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Re: Marsupial forelimbs... or rather hindlimbs
Well, it'll be nice to see this paper when it comes out. Obviously, to
speak about a heterochrony you need to compare ontogenies across
phylogeny, and though there are studies that have looked at this
question (and similar ones) for these groups, this is certainly the
largest placental-marsupial developmental comparison to date.
I don't know that this is especially novel in terms of thinking,
though; there are papers that have suggested deceleration of hindlimbs
is what is going on in marsupials (although the majority do suggest it
is acceleration of forelimbs). It will be interesting to see their
methods: do they hold timing of development of hindlimbs constant
relative to a third event, such as brain development? Or constant to
time/developmental stage? A lot of the current literature holds brain
development constant and compares limb or facial development relative
to that. I'm especially curious about which outgroups they used and
what's going on in them.
In terms of comparing forelimb to brain development, there is a large
body of literature that suggests that marsupials are indeed developing
their forelimbs earlier than placentals in both absolute and relative
terms. In several cases (e.g., Monodelphis, Dasyurus) you get
development and ossification of the facial and forelimb bones (and
significant development of their associated muscles) while the brain
is still undifferentiated neuroepithelium (and while big chunks of the
rest of the body is undifferentiated mesenchyme). The
forelimb-hindlimb timing is interesting, but think the story with the
timing of brain/braincase and limbs is more interesting.
For some really cool research by someone who completely gets
heterochrony, and who works on limb and craniofacial development in
marsupials and placentals, check out Kathleen K Smith's website:
Most of her papers are available as free pdf's.
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University of California, Berkeley
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On 11/26/07, David Marjanovic <firstname.lastname@example.org> wrote:
> Here's the promised SVP meeting abstract, _emphasis_ (italics) in the
> Vera Weisbecker, Anjali Goswami, Marcelo Sánchez-Villagra & Stephen Wroe:
> Postcranial sequence heterochrony and the marsupial-placental dichotomy, SVP
> meeting abstracts 2007, 164A
> It has long been recognized that there is a deep dichotomy between marsupials
> and placentals in terms of taxonomic and ecological diversity. Marsupial
> species known today comprise a small percentage of the number of placental
> species, and have no extremely specialized (e.g. aerial or fully aquatic)
> representatives. The comparative lack of marsupial diversity is particularly
> eident in the marsupial forelimb, which is much less diverse compared to that
> of placentals. Marsupials are born in an extremely altricial state, and in
> most species, the neonate actively crawls to the pouch using its forelimbs.
> Current consensus holds that the crawl to the pouch is related to
> heterochronic acceleration of the marsupial forelimb. Howeve, this hypothesis
> has only experienced limited formal testing. We analysed postcranial ontogeny
> in a new dataset of ossification sequences of 13 placentals, 11 marsupials,
> and three outgroup taxa, considering 25 events. Data were obtained from
> g and/or
> micro-computed tomography of ontogenetic series, and in some cases from the
> literature. The ossification sequence data were analyzed using the
> event-pair[-]based Parsimov analysis. To examine their potential phylogenetic
> signal, a parsimony analysis of the event-pair data was conducted. This did
> not retrieve any of the recognized mammalian clades except for monophyletic
> marsupials (except *Petaurus*) nested within placentals. This suggestes that
> postcranial ossification sequences carry some limited phylogenetic signal for
> marsupials, although [this is] not sufficient to separate them from
> placentals. Event distributions within species suggest that a larger amount
> of events occurs in younger animals compared to older specimens, a pattern
> particularly obvious in marsupials. We expect[ed?] that this is due to fast
> appearance of the forelimb long bones, shoulder girdle, cervical and thoracic
> elements in the youngest marsupials. Parsimov analysis reports that
> ossification events!
> in the
> hindlimb of marsupials are _decelerated_ with respect to forelimb long bone
> ossification events. This novel ypothesis challenges the long-held tenet of
> an ontogenetic "bottleneck" on marsupial forelimb diversity.
> So, it's not the brain that the placentals develop while the marsupials are
> developing their forelimbs. It's the hindlimbs.
> Marsupials don't develop the forelimbs earlier than placentals, they develop
> the hindlimbs later than placentals.
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