Re: Fossil calibration

David Campbell wrote (in response to my questions):
> >Dating the deep divergence times between higher taxonomic levels, however, still seems to be a serious problem, as the "molecular clock" almost never seems to run very clock-like. A calibration by means of well-dated fossils is required for every evolutionary divergence point, with the big problem being a run-away propagation of statistical and systematic errors.
> >In a recent paper, Graur D., Martin W., "Reading the entrails of chickens: molecular timescales of evolution and the illusion of precision", Trends in Genetics 20 (2004), 80-86, wrote (their abstract):
> >"For almost a decade now, a team of molecular evolutionists has produced a plethora of seemingly precise molecular clock estimates for divergence events ranging from the speciation of cats and dogs to lineage separations that might have occurred ~4 billion years ago. Because the appearance of accuracy has an irresistible allure, non-specialists frequently treat these estimates as factual. In this article, we show that all of these divergence-time estimates were generated through improper methodology on the basis of a single calibration point that has been unjustly denuded of error. The illusion of precision was achieved mainly through the conversion of statistical estimates (which by definition possess standard errors, ranges and confidence intervals) into errorless numbers. By employing such techniques successively, the time estimates of even the most ancient divergence events were made to look deceptively precise. ...
> >I wonder whether Graur and Martin describe a general problem, or whether they are just pointing to some excesses. How do you evaluate the situation?<
>
> The problem is very widespread in molecular clock papers, although a few do use multiple calibration points across many lineages and nonlinear models of rates of change. At worst, some molecular clock papers simply lift a rate from another molecular clock paper. Barely better is lifting a date or two from another molecular clock paper. Many confounding factors are also known. As an example, the 18S gene has major differences within the bivalve family Cardiidae, with groups that diverged during the past 65 million years, whereas the one crassatellid and astartid that have been sequenced show no divergence over ca. 300 million years. Some molecular biologists seem clueless about the fossil record, if not about organisms in general.
>
> >Even more, however, I would be interested in some reliable fossil dates (including realistic error margins and references to the primary literature) which could reasonably be used as bounds for divergence time estimates with respect to higher taxonomic levels.<
>
> Michael Benton, The Fossil Record II, is the most comprehensive source for that sort of thing that I know of.

Thank you David, this confirms my practice of using Benton's Fossil
Record 2 (1993) for the previous 10 years, and its earlier version 1 of
1967, edited by Harland et al., before that.

However, I was wondering about how to get at reliable newer dates. Of
course, the reliability of papers dedicated to dating particular fossils
usually can be assessed reasonably well. But how about molecular
phylogenies etc.?

I was struck by the fact that the papers most criticized by Graur and
Martin were published in such respectable journals as Nature and
Science, and Bromham et al. published their range-less dates (without
any literature references!) in the equally respectable PNAS: 95 (1998),
12386. How reliable are the peer review processes of these leading
journals? If I remember correctly a recently published figure, they
reject up to 90% of the manuscripts received.

Peter

-- 
Dr. Peter Ruest, CH-3148 Lanzenhaeusern, Switzerland
<pruest@dplanet.ch> - Biochemistry - Creation and evolution
"..the work which God created to evolve it" (Genesis 2:3)