[Taxacom] Your straw man argument (was: Galapagos tortoise bedtime story)

John Grehan calabar.john at gmail.com
Sun Jul 7 21:55:05 CDT 2019


Hi Evan,

Great to have that update on systematic information. Always great to catch
up with the latest in any group and all too easy to miss.

Regarding your comment "The oldest known island in the Galapagos is 2.9 Ma
old, and they say that there might also be older islands. And that
apparently the origin of the Galapagos tortoise lineage slightly predates
the age of the oldest known island." - it is quite definitive geologically
that there have been islands present at the Galapagos in a continuous
manner for at least 16.5 Ma (earlier than that is subducted).

As for the age of the tortoise lineage, if you are referring to island or
fossil calibrated molecular ages there are huge problems (this the subject
of some Taxacom discussion). The first presupposes that taxa cannot be
older than the island they occupy, the second misrepresents fossil
calibrated ages as actual or maxmimal rather than minimal.

John Grehan

On Sun, Jul 7, 2019 at 10:37 PM Evangelos Vlachos <evlacho at gmail.com> wrote:

> Hi John and Ken, I jump right in as I see that I have been "summoned" in
> this thread. Thank you for citing our abstract, but I do not think that we
> can offer, with gringorum, a lot to these intriguing questions. We study
> all the Chelonoidis gringorum specimens and the truth is that it is
> difficult to establish well its position within Chelonoidis when a broader
> testudinid taxon sampling is made. It is probably closer to chilensis than
> to carbonaria+denticulata, but it could even be basal to all these.
> Hopefully, soon we will have some results to share.
>
> And the whole story might be even more complicated. It is not only about
> the Galapagos tortoises, but those of the West Indies as well.
>
> Check this recent paper
> https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2016.2235 by
> Kehlmaier et al. reporting the ancient DNA of a Holocene Chelonoidis from
> the West Indies. Its position is well-resolved and supported as the sister
> of the Galapagos+chilensis clade, and divergence dates place those splits
> in the Miocene.
>
> Check also their Fig. 2 on the biogeographical evolution.
>
> I still cannot tell where and when Ch. gringorum enters this story, but it
> seems that members of this clade defined by chilensis have been able to
> disperse to the West Indies and to the Galapagos as well.
>
> Note also that the uplift of the Andes was gradual and at least the
> northern parts of the cordillera appeared towards the end of the Miocene,
> and at the time of those basal splits the Panama landbridge was not, at
> least, fully formed or was non-existent. It is even possible that currents
> were different at that time, so the whole Humbolt current story might not
> be relevant at all; if understand correct the Panama current is as well
> relevant for the situation in the Galapagos. The oldest known island in the
> Galapagos is 2.9 Ma old, and they say that there might also be older
> islands. And that apparently the origin of the Galapagos tortoise lineage
> slightly predates the age of the oldest known island.
>
> There are so many scenarios that could have happened, but for me it is
> likely (difficult to prove) that the ancestor of any of these insular
> clades appeared in the continent, was distributed in the coastal areas
> (prior to both Andes uplift and/or formation of Panama bridge) and from
> that made the "jump". But this is speculation, of course.
>
> But let's not underestimate these tortoises. Apparently, Chelonoidis
> tortoises and those of the more inclusive clade (the old-fashioned
> 'Geochelone') are capable of remarkable transoceanic dispersals, for which
> we even have some actualistic evidence. They are so "badass" that they
> could even go around the Horn (a joke of course).
>
> All the best,
>
> Evan Vlachos
>
>
>
> On Sun, 7 Jul 2019 at 22:34, John Grehan via Taxacom <
> taxacom at mailman.nhm.ku.edu> wrote:
>
>> Hi Ken,
>>
>> But of course "they did not suggest such tortoises rafted around the tip
>> of
>> South America". That is what I said. Well Caccone et al were a bit vague
