[Taxacom] How old?

[John Grehan]

As a courtesy to Ken for taking an interest in these matters, and assuming
he (or others on Taxacom) may not have access to Heads’ book, I have
excerpted a section address some of Goswami and Upchurch (apologies for any
typos on my part):

Heads (2012 p. 132)

In their critique of the model presented here, Goswami and Upchurch (2010)
wrote:

While certainly some people may mistakenly treat fossil or
fossil-calibrated molecular dates as maximum divergence estimates, the vast
majority of workers realize that they are in fact minimum divergence
dates…However, we can constrain a realistic range of divergence times for a
clade provided we have a suite of phylogenetic nodes where the minimum
divergence time has been estimated. For example, if the earliest known
euprimate fossil is the approximately 56 million years old (Mya)
Altiatlasius koulchii from Morocco…, and the earliest known eutherian
mammal is the 125 Mya Eomaia scansoria from China (Ji et al. 2002), *it
seems probable that the first true primate originated somewhere between *56
and 125 Mya. (italics added)

This is not strictly logical. If both the fossil dates are minimum dates,
as Goswami and Upchurch acknowledged, there is no reason to assume that the
first primate evolved between 56 and 125 Ma. The authors pointed out that
the line of reasoning they cited “has led to the development of
sophisticated Bayesian methods…” but this does not address the basic flaw
in the argument.

Goswami and Upchurch (2010) also suggested that the ages of tectonic events
proposed by geologists were less accurate than the ages of clades based on
fossils. They wrote: “…substantial margins around the dates of tectonic
events contrast with many fossil-based calibration points that can be dated
accurately to the nearest million or even a few hundred thousand year (p.
3). But the fossil-based clade ages (and nodes in phylogenies calibrated
with them) are minimum dates only, whereas the tectonic dates (and nodes
calibrated with these) are estimates of absolute dates established using
radiometric methods, not fossils. Fossil-calibrated clade dates can only be
compared with tectonic dates if the former are transmogrified into
estimates of absolute dates, and this is what Goswami and Upchurch have
done here, implicitly. Earlier in their note they described my suggestion
that fossil-calibrated divergence times are generally treated as absolute,
rather than minimum, dates as “something of a ’straw man’”, but here they
are reasoning in the same way as the straw man.


On Thu, Jun 14, 2018 at 1:06 PM, John Grehan <calabar.john at gmail.com> wrote:

> OK. Interesting. Looks like the authors did not include that, unless they
> did in the article. But as you say, if the spores are land plants then not
> 100 Ma difference.
>
> John Grehan
>
>
> On Thu, Jun 14, 2018 at 12:57 PM, Kenneth Kinman <kinman at hotmail.com>
> wrote:
>
>> John,
>>
>>        That article is not really accurate.  It was proposed back in 2003
>> (an article in Nature) that they had found fossil spores of the earliest
>> land plants which were dated to 475 million years ago.   So 500 million
>> years ago based on molecular data isn't much different from 475 million
>> years ago based on fossil spores.  Needless to say these dates are
>> somewhat controversial, but they certainly are not 100 million years
>> apart.
>>
>> Weblink to the 2003 article in Nature:
>>
>> https://www.nature.com/articles/nature01884
>>
>>
>>                       ------------------------Ken
>>
>>
>> ------------------------------
>> *From:* Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of John
>> Grehan <calabar.john at gmail.com>
>> *Sent:* Thursday, June 14, 2018 11:24 AM
>> *To:* taxacom
>> *Subject:* [Taxacom] How old?
>>
>> In relation to Ken's recent objections about gaps being too big, here's a
>> whoppa:
>>
>> "Now, new research suggests the first land plants began taking root 500
>> million years ago, 100 million years earlier than scientists thought.
>> Until
>> now, scientists used the oldest known land plant fossils, roughly 420
>> million years old, to date their arrival on Earth's continents. In the
>> latest study, scientists used molecular clock analysis to more accurately
>> pinpoint the origin of the earliest land plants.
>> https://www.upi.com/Land-plants-are-older-than-scientists-
>> thought/2731519132728/
>>
>>
>> So, these molecular theorists propose an origin that requires land plants
>> to have stuck around for at least 100 Ma with no fossil record.
>>
>> John Grehan
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>> Nurturing Nuance while Assaulting Ambiguity for 31 Some Years, 1987-2018.
>>
>
>