Permian Extinction Recovery Story Stretches Credibility
It goes without saying that Darwin’s theory fits hand in glove with the geological dating scheme, but how reliable is the latter? The textbook age names – Permian, Triassic, Jurassic, Eocene and all the rest – have taken on their own life as assumed truths. Every once in awhile, though, papers are published that require heavy doses of credulity to keep the scheme intact. The Permian extinction is a case in point. The textbook story is that 80 to 85 percent of marine organisms perished at the Permian-Triassic boundary (PTB). A new kink in the story requires believing that cephalopods, those most affected by the crisis, recovered spectacularly within one million years of the extinction, but everything else took five times as long, as measured by species diversity.
Charles Marshall, the “Master of Disaster” of the Harvard Museum who tackled the Cambrian explosion problem in 2006 by saying that animals evolved because they evolved (04/23/2006), tackled the Permian extinction with David Jacobs of UCLA in Science last week.1 They were commenting on a paper in the same issue by Brayard et al who presented evidence that ammonites (a kind of shelled squid) recovered much faster than everything else.2 The two papers invoked copious amounts of hand-waving to explain the evolutionary difference. Many statements amount to references to the Stuff Happens Law (i.e., the negation of explanation; see 09/15/2008 commentary). For instance, Brayard et al entitled their paper, “Good Genes and Good Luck.” Here are some example quotes from Marshall and Jacobs that cast doubt on scientific confidence in the Permian extinction story, both its causes and its effects:
- Two hundred and fifty-two million years ago, the Paleozoic Era came to a cataclysmic close with the end-Permian mass extinction, when as much as 85% of readily fossilizable marine species became extinct. It took 5 million years for the biosphere to begin to recover from the event. At least this has been the conventional view. However, on page 1118 of this issue, Brayard et al. show that ceratitid ammonoids (see the figure, panel A) recovered much faster than did most other marine groups, attaining considerable diversity just 1 million years after the mass extinction. Moreover, these mollusks reached a peak in their diversity at the end of the Early Triassic, when the diversity and body size of most other groups (particularly bivalves and gastropods) was still depressed.
- The cause of the end-Permian mass extinction has long been controversial. There is increasing agreement that toxic waters decimated bottom communities in shallow waters, but it remains unclear whether the kill mechanism was hypercapnia (high CO2 levels), euxinia (anoxic water infused with H2S), or something else. There is even less agreement on what might have caused the toxicity.
- Whatever the ultimate cause(s) of the extinction, the proximal cause appears to have been the inability of many species to handle the physiological demands of a changed ocean chemistry. Evidence that conditions remained difficult for 5 million years after the extinctions comes mainly from the observation that the diversity and size of fossil bivalves and gastropods remained low, indicating stressed conditions. Furthermore, the carbon cycle was unusually volatile, although the exact meaning of this volatility is not understood.
- The ammonoid data reported by Brayard et al. suggest a much more rapid recovery, at least for part of the biosphere. Unlike the bottom-dwelling gastropods and bivalves, ammonoids live in the water column. Thus, Brayard et al.‘s study suggests that conditions in the water column were better than those on the bottom. Or does it?
- To better understand the meaning of Brayard et al.’s data, we need to know more about the biology and physiological tolerances of ammonoids in general, and of ceratitids in particular.
- These species lie deep in the evolutionary trees of living coleoids and living cephalopods, respectively, suggesting that a tolerance for low oxygen was ancestral for living cephalopods.
Their Perspectives article did little more than to suggest this and that, and then to say more work needs to be done. How about the other paper? Did Brayard et al have anything more solid to lean on? Keep in mind that classic Darwinian evolution explains diversification as gradual and continuous.
- One problem has been a lack of absolute age calibration of evolutionary trends across the PTB.
- It has usually been assumed that the end-Permian mass extinction affected ecological assemblages so deeply that the postcrisis biotic recovery spanned the entire Early Triassic [~5 million years (My)], if not more.
- The Triassic part of the time series consists of four successive diversity oscillations of declining magnitude, probably primarily shaped by global climatic and oceanographic changes.
- In the first oscillation…only 1 to 2 My after the PTB, based on the available radiometric ages and associated uncertainties—ammonoid diversity reached values equal to, if not higher than, those for the Permian (~85 sampled genera) and then were followed by still higher values …. This late Early Triassic generic richness is unsurpassed during the Middle and Late Triassic, where diversity oscillated around an average value…close to the Middle Permian maximum. This rapid recovery less than 2 My after a mass extinction is also seen for Early Jurassic ammonoids.
- The Early Triassic rapid ammonoid diversification diverges from delayed recovery after the PTB suggested for many benthic groups…. Apparently, recovery rates strongly varied across marine clades, and ammonoids boomed well before the oceanic realm returned to a long-term steady state.
- Extreme contraction of survivorship and prenascence contour lines is diagnostic of high evolutionary rates, as echoed by the simultaneously high numbers and rates of Early Triassic originations and extinctions (Fig. 3).
- Ammonoid diversification during the Early Triassic produced more than 200 genera in less than ~5 My and was accompanied by a progressive change from cosmopolitan to latitudinally restricted distributions of genera.
- This trend was not a gradual, continuous, and smooth one.
- How did these cephalopods flourish in the presumably unstable and harsh environmental conditions prevailing at that time? The same question applies to conodonts, whose Early Triassic diversity dynamics tend to parallel that of ammonoids.
