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Re: [dinosaur] Fossorial Origin of the Turtle Shell



This is a fascinating idea - that digging behavior can lead to
swimming behavior.  In both activities the forelimbs are used to
propel the body forward by the displacement of the surrounding media
(soil vs water), as Lyson &c put it.  So the osteological adaptations
associated with one behavior (digging) were exapted toward the other
behavior (swimming) in the line leading to turtles.

The same transition, in the opposite direction, was previously
proposed for another group of oddball tetrapods: the monotreme
mammals.  The echidnas (or spiny anteaters), which are fossorial and
terrestrial, are hypothesized to have evolved from aquatic
platypus-like ancestors, thereby making echidnas secondarily
terrestrial (Phillips et al., 2009; www. pnas.org cgi doi 10.1073
pnas.0904649106.)

For stem turtles, Lyson &c propose that the anteroposterior
(=craniocaudal) expansion of the dorsal ribs of _Eunotosaurus_ and
_Pappochelys_ was a fossorial adaptation.  The broad ribs provided a
stable base on which to operate the digging mechanism of the powerful
shoulders and forelimbs.  Fossorial edentates (xenarthrans),
especially the giant anteater, are cited as an analog, referring to
Farish Jenkins' 1970 work on the expanded ribs of certain extant
mammals.

It's worth noting that Jenkins looked at a range of xenarthrans, as
well as primates.  Jenkins hypothesized that anteroposterior
broadening of the ribs increases the stability of the thorax, which,
in turn, increases the stability of the vertebral column.  Jenkins
proposed that the expanded ribs (and robust intercostal muscles) of
the giant anteater and the tamandua provide thoracolumbar stability
when digging into termite mounds or termite-infested wood.  The
powerful digging stroke displaces the substrate, while the body stays
in place. But the same expansion of the ribs is also seen in some
non-digging mammals - like the silky anteater, which is strictly
arboreal.  If the silky anteater evolved from terrestrial or
scansorial ancestors (making it secondary arboreal), the expanded ribs
would be a plesiomorphic character that it inherited from ancestral
fossorial/digging anteaters.  However, the ribs of the silky anteater
are actually *more* expanded than those of the giant anteater and
tamandua; the ribs of the silky anteater are so expanded (due to
posteriorly extended flanges) that they actually overlap one another
(imbricate).  Jenkins associated this increased thoracolumbar
stability and rigidity with "methodical arboreal locomotion" (slow
climbing and bridging), by which the trunk is held horizontally as the
forelimbs reach for a new branch, while the hindfeet and tail anchor
the hindquarters to another branch.

Since Jenkins' work, expanded ribs (to varying degrees) have also been
described in various other arboreal mammals aside from certain
xenarthrans and 'prosimian' primates, including some bats, tupaiids,
and didelphid marsupials.  However, Jenkins also suggested that the
increased thoracolumbar stability and rigidity assisted the silky
anteater in adopting a defensive posture.  This is essentially the
same posture used during bridging, with the forelimbs free and poised
to strike.  So there is no evidence that expanded ribs serve a
protective role (ventral shield, etc) in mammals.

I don't mean to imply that stem turtles were arboreal(!).  The other
osteological characters in _Eunotosaurus_ are consistent with digging
and burrowing (as in extant gopher turtles).  But the link between
anteroposterior expansion of the ribs and fossorial/digging behavior
is not at all straightforward.  The increased thoracolumbar
stability/rigidity conferred by anteroposterior expansion of the ribs
can be employed for other behaviors, as it is in many non-digging
mammals.



On Fri, Jul 15, 2016 at 3:44 AM, Ben Creisler <bcreisler@gmail.com> wrote:
>
> Ben Creisler
> bcreisler@gmail.com
>
> A new paper:
>
> Roger M.H. Smith, Jennifer Botha-Brink & G.S. Bever (2016)
> Fossorial Origin of the Turtle Shell.
> Current Biology (advance online publication)
> DOI: 
> https://urldefense.proofpoint.com/v2/url?u=http-3A__dx.doi.org_10.1016_j.cub.2016.05.020&d=DQIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=4f7Ki3pDG6JZg-TANRgGIqOcb06cyOJHLcpcQU4qFi0&s=uTdndsvKxfGUKpEJP63DxsoC2VgzOELSeaK3ybVhkiM&e=
>  
> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.cell.com_current-2Dbiology_fulltext_S0960-2D9822-2816-2930478-2DX&d=DQIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=4f7Ki3pDG6JZg-TANRgGIqOcb06cyOJHLcpcQU4qFi0&s=FyAfDhfgt_42xkjzITKr4oF3s2BtP0hw7jvTqeR-vt8&e=
>  
>
>
>
> Highlights
>
> Recently discovered stem turtles indicate the shell did not evolve for
> protection
> Adaptation related to digging was the initial impetus in the origin of the
> shell
> Digging adaptations facilitated the movement of turtles into aquatic
> environments
> Fossoriality likely helped stem turtles survive the Permian/Triassic
> extinction
>
>
> Summary
>
> The turtle shell is a complex structure that currently serves a largely
> protective function in this iconically slow-moving group. Developmental and
> fossil  data indicate that one of the first steps toward the shelled body
> plan was broadening of the ribs (approximately 50 my before the completed
> shell. Broadened ribs alone provide little protection and confer significant
> locomotory and respiratory costs. They increase thoracic rigidity, which
> decreases speed of locomotion due to shortened stride length, and they
> inhibit effective costal ventilation. New fossil material of the oldest
> hypothesized stem turtle, Eunotosaurus africanus (260 mya) from the Karoo
> Basin of South Africa, indicates the initiation of rib broadening was an
> adaptive response to fossoriality. Similar to extant fossorial taxa, the
> broad ribs of Eunotosaurus provide an intrinsically stable base on which to
> operate a powerful forelimb digging mechanism. Numerous fossorial correlates
> are expressed throughout Eunotosaurus’ skeleton. Most of these features are
> widely distributed along the turtle stem and into the crown clade,
> indicating the common ancestor of Eunotosaurus and modern turtles possessed
> a body plan significantly influenced by digging. The adaptations related to
> fossoriality likely facilitated movement of stem turtles into aquatic
> environments early in the groups’ evolutionary history, and this ecology may
> have played an important role in stem turtles surviving the Permian/Triassic
> extinction event.
>
> ***
>
> News:
>
> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.theatlantic.com_science_archive_2016_07_the-2Dturtle-2Dshell-2Dfirst-2Devolved-2Dfor-2Ddigging-2Dnot-2Ddefence_491087_&d=DQIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=4f7Ki3pDG6JZg-TANRgGIqOcb06cyOJHLcpcQU4qFi0&s=yEpTLwNGIJG83VvXNKTjr1XfiKyptcf7kPQ3IyY0ccM&e=
>  
>