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

John Grehan calabar.john at gmail.com
Sun Jul 7 21:56:01 CDT 2019


Oops - forgot to ask what is the actualistic evidence for the trans oceanic
dispersals?

Thanks, 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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>


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