>> as
>> to how chilensis, which is to the east of the Andes, made it to the other
>> side to hop on a raft to the Humboldt current. Admittedly they did not say
>> anything about how they reached the current from the east side of the
>> Andes. Maybe the hiked over the mountains (although they are elevationally
>> challenged). If Caccone et al did mean it to raft all the way (and they
>> did
>> not indicate otherwise) then going around the Horn of South America seemed
>> the option, although admittedly they could have rafted across the Indian
>> Ocean and Pacific. So agreed, I was inferring what proceeding event was
>> implied by Caccone et al. Its a pity they were not explicit about the
>> origin of the Humboldt ride. That might have made for some imaginative
>> reading.
>>
>> Thanks for the added info about the fossil. Most appreciated. I will
>> acknowledge your assistance in an article addressing this taxon.
>>
>> John Grehan
>>
>> On Sun, Jul 7, 2019 at 9:12 PM Kenneth Kinman via Taxacom <
>> taxacom at mailman.nhm.ku.edu> wrote:
>>
>> > John,
>> >         Well, it's easy to argue against a "straw man" that doesn't
>> exist
>> > and then label it a bedtime story.   They did not suggest such tortoises
>> > rafted around the tip of South America, so you're arguing against
>> something
>> > that they didn't say.   It is perfectly reasonable to suggest that one
>> or
>> > more such rafts did ride the Humboldt Current from the mainland of
>> western
>> > South America.
>> >          I did some more checking, and it seems likely that the Miocene
>> > species Ch. gringorum of Patagonia gave rise not only to Ch. chilensis
>> east
>> > of the Andes, but also to the early ancestors of the Galapagos
>> tortoises to
>> > the west of the Andes.  The Strait of Magellan had absolutely nothing
>> to do
>> > with it (and they did not suggest that it did).  It is interesting that
>> Ch.
>> > gringorum may have also given rise to (or was basal to) other species of
>> > the genus as well.  I would suggest you read that paper (quote and
>> weblink
>> > below):
>> >
>> > A 2017 Conference paper says:
>> > "Chelonoidis gringorum (Simpson, 1942) is a medium-sized testudinid
>> taxon
>> > known mainly from the Early-Middle Miocene of Patagonia. Most fossil
>> > specimens come from the deposits in the area of Trelew – Gaiman –
>> Dolavon
>> > (Chubut Province, Argentina). Besides the type material (a partial
>> shell),
>> > several specimens have been referred to Ch. gringorum over the years,
>> > constituting the best-known fossil testudinid species in South America.
>> > Most phylogenetic analyses place Ch. gringorum as basal to the extant
>> clade
>> > that includes Ch. chilensis and the Galápagos tortoises, or as basal to
>> all
>> > extant species of Chelonoidis."
>> >
>> >
>> >
>> https://www.researchgate.net/publication/321289805_Chelonoidis_gringorum_unshelled_a_state-of-the-art_of_the_southernmost_tortoises_of_the_world
>> >
>> >
>> > ________________________________
>> > From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of John
>> > Grehan via Taxacom <taxacom at mailman.nhm.ku.edu>
>> > Sent: Sunday, July 7, 2019 12:06 PM
>> > To: taxacom
>> > Subject: [Taxacom] Galapagos tortoise bedtime story
>> >
>> > Here's another for those who believe in such things. Caccone et al 1999
>> > found that the sister group to the Galapagos tortoise is Chelonoidis
>> > chilensis. This species is distributed east of the Andes mostly in
>> > Argentina (north of the Patagonia) and Bolivia. The only tortoise in the
>> > genus that is on the Pacific Coast is C. carbonarius (Panama). Caccone
>> et
>> > al attribute the origin of the Galapagos tortoise to the Humboldt
>> current,
>> > but left out the more difficult question of how the tortoises found
>> large
>> > enough rafts (if such things could be produced in the scrublands or dry
>> > forest habitats) that would remain stable enough to navigate around the
>> tip
>> > of South America, presumably via the Strait of Magellan, and AGAINST the
>> > Cape Horn and Circumpolar Currents. Its wonder they didn't end up in
>> South
>> > Africa. Believe it or not.
>> >
>> > John Grehan
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