Marsupial forelimbs... or rather hindlimbs

[David Marjanovic <david.marjanovic@gmx.at>]

Here's the promised SVP meeting abstract, _emphasis_ (italics) in the original:

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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 clear-stainin!
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.

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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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