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

Evangelos Vlachos evlacho at gmail.com
Sun Jul 7 21:59:43 CDT 2019


It is this case of a tortoise that traveled from Aldabra to the continent
(see attached).

Calibrations: I was referring to the fossil calibrated molecular ages.

All the best,

Evan

On Sun, 7 Jul 2019 at 23:56, John Grehan <calabar.john at gmail.com> wrote:

> 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
>>> > _______________________________________________
>>> > Taxacom Mailing List
>>> >
>>> > Send Taxacom mailing list submissions to: taxacom at mailman.nhm.ku.edu
>>> > For list information; to subscribe or unsubscribe, visit:
>>> > http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
>>> > You can reach the person managing the list at:
>>> > taxacom-owner at mailman.nhm.ku.edu
>>> > The Taxacom email archive back to 1992 can be searched at:
>>> > http://taxacom.markmail.org
>>> >
>>> > Nurturing nuance while assaulting ambiguity for 32 some years,
>>> 1987-2019.
>>> > _______________________________________________
>>> > Taxacom Mailing List
>>> >
>>> > Send Taxacom mailing list submissions to: taxacom at mailman.nhm.ku.edu
>>> > For list information; to subscribe or unsubscribe, visit:
>>> > http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
>>> > You can reach the person managing the list at:
>>> > taxacom-owner at mailman.nhm.ku.edu
>>> > The Taxacom email archive back to 1992 can be searched at:
>>> > http://taxacom.markmail.org
>>> >
>>> > Nurturing nuance while assaulting ambiguity for 32 some years,
>>> 1987-2019.
>>> >
>>> _______________________________________________
>>> Taxacom Mailing List
>>>
>>> Send Taxacom mailing list submissions to: taxacom at mailman.nhm.ku.edu
>>> For list information; to subscribe or unsubscribe, visit:
>>> http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
>>> You can reach the person managing the list at:
>>> taxacom-owner at mailman.nhm.ku.edu
>>> The Taxacom email archive back to 1992 can be searched at:
>>> http://taxacom.markmail.org
>>>
>>> Nurturing nuance while assaulting ambiguity for 32 some years, 1987-2019.
>>>
>>


More information about the Taxacom mailing list