Earlier this year, in March, Nature reported that soft-bodied worms from the Burgess Shale fossil beds in Canada, given the name Spartobranchus tenuis, have been identified as ancient examples of acorn worms. They were hailed as a "missing link" in the vertebrate family tree: "a crucial evolutionary link between two distinct living groups of animals: enteropneusts and pterobranchs." The evidence supporting this was said to be the tubes constructed by Spartobranchus tenuis. Living enteropneusts (acorn worms) do not have tubes, whereas living pterobranchs (minute colonial organisms) do. Professor Simon Conway Morris affirmed the significance of the newly discovered fossil tubes with these words: "By finding enteropneusts in tubes we begin to bridge this evolutionary gap." At the time, these issues were discussed in a blog here, and questions were raised about the evolutionary narrative. More now needs to be said, as a recent paper in Nature Communications has documented modern tube-forming acorn worms found in Antarctic benthic communities.
Figure 1b: Large purple morphotype in secreted tube with proboscis expanded. (Source here)
Recent work surveying life in ocean basins has identified a new family of deep-sea enteropneusts, the Torquaratoridae. During oceanographic cruises in the Antarctic, between 2008 and 2013, two new enteropneusts were discovered at depths ranging from 531 to 1,111m. These species secrete translucent tubes in which they live, although they have also been observed abandoning their tubes. The authors recognize that their discovery is directly relevant to the interpretation of the Burgess Shale worm tubes. This is their discussion:
"Foremost, our discovery provides context for the recently re-described Middle Cambrian fossils, S. tenuis from the Burgess Shale Formation. These acorn worm fossils look remarkably like present-day enteropneusts, except many are observed to be within a 'fiberous' tube. The persistence of tubes in Antarctic worms suggests that the tubes contain proteinaceous components functioning as binding agents. Some tubes were lightly covered with sediment giving them a 'ribbed' appearance, similar to those reported for S. tenuis. [. . .] Our observations of Antarctic tubicolous worms and faecal casings imply that the worms often turn and zigzag during movement or may even double back on themselves as reported elsewhere. If buried quickly in a single obtrusion event, as suspected for each of the mudstone beds of the Greater Phyllopod Bed (that is, Walcott's quarry), such tubes could appear helical or even circular as reported for S. tenuis [. . .]. The fossil tubes were interpreted as 'fiberous' based on apparent tearing of the tubes, but a similar phenomenon occurs with the present-day tubes. Given the similarity in tube design between S. tenuis and the Antarctic torquaratorids, similar behavioural repertoires (for example, tube building, vacating tubes and meandering epibenthic movements) appear to have been conserved ~500 million years."
So, the discovery of modern tube-forming acorn worms has assisted the interpretation of the fossil tubes: the fibrous texture is an artifact of preservation and not part of the constructed tube, and the doughnut shapes are likely to be formed by a rapid depositional process and is not representative of the original structure in life. The evidence we have is of stasis in behavioral repertoires as well as in morphology.
This leads to a rather different conclusion about the implications for evolutionary theory. The idea that S. tenuis is a link between the enteropneusts and the pterobranchs lacks credibility. The argument is explained very clearly in the research paper:
"Fossil evidence reveals that graptolites, and even rhabopleurids and cephalodiscids (modern pterobranch lineages), were present in the Middle Cambrian, making S. tenuis contemporary with established pterobranch (including graptolite) lineages. As the split between enteropneust and pterobranch lineages would have been before the Middle Cambrian, the tube of S. tenuis was not a precursor to the pterobranch coenecium. Lack of synapticulae and hepatic sacs were also argued to ally S. tenuis with harrimaniid enteropneusts. However, torquaratorid enteropneusts, like harrimaniids, lack synapticles, and assessing the presence of hepatic sacs often requires microscopy in modern species, much less in fossilized ones. Given the position of torquaratorids and pterobranchs in hemichordate phylogeny, the last common hemichordate ancestor may have been able to build tubes, raising the question whether this ability was present in the last common deuterostome ancestor."
The implication is that the last common hemichordate ancestor lived before the Middle Cambrian and should be located before the Cambrian Explosion. The problem for this hypothesis is that there is a paucity of fossil data: we do not have anything other than speculation for hypothetical ancestors of the animal phyla. Instead of evolutionary theory having anchors in fossil evidence about the past, the Precambrian is effectively a blank sheet where inferences are drawn from selected phylogenetic data and produce conflicting evolutionary trees.
There is a pattern in the way fossil discoveries are reported. If they are deemed to fill in the gaps in a branch of the evolutionary tree, they generally get massive exposure and are hailed as milestones in developing an understanding of life on Earth. However, when new data comes to light that shows the original thinking to be wrong, the exposure is far less and the media show little interest. With acorn worms, we have a case to reflect on worthy of our time. The message we should be taking away is that the fossil record brings us evidence of stasis, not evolutionary transformation. We need a radical rethink of the presuppositions we bring to the story of life on Earth, because the present hegemony of Darwinism has no adequate explanation for the origin of biological information.
Modern Antarctic acorn worms form tubes
Kenneth M. Halanych, Johanna T. Cannon, Andrew R. Mahon, Billie J. Swalla and Craig R. Smith
Nature Communications, 4, No. 2738, 07 November 2013 | doi: 10.1038/ncomms3738
Abstract: Acorn worms, or enteropneusts, are vermiform hemichordates that occupy an important position in deuterostome phylogeny. Allied to pterobranch hemichordates, small colonial tube dwellers, modern enteropneusts were thought to be tubeless. However, understanding of hemichordate diversity is poor, as evidenced by absence of reports from some oceanic regions and recent descriptions of large epibenthic deep-water enteropneusts, Torquaratoridae. Here we show, based on expeditions to Antarctica, that some acorn worms produce conspicuous tubes that persist for days. Interestingly, recent fossil descriptions show a Middle Cambrian acorn worm lived in tubes, leading to speculation that these fossils may have been pterobranch forbearers. Our discovery provides the alternative interpretation that these fossils are similar to modern-day torquaratorids and that some behaviours have been conserved for over 500 million years. Moreover, the frequency of Antarctic enteropneusts observed attests to our limited knowledge of Antarctic marine ecosystems, and strengthens hypotheses relating more northern deep-sea fauna to Antarctic shelf fauna.
Birchfield, C. Auburn University researchers make deep sea creature discovery and set sail for Antarctica, Auburn University News, 20 November 2013.
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