- Ammonoids are morphologically and taxonomically so diverse that it is likely that they occupied a great variety of niches and exploited various food resources. Their high diversity and abundance suggest that diversified and abundant food resources were already available less than 2 My after the PTB. Consequently, even if Early Triassic trophic webs were possibly less complex than Permian and Middle-Late Triassic ones, they were far from devastated. At least some sizeable, while still unknown, primary production made it possible for these two clades to diversify profusely and rapidly despite short-term fluctuations of environmental conditions.
- The Early-Middle Triassic transition was again marked by a severe drop in ammonoid diversity. In this case, a fall in global sea level is implicated.
- In addition, the empirical (log) richness-rates relationships (table S4) illustrate a possible niche incumbency effect. This hypothesis, which predicts that richness and extinction rates are independent, allows the estimate of an average steady-state generic niche saturation level of ~85% under the hierarchical model, compatible with species niche saturation levels previously published for various clades of marine organisms.
- Numerous Lazarus taxa3 among benthic and pelagic mollusks reappear during the Smithian.
- Coupled with the Triassic ammonoid nondelayed diversity dynamics evidenced here, this suggests that complex trophic webs based on abundant and diversified primary producers were already functioning less than 2 My after the PTB and opens the possibility that heterotrophic taxa other than ammonoids also rapidly recovered.
- This phased scenario for the Triassic biotic recovery accounts well for its generally accepted delayed character, which may reflect still inadequate sampling and time resolution and/or biased diversity estimates due to the lack of sampling standardization in the first million years after the PTB.
- Recoveries obviously show environment- and clade-specific dynamics. Nevertheless, our results indicate that the time duration of the post-PTB recovery is likely overestimated, at least for some marine taxa.
It should be noted that the statistics of biodiversity on which they relied for their graphs and charts depend heavily on sampling – a human enterprise. The fossils, in other words, do not speak for themselves. This was clear from several paragraphs in the paper that explained why Brayard et al leaned on some data sets but rejected others.
1. Charles R. Marshall and David K. Jacobs, “Paleontology: Flourishing After the End-Permian Mass Extinction,” Science, 28 August 2009: Vol. 325. no. 5944, pp. 1079-1080, DOI: 10.1126/science.1178325.
2. Brayard, Escargue, Bucher, Monnet, Br�hwiler, Goudemand, Galfetti, and Guex, “Good Genes and Good Luck: Ammonoid Diversity and the End-Permian Mass Extinction,” Science,28 August 2009: Vol. 325. no. 5944, pp. 1118-1121, DOI: 10.1126/science.1174638.
3. Lazarus taxa: resurrected extinct groups or “living fossils” – see 03/10/2006 and 12/04/2007.
Who else but CEH is revealing, line by line, in detail, the arbitrariness of story generation in the evolutionary scientific literature? The Framework is never called into question, no matter how many anomalies are found, and no matter how many suspensions of disbelief are required. The Stuff Happens Law is everywhere – “good genes and good luck.” There is no pattern or sense to any of this. Here is the story in a nutshell:
Through causes we don’t understand, something happened at some uncalibrated time, and, if our sampling methods are not completely biased, some groups of animals, based on some method of deciding what constitutes a species or genus among extinct animals we cannot observe except by their shells, using controversial measures of classification and sampling, recovered much faster than others, through reasons we also don’t understand, perhaps due to their level in the water column, or climate, or availability of food, or tolerance to carbon dioxide and hydrogen sulfide, or a number of other possibilities. This points out that their evolutionary potential, whatever that means, was greater than that of shellfish, because of mechanisms not well understood, i.e., some sizeable, while still unknown, primary production that made it possible for ammonites and conodonts to diversify profusely and rapidly compared to their depressed contemporaries, despite rapid fluctuations and oscillations in their environment, illustrating their ability to occupy a variety of ecological niches, though stressed by the unknown extinction event of unknown duration or cause–perhaps volcanoes, which surprisingly killed almost everything on the sea floor (which one would think more robust against calamities in the climate or on the surface, but whatever). Yet some of them, nevertheless, somehow, resurrected like Lazarus (but we don’t want this to get anyone started thinking about the Bible or miracles, which is forbidden; only Darwinian miracles are allowed). So whatever the cause, or causes, or no cause at all, while all we have is confusing data and a Framework to put it in bequeathed to us by Saint Lyell, we at least came up with a “scenario”, illustrated with a few graphs and charts and math, that was good enough to get published by the Keepers of the Darwinian Flame in Science, even though we diverged a little bit from Saint Darwin’s concept of gradual, smooth, continuous change, because we know his heirs have become more tolerant of unexplained hiccups in the geological record, or the biological record, or in evolutionary theory itself, because of the need to keep Evolution reigning supreme in the public eye, by sounding sophisticated with terms like “diversity dynamics” (which we don’t have to define or explain), but that doesn’t matter because it sounds scholarly, and helps to keep at bay the constant threat from those rascally Creationists, who might expose our methods and threaten our jobs and funding unless we present a unified front and an air of confidence in the journals and cooperative science news outlets.
Abbreviated version: Something happened. We’re not sure what, when, or how, or why, or even if something happened at all, but some day we may figure it out. Praise Darwin for modern science!
Welcome to modern evolutionary biology. Stuff happens. Evolution happens. Diversity happens. Niches magically get filled. Rates of change vary with no known reason. Facts are convenient props, but keeping the Framework intact while weaving more intricate stories is the name of the game. Don’t even THINK about criticizing us. We are scientists. Don’t even think.