Ediacaran-to-Cambrian gap. One sponge in particular, Namapoikia rietoogensis, found by Wood et al. in 2002 and dated at 550 million years ago, “has been featured prominently,” comments Shuhai Xiao from Virginia Polytechnic, “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.

The celebrations appear to have been premature. Now, Mehra et al., publishing in PNAS,2 claim it is not a sponge at all. In their view, it is only a “microbial construction” like stromatolites and thrombolites. This means that the interpretation of putative “reefs” built by N. rietoogensis falls like a house of cards:

Given our reconstructions, which lack the regularity expected of sponges or, more generally, animals, we suggest that Namapoikia was not a metazoan. Namapoikia’s morphological expression, which can be summarized as widely spaced, meandering partitions that split and merge both transversely and longitudinally…, likely lacked the structural integrity to stay upright without external support (i.e., much like trying to stand playing cards up on their sides). As a result, Namapoikia probably had low emergent synoptic relief…. We suggest that such a morphological expression can be explained by the growth of partially or totally microbially mediated structures.

Xiao, writing about this two weeks later in PNAS,3 seems disappointed if not upset by this interpretation. He explains why it is important for evolutionary phylogeny to keep Namapoikia rietoogensis classified as a sponge.

Why does it matter whether N. 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 or a monophyletic clade constituting a sister group of all other animals. Molecular fossils or biomarkers indicate that one of the modern sponge classes, the demosponges, diverged no later than ca. 650 Ma in the Cryogenian Period. 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. The abundant evidence of Ediacaran eumetazoans and even bilaterian animals, which diverged after the sponges, also dictates the presence of at least total-group sponges in the Ediacaran Period or earlier. 

Evolutionists need sponges to be there! Xiao doesn’t want this prize fossil taken away from the Darwin trophy case, because there isn’t a better candidate.

However, the first unequivocal sponge fossils do not appear until in the Cambrian Period, with disarticulated sponge spicules around ca. 535 Ma and fully articulated sponge bodies shortly after. 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.

Putative or Purported?

In his discussion, Xiao further confesses the lack of evidence for Ediacaran sponges, compared to their clear identification after the Cambrian explosion.

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. Certainly, none of the purported Ediacaran sponges have unambiguous spicules, in sharp contrast to Cambrian sponges that are mostly identified on the basis of their biomineralized spicules.

This means that the most diagnostic aspect of sponges — spicules — appear at the base of the Cambrian, not before. To rub it in, spicules (tiny glassy shards characteristic of sponges) are not much to brag about. These skeletal-like fragments of calcium carbonate or silicon dioxide give sponges their structural support but are otherwise unremarkable; they are nothing like guts or articulated limbs and eyes found in the Cambrian bilaterian animals. 

Sponges (phylum Porifera, or “pore-bearing” animals known by their holes!) are not exactly the most exotic creatures to burst upon the evolutionary stage, although some later sponges, like the Venus Flower Basket, are quite beautiful. Xiao chuckles,

With an estimated 15,000 living species, 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.

Take note of this predicament for Darwinists. The annelid and the arthropod reported above, loaded with complex organs and systems, burst onto the scene in the early Cambrian, fully operational from the beginning. This is no help for Darwin’s doubt. Then, the one hopeful candidate for an “animal” to have emerged earlier in the Ediacaran — a purported, putative sponge, an unassuming, non-bilaterian, sessile, boring pet — could well just be a colony of microbes. 

The Cambrian explosion remains a canyon too wide for the short ropes evolutionists are desperately trying to stretch into a bridge.


  1. Chen, H., Parry, L.A., Vinther, J. et al. A Cambrian crown annelid reconciles phylogenomics and the fossil record. Nature 583, 249–252 (11 June 2020). https://doi.org/10.1038/s41586-020-2384-8.
  2. Mehra, Watters, Grotzinger, Maloof. Three-dimensional reconstructions of the putative metazoan Namapoikia show that it was a microbial construction. PNAS August 18, 2020, 117 (33) 19760-19766. https://doi.org/10.1073/pnas.2009129117
  3. Xiao, Shuhai. Ediacaran sponges, animal biomineralization, and skeletal reefs. PNAS September 1, 2020 117 (35) 20997-20999. https://doi.org/10.1073/pnas.2014393117