Controversial.
"Spriggina's affinity is currently unknown; it has been variously classified as an annelid worm, a rangeomorph-like frond, a variant of Charniodiscus, a proarticulatan, or an arthropod perhaps related to the trilobites, or even an extinct phylum. Lack of known segmented legs or limbs, and glide reflection instead of symmetric segments, suggest an arthropod classification is unlikely despite some superficial resemblance."
(Wikipedia "Spriggina")
"Trilobites made a sudden appearance in the fossil record. There appears to be a considerable evolutionary gap from possible earlier precursors such as Spriggina, which is found in the 550-million-year-old Ediacaran-age rocks of Australia, and thus predates trilobites by some 30 million years ...
Morphological similarities between trilobites and earlier arthropod-like creatures such as Spriggina, Parvancorina, and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make a detailed analysis of their ancestry complex."
(Wikipedia "Trilobite")
"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)
Seems like a disingenuous response for reasons noted in my prior post. Also, spriggina is generally considered bilaterally symmetrical. And it may very well have had a mouth and eyes.
But regardless, there are many reasons that the above argument just isn't reasonable. One being that, if critics of evolution have to dwelve into the deep depths of the precambrian, where fossils transition to microscopic soft bodied species, it just looks bad for their position in light of the next 600 million years of fossils.
It would be like going through recordings of Michael Jordan's life and finding a video of when he was 10 years old, watching him miss a free throw, and then saying that he was never a good basketball player.
And what that does is, it put God in a box that is ever decreasing in size. Because what happens is, and this keeps happening hundreds of times over, people do find indisputable ancestors to various species. And then critics say, well what's the ancestor of that ancestor? Until they go so far back that they start arguing about precambrian (typically metamorphosed and ancient billion year old rock, the oldest of the old) microscopic soft bodied organisms (that don't have bones and so are less likely to be fossilized and are so tiny that they're less likely to be seen at all).
Meanwhile, in the precambrian, you still have a whole collection of bilaterians, cnidarians, mollusks, sponges, annelids etc. And the best counter you find is "well that animal doesn't have an anus!" Even though it's actually likely that spriggina did in fact have an anus, and you get people saying that it wasn't bilaterally symmetrical, yet anyone with two eyeballs can draw a line down the center of it's body and see that it is symmetrical on either side of the line.
But critics would rather argue the minutia of details, unknowingly pushing God into a smaller and smaller and smaller box as more ancestral fossils continue to pour out of the fossil record.
Remember, in Darwin's time, there were no transitional fossils. Literally not a single one was known.
So the question is, do you want to keep pushing God into a smaller and smaller box, or do you want to just give God credit for the fossil succession? I think the latter is the inevitable conclusion for theists