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Was Spriggina an Evolutionary Ancestor of Arthropods?

Photo: Spriggina, by Daderot, CC0, via Wikimedia Commons.

A commenter at our new Science Uprising video on the fossil record asks whether a Precambrian fossil from the Ediacaran fauna called Spriggina could have been an evolutionary ancestor of arthropods, purportedly contradicting a claim by Stephen Meyer. In fact, this is a claim Meyer addressed long ago in Darwin’s Doubt, where he explained why various authorities do not believe it was an evolutionary ancestor of arthropods or other Cambrian animal phyla:

Similar disputes have characterized attempts to classify Spriggina. In 1976, Martin Glaessner, the first paleontologist to study the Ediacaran in detail, described Spriggina as a possible annelid polychaete worm based largely upon its segmented body. Nevertheless, Simon Conway Morris later rejected that hypothesis because Spriggina shows no evidence of the distinguishing “chaetes,” leg-like bristled protrusions that polychaete worms possess. Glaessner himself later repudiated his original hypothesis that Spriggina was ancestral to polychaetes, noting that Spriggina “cannot be considered as a primitive polychaete, having none of the possible ancestral characters indicated . . . by specialists on the systematics and evolution of this group.”

In 1981, paleontologist Sven Jorgen Birket-Smith produced a reconstruction of a Spriggina fossil showing that it possessed a head and legs similar to those of trilobites, though examinations of subsequent Spriggina specimens have shown no evidence of it possessing limbs of any kind. In 1984, Glaessner weighed in on this discussion as well He argued that “Spriggina shows no specific characters of the arthropods, particularly of the trilobites.” He also noted that the body segmentation of Spriggina, and “its known appendages are at the level of polychaete annelids” (although, as noted, by this time he had rejected Spriggina as a possible polychaete ancestor). Instead, he proposed that Spriggina represented a side branch on the animal tree of life—one that resulted, “metaphorically” perhaps, in “an unsuccessful attempt to make an arthropod.”

In a presentation to the Geological Society of America in 2003, geologist Mark McMenamin revived the idea that Spriggina might represent a trilobite ancestor. He argued that several features present in Spriggina fossils are comparable to those in trilobites such as “the presence of genal spines” and an eff aced head or “cephalic region.” Nevertheless, many Ediacaran experts, including McMenamin, have also noted that Spriggina specimens show no evidence of eyes, limbs, mouths, or anuses, most of which are known from fossil trilobites. Other paleontologists remain skeptical about whether Spriggina does in fact exhibit genal spines, noting that good specimens seem to show relatively smooth edges with no protruding spines. In addition, analysis of the best recent specimens of Spriggina shows that it does not exhibit bilateral symmetry, undermining earlier attempts to classify it as a bilaterian animal, and by implication an arthropod. Instead, Spriggina exhibits something called “glide symmetry” in which the body segments on either side of its midline are off set rather than aligned. As geologist Loren Babcock of Ohio State University notes, “The zipper-like body plans of some Ediacaran (Proterozoic) animals such as Dickinsonia and Spriggina involve right and left halves that are not perfect mirror images of each other.” The lack of such symmetry, a distinctive feature of all bilaterian animals, and the absence in Spriggina specimens of many other distinguishing features of trilobites, has left the classification of this enigmatic organism uncertain.

Darwin’s Doubt, pp. 82-83

Meyer’s Point Is Vindicated

That was published in 2013. Five years later, Günter Bechly noted a paper published by Daley et al. (2018)which vindicated Meyer’s point that the symmetry of strange non-bilateral symmetry of Spriggina makes it a thoroughly implausible ancestor to arthropods. That paper stated:

Spriggina, for example, does not possess bilateral symmetry, but instead has a marked offset along the midline, and this alone is sufficient to reject a euarthropod affinity … No euarthropod claim from the Ediacaran biota can therefore be substantiated.

Missing: Arthropod Ancestors

Daley et al. (2018) further found that Precambrian strata should have been capable of preserving stem arthropods that were ancestors to true arthropods that appear in the Cambrian. Yet arthropod ancestors are missing:

Modes of Fossil Preservation Are Comparable in the Cambrian and Precambrian 

…Hypotheses that regard Precambrian preservation as insufficient to preserve euarthropods can no longer be sustained, given the abundant lagerstätten from the Ediacaran Period. Similarly, claims that euarthropods evolved as a tiny and soft-bodied meiofauna that escaped preservation cannot be substantiated because of how commonly the phosphate window is found in the Ediacaran and lower Cambrian, with microscopic euarthropods not appearing until 514 Ma.

An accompanying Oxford University news release at Science Daily emphasized this point in plain language:

“The idea that arthropods are missing from the Precambrian fossil record because of biases in how fossils are preserved can now be rejected,” says Dr. Greg Edgecombe FRS from the Natural History Museum, London, who was not involved in the study. “The authors make a very compelling case that the late Precambrian and Cambrian are in fact very similar in terms of how fossils preserve. There is really just one plausible explanation — arthropods hadn’t yet evolved.”

All of this confirms what the Dutch evolutionary ecologist Marten Scheffer wrote in a Princeton University Press book in 2009:

The collapse of the Ediacaran fauna is followed by the spectacular radiation of novel life-forms known as the Cambrian explosion. All of the main body plans that we know now evolved in as little as about 10 million years. It might have been thought that this apparent explosion of diversity might be an artifact. For instance, it could be that earlier rocks were not as good for preserving fossils. However, very well preserved fossils do exist from earlier periods, and it is now generally accepted that the Cambrian explosion was real. 

Marten Scheffer, Critical Transitions in Nature and Society (Princeton University Press, 2009), pp. 169-170

So Where Does This Leave Us? 

While analyzing Daley et al. (2017), Bechly shows that we’re left with a situation where arthropods appear abruptly in the Cambrian period, without evidence of evolutionary precursors — a timeline too short for arthropods to evolve by standard neo-Darwinian mechanisms:

[T]he paper by Daley et al. confirms that the Cambrian explosion implies a very acute waiting time problem, again as elaborated by Meyer (2013). Based on their postulated ghost lineages and on molecular clock data, the authors suggest that euarthropods originated about 541 million years ago. They conclude, “Rather than being a sudden event, this diversification unfolded gradually over the ∼40 million years of the lower to middle Cambrian, with no evidence of a deep Precambrian history.” However, this conclusion is totally speculative and an artifact of their methodological assumptions. It is not based on actual fossil evidence (see above). The latter indeed suggests that the euarthropod body plan appeared with trilobites in the Lower Cambrian, as if out of thin air without any known precursors and without any fossil evidence for a gradual step-wise generation of this body plan. 

Far from being a refutation of the abruptness of the Cambrian explosion, this study actually confirms it and makes the abruptness of the event even more acute. Here is why: since the authors refute the existence of stem group arthropods in the Ediacaran period before 550 million years, and euarthropods are documented already for the Lower Cambrian at 537 million years, there remains a window of time of only 13 million years to evolve the stem arthropod body plan from unknown ecdysozoan worm-like ancestors and to make the transition from lobododian pro-arthropods to the fully developed euarthropod body plan, with exoskeleton, articulated legs, compound eyes, etc. Since the average longevity of a single marine invertebrate species is about 5-10 million years (Levinton 2001: 384, table 7.2), this available window of time equals only about two successive species. Considering the implied enormous re-engineering involved, this time is much too short to accommodate the waiting times for the necessary genetic changes to occur and spread according to the laws of population genetics.

For those wedded to an evolutionary interpretation of life’s history, the fossil and genetic evidence leave the origin of arthropods a major mystery.