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News from the Ediacaran and Cambrian

Evolution News

The Cambrian explosion remains one of the severest evidential challenges to Darwinian evolution. Recent fossil finds adduced to support evolution deserve a closer look.

Ediacaran Fossils

Since our recent posts about the “Ediacaran Explosion” and the enigmatic Dicksonia fossils, a couple of news items have appeared about Ediacaran organisms.

Rangeomorphs. At New Scientist, Andy Coghlan invites readers to “See inside the 580-million-year-old creature no one understands” – the rangeomorphs that resemble large petals or leaves. Most fossils of these creatures appear as flattened impressions in the rock, showing only their outer surfaces. Now, for the first time, University College London scientists performed CT scans of rangeomorphs found in their original 3-D condition in Namibia. This is the first look “inside” these organisms. What was found?

[Alana] Sharp and her colleagues think all six fronds may have been inflated like long balloons. They may even have touched one another – meaning that a horizontal section through Rangea would have looked more like a slice through an orange rather than one through a starfruit.

“Our work supports a lifestyle of absorption of nutrients through membranes inflated to the maximum, increasing the surface area across which these organisms seemed to feed,” says Sharp. [Emphasis added.]

In other words, these creatures had no organs, no systems, and no body cavities. The researchers found a central stalk filled with sediment that may have helped “support the creature like a primitive skeleton.” But it isn’t a skeleton; it’s just a “cone-shaped channel.” More importantly, rangeomorphs looked nothing like the true animals that appeared later in the Cambrian explosion. Sharp added, “they are the first of the truly large, multicellular organisms that radiated broadly before the first true animals evolved.”

Parvancorina. In June, two researchers from Vanderbilt University and University of Oxford tried to get a grip on another Ediacaran animal named Parvancorina. That looks a little like a letter T with a curved top. In our last mention of Parvancorina in May (see fossil photo at top), we cited a paper that concurs with Stephen Meyer’s assessment that it does not represent a transitional form to any Cambrian animal, including trilobites. The new work, described in The Conversation, adds little, but confesses much.

When large, complex fossils were discovered in the Ediacaran, researchers naturally expected that many of them would represent early relatives of the same animal groups that had been recognized in the Cambrian. But these Ediacarans seem completely distinct from modern animals.

They mention rangeomorphs and Tribrachidium as examples of creatures distinct from the Cambrian animals.

So how do oddballs like these fit in with what came before and what came after? We just haven’t been able to place them on any evolutionary tree.

In order to better understand these organisms, paleontologists have been forced to adopt a different approach. We’ve abandoned all assumptions about what they might be related to, and instead tried to answer more fundamental questions. For instance, did they move? How did they feed? How did they reproduce? By answering these questions, we can begin to understand their biology and ecology, which in turn may provide hints as to how these organisms are related to other multicellular lifeforms.

All they did was study fluid flow around models shaped like varieties of Parvancorina, seeing if it provides any clues about whether they moved or not. The fluid dynamics may have favored capturing food if the organisms were able to orient in one direction. This could mean, they reasoned, that the organisms were “better adapted to life as a mobile, rather than a sessile, organism.” They make a big deal out of this tidbit of data, but once again, the confessions may be more important than the findings:

First, so little is currently known about Parvancorina that any additional information is crucial. The knowledge that it was mobile will help us work out where this fossil fits in the tree of life.

Second, the inference that Parvancorina was mobile, but nonetheless left no trace of its movement, is important – it means that many other Ediacaran fossils that we’ve assumed were sessile may actually have been mobile as well. This may require us to reimagine Ediacaran ecosystems as much more dynamic and, by extension, much more complex than we previously thought.

Opposite conclusions are possible. Knowledge that Parvancorina left no trace of its movement might mean that other Ediacaran fossils thought to be mobile were really sessile. Perhaps the best part of the article is the title: “Reverse engineering [a form of intelligent design] mysterious 500-million-year-old fossils that confound our tree of life.”

Cambrian Fossils

Three species have collapsed into one, based on work by Chinese researchers who scoured the rocks in the Chengjiang biota and found 85 more specimens. The title of their paper in PNAS reads, “Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity.” The authors resolve what they call, in interesting jargon, “three early Cambrian problematica.” Try using that last word in your daily conversation whenever you don’t understand something.

New material from the Chengjiang fossil Lagerstätte clarifies the identity of three early Cambrian problematica. The presumed earliest hemichordate Galeaplumosus abilus and the putative ancient sea pen Chengjiangopenna wangii are in fact fragments of Xianguangia sinica. Here we demonstrate that X. sinica possessed a polypoid body, a blind gastric cavity partitioned by septum-like structures, a holdfast that contained an additional cavity functioning as a hydroskeleton, a basal pit used for anchorage, and a radial whorl of feather-like tentacles for ciliary suspension feeding. Phylogenetic analyses based on the new findings suggest that X. sinica represents an extinct body plan most closely allied to cnidarians and thus sheds light on their early evolution.

While helpful to fill in the blanks about X. sinica, this research does nothing to de-fuse the Cambrian explosion. How often do you see ‘weirdo’ in a scientific paper?

Fossil problematica are extinct taxa that have defied unambiguous phylogenetic interpretations. They are enigmatic weirdos that have caused taxonomic headaches or have been unsatisfactorily shoehorned into one or another extant group. Problematica commonly occur in the fossil record of the Ediacaran and Cambrian (635–484 Mya), during which metazoans underwent a dramatic diversification. Therefore, deciphering fossil problematica from this crucial interval of evolution might provide pivotal insights into the origin and early radiation of metazoan body plans.

Now that three “problematica” have collapsed into one weirdo, the hoped-for insight is still on back order.

These putative species have been assigned to three different animal groups: hexacorals octocorals and hemichordates respectively. However, alternative views suggest that X. sinica was a ctenophore, a lophophorate, an Ediacaran survivor, or a metazoan of unknown affinity. Moreover, the phylogenetic placement of C. wangii as an ancient sea pen remains ambiguous and the assignment of G. abilus to hemichordates has also been questioned. In this study, we demonstrate that these three problematica are in fact conspecifics. The assemblage of this Cambrian puzzle revealed the existence of a “feathered polyp,” most likely representing an offshoot of the cnidarian stem lineage that diverged early during the Cambrian radiation of animal body plans.

So if it represents a cnidarian, that’s a member of a complex group that today includes corals, sea pens, sea anemones, and three groups of complex jellyfish. The members of this diverse phylum of some 10,000 marine organisms are united by possession of cnidaria or nematocysts, stinging cells that help them catch prey. The researchers did not find stinging cells with X. sinica, but that fact does not make them transitional; they could have lost them. Whatever the creature represents, it went extinct and does not provide insight into the explosion. Interestingly, Meyer’s chart of new phyla (p. 32) lists cnidaria with a question mark as making a possible first appearance in the Precambrian, along with Mollusca (questionable) and Porifera (sponges that are found in embryonic form in the Precambrian). Some 20 other animal phyla appear suddenly in the Cambrian.

If the resolution of these “problematica” helped Darwinism, the authors would not have needed to compare and contrast two alternative scenarios for how they evolved. They end by saying:

Whichever of the two scenarios is favored, the revealed body plan embodied by X. sinica suggests that the feeding behavior of stem-group cnidarians may have been strikingly different from that of their crown-group descendants. The distinctive suite of characters revealed in X. sinica significantly augments the morphological disparity of total-group Cnidaria.

In layman’s terms, finding another kind of weirdo does not help make existing weirdos normal.