New Fossils Before and After the Cambrian, What Do They Show?
Evolution News & Views July 16, 2015 4:01 AM
To understand a fossil bed, it's important to know what came before and after it (or above and below it, to be more precise). The explosive burst of new body plans called the Cambrian explosion is undisputed among paleontologists, even those who disagree with the ID understanding Stephen Meyer argues for inDarwin's Doubt. Recently, some new fossils before and after the explosion came to light. Let's see what kind of context they provide -- and evaluate the evolutionary spin given in the reports.
Precambrian Sponge
A headline at PhysOrg sounds promising: "Tiny sponge fossil upsets evolutionary model." Below the headline, though, evolution appears to score points by stretching out the Cambrian Explosion into the Precambrian by tens of millions of years. Dribbling for this goal is David Bottjer, whom we last saw promotingVernanimalcula as a bilaterian ancestor before the explosion -- a view later debunked by his colleagues (see Casey Luskin's 2012 critique here, and Meyer's discussion in Darwin's Doubt, pp. 90-92). That fossil was 580 million years old; Bottjer's tiny new sponge is dated 600 million. Here's what he says about it:
Though some evidence, including molecular clocks, has already pointed to sponges evolving earlier, this fossil shows that the Cambrian explosion might not be a period when a large number of new traits emerged, but a period when a large number of fossils could be preserved, as animals during the Cambrian grew larger and gained skeletons."This specimen is of an animal that had already evolved a number of fundamental sponge traits," Bottjer said. "It implies that by the time this animal was living, most of the developmental genes for sponges had evolved."This raises the possibility that some aspects of early animals' evolution, a good deal of which happened during the Cambrian explosion, happened even more gradually.With an international team of colleagues, Bottjer discovered thatthe millimeter-wide, 600-million-year-old fossil hascharacteristics that many thought emerged in sponges only 540 million years ago."Fundamental traits in sponges were not suddenly appearing in the Cambrian Period, which is when many think these traits were evolving, but many million years earlier," Bottjer said...."These organisms don't have all the bells and whistles that modern creatures do," Bottjer said. "But this particular fossil has enough complexity that we can say we hadn't been dating the early evolution of animal traits properly." [Emphasis added.]
So that's the upset: the Cambrian wasn't really an explosion after all. Bottjer's team paper was published in PNAS last March. Let's take a look at it.
The paper claims that the existence of this sponge at 600 million years ago supports the notion that sponges diverged from a eumetazoan ancestor much earlier, such that this sponge already had most of the "metazoan genetic toolkit" associated with complex animals very early on. Bottjer also continues to promote Vernanimalcula as a bilaterian ancestor, despite the naysayers:
Adult forms have been reported only rarely, which has increased the difficulty of interpreting the putative fossilized embryos. However, reported Doushantuo microfossils include small tubular cnidarians and a small bilaterian form, Vernanimalcula, of which multiple fossils have been recovered. Alternative interpretations have been proffered for virtually all of the Doushantuo microfossils. The present report, which describes an unmistakable adult animal form, will alter the structure of this debate, although the full force of the implications will not be realized until more than this single specimen becomes available.
It's hard to see what the fuss is about, though, considering that Meyer acknowledges in his book that sponges existed in the Ediacaran period (635-540 mya). In fact, sponge embryos provided his evidence that the Precambrian strata were fully capable of preserving fossils of the Cambrian ancestors, had they existed. It's clear that if sponge embryos are found, adults were present to spawn them, so what's the surprise there? Moreover, Meyer demonstrated that the Ediacaran period had its own mini-explosion, a "pow" before "biology's big bang" (p. 87). Bottjer's sponge is a case in point; "It displays the unmistakable gross anatomy of an adult sponge-grade animal, but beyond this finding, several distinct cell types and cellular structures can be clearly recognized, as displayed in the figures accompanying this report."
Sponges (phylum Porifera) are among the simplest of multicellular animals, hardly more complicated than other Ediacaran organisms. Bottjer's tiny fossils look like little more than hollow tubes or cups. They are not bilaterians. The paper acknowledges that "sponges do not display tissue-grade anatomical characters or organs beyond a general gross, spongelike structure."
From this unimpressive data, grand scenarios are drawn, ending with this promissory note:
Thus, just as implied by the current temporal extrapolations of phylogenomics, the "calibration point" afforded by this fossilsuggests that the shared metazoan genetic toolkit must have originated in the Cryogenian. Furthermore, if a relatively advanced sponge existed 600 Ma, then so did coeval animals of the eumetazoan lineage that also descended from the same last common poriferan/eumetazoan ancestor. Thus, it is a clear prediction that fossilized organisms of eumetazoan affinity fromsimilarly deep in time await paleontological discovery, and some such may already have been seen in the Doushantuo animal microfossils cited above.
-- i.e., Vernanimalcula. No other eumetazoans are mentioned. So Bottjer is not backing down from his claim, even after the stinging criticism by others that he is only seeing what he wants to see. Now he sees slow, gradual evolution in this sponge. Convincing?
Ordovician Arthropods
More exciting are the discoveries from Morocco. Fossils of exceptional preservation in a remote region called the Fezouata formation, dated to the Ordovician period (485-443 Ma) that followed the Cambrian, have come to light in the last few years. Live Science has photos of some of these beautiful fossils, including horseshoe crabs that look identical to modern ones.
What's interesting is that many of the Burgess Shale type animals are found here: trilobites, marrella-like arthropods, and anomalocaridids of enormous size (see example on here). A companion article on Live Science tells the story of their discovery by adventure and luck.
So what context do these fossils provide about the Cambrian explosion? Some diversification is evident, but mostly, it's a story of stasis. Animals appeared abruptly in the Cambrian, then remained largely unchanged for tens of millions of years into the Ordovician. In the case of horseshoe crabs, that's hundreds of millions of years on the evolutionary timeline. Not only do the fossils look thoroughly modern, they appear 20 million years earlier than thought (see this from the Geological Society of London). Live Science's photo caption says, "It shows a sub-adult that has fused segments at its rear, a characteristic that living horseshoe crabs still have today."
Facts versus Spin
Most of these articles mention the Cambrian explosion, and spin the story to suggest the fossils are shedding light on evolution. "Spectacular Moroccan fossilsredefine evolutionary timelines," the Geological Society of London says. "As well as demonstrating the longevity of fauna thought to have been extinct millions of years previously, the Fezouata proves that other creatures evolved far earlier than previously thought."
'Horseshoe crabs, for example, turn out to be at least 20 million years older than we thought. The formation demonstrates how important exceptionally preserved fossils are to our understanding of major evolutionary events in deep time' says Peter Van Roy, also of Yale, who first recognised the scientific importance of the Fezouata fauna and is lead author of the study, part of a project funded by the National Science Foundation.
Live Science adds, "During the past few years, these newfound Fezouata fossils have rewritten evolutionary textbooks," as if that is good. But what if the revision says, "Animals appeared suddenly without evidence of transitions, and stayed the same without evolving"? It would be like your accountant saying cheerfully, "Your IRA just lost 50 percent of its value; this will rewrite your retirement."
Fossils are great; the more the better. Experience from previous spectacular discoveries (e.g., Marble Canyon, Chengjiang) assures us that no surprises will change Darwin's Doubt, once the spin is scrubbed off the data. New fossils tend to fall into the same bins. They don't increase "our understanding of major evolutionary events in deep time"; instead, they add more data points to the same story: sudden appearance of complex body plans, followed by diversification and stasis. That's not Darwinism; that's design.
