Watch This Space:  What, and When, Was the Ediacaran Biota?

first_imgEvolutionary paleontologists are understandably very interested in the Ediacaran period (recently added to the geologic column) because, to them, it incorporates “the most ancient complex organisms on Earth.” As classified, this Precambrian period (dated 580 to 543 million years old) precedes the Cambrian explosion by some 20 million years, yet “remains one of the greatest enigmata within evolutionary paleobiology.”2 The type section for which it is named, discovered in 1946, is in the Flinders Ranges, Australia. Paleontologists had classified several species from the original Australian deposits and others in England and Russia. Some thought their frond-like shapes indicated they were possible ancestors of sea pens or even jellyfish, that arose later in the Cambrian. A new sample of exquisitely-preserved Ediacaran fossils was uncovered in Newfoundland, and reported by Guy M. Narbonne in the Aug. 20 issue of Science.1 Martin Brasier and Jonathan Antcliffe analyze the samples in the same issue2, but feel the time has come to “raise difficult questions about the methodology used to analyze Ediacaran fossils.” They take issue with classification by analogy, the idea that because some of the fossils resemble sea pens, they are related by evolution. An alternative view is likely:Paleontologists eagerly sought relationships between Ediacaran fossils and living seapens and worms, jellyfish and crabs. This “great ancestral” view has held sway for almost 40 years, but a growing number of paleontologists argue that Ediacaran creatures were not ancestral to Cambrian life at all. They suggest that members of the Ediacara biota were uniquely fashioned beasts that met their doom at the end of the Precambrian.Narbonne seems perplexed just how to classify these animals. “It is difficult to relate rangeomorphs [a clade of Ediacaran animals] to any modern group of macroscopic organisms, and they appear to represent a ‘forgotten’ architecture and construction that characterized early stages in the terminal Neoproterozoic evolution of complex multicellular life,” they conclude their paper. Brasier and Antcliffe also take issue with the practice of classifying forms into different species without considering the possibility they may be stages of development of a single species:Our concern is that the current “Ediacaran species concept” is no longer tenable. It is based on a “typological” approach using type specimens rather than populations, and on an “analog” approach that compares fossil morphologies with modern organisms according to assumed similarities. But these similarities could well have evolved independently. This approach is therefore unsound for deciphering long-extinct groups and, unlike cladistics, is an insecure basis for classification. We need quantitative studies of fossil populations, with analysis of morphological gradients [i.e., transitional forms–ed.] in the same geological successions and bedding planes, as well as detailed analyses of growth programs (morphospace), life history (ontogeny), and evolutionary history (phylogeny). It is premature to put forth any evolutionary history for fossils whose diagnosis has been conceived without reference to a postulated growth program observed through successive stages of ontogeny. Without such reference, both the taxonomic pattern and the evolutionary processes responsible for it will remain obscure.They point out several differences between Ediacaran animals and living sea pens and corals. They note also that many of these alleged “species” overlap each other in the strata. Reading the history of Ediacara is like reading hieroglyphics, they say, but a “Rosetta stone” is lacking. The only way they can fit an evolutionary account to the data is to suggest that speciation occurred by heterochrony: i.e., “architectural novelty arose through accentuation of adult or juvenile growth stages.” For his part, Narbonne simply assumes that ancestors for the Cambrian explosion existed in the Ediacaran period, but it wasn’t these creatures: “It is probable that the Ediacara biota included stem groups for the Cambrian explosion of animals, but there are no obvious analogs for rangeomorph architecture and construction among modern taxa.”1Guy M. Narbonne, “Modular Construction of Early Ediacaran Complex Life Forms,” Science, Vol 305, Issue 5687, 1141-1144, 20 August 2004, [DOI: 10.1126/science.1099727].2Martin Brasier and Jonathan Antcliffe, “Paleobology: Decoding the Ediacaran Enigma,” Science, Vol 305, Issue 5687, 1115-1117, 20 August 2004, [DOI: 10.1126/science.1102673].Yes, it is time to ask difficult questions. They just admitted that these fossils appear suddenly, then disappear, with no clear relationship to the Cambrian fossils that followed. As such, they are no help to explaining the Cambrian explosion. They already had complexity, forming leaf-like fronds with three levels of fractal patterning. Yes, we agree; they appear to be “uniquely fashioned groups.” One sentence demands another look: “We need quantitative studies of fossil populations, with analysis of morphological gradients in the same geological successions and bedding planes, as well as detailed analyses of growth programs (morphospace), life history (ontogeny), and evolutionary history (phylogeny).” In plain English, this means: we can’t tell an evolutionary story if we have no transitional forms to connect the dots. Now for some difficult questions of our own. Does anyone see an evolutionary picture in the Ediacaran biota? Is anyone convinced by the dates attached to the strata, which have been stitched together from four continents? Is anyone impressed by giving a just-so story a fancy name like heterochrony? The conclusion of their article teases, “If this sequence of evolutionary development (heterochrony) is correct, then perhaps we are about to break the code to the evolution of the Ediacara biota, the earliest animals. Watch this space.” Interesting ending: “Watch this space.” This implies that there nothing to watch except space: i.e., emptiness, a void, a vacuum. If, after 58 years of speculation about the Ediacaran biota, the evolutionary story has left nothing but a space, asking us to watch it as if something important is about to happen sounds like an empty promise from a used Darwinmobile salesman. Last question (an easy, not difficult one): any takers?(Visited 22 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

Leave a Reply

Your email address will not be published. Required fields are marked *