I found this little ditty in this month’s issue of the journal Evolution. It’s entitled Ossification heterochrony in the therian postcranial skeleton and the marsupial-placental dichotomy, and does a very good job of exploring the differential rates of bone formation in marsupial (the fetus is nourished by a yolk-like sac) v. placental (the fetus is nourished by a placenta) mammals. The reason this is interesting (it’s not obvious, don’t worry), is that marsupials show very little morphological diversity in their postcranial skeleton, and most particulalry in their forelimbs. Observing the rates at which these bones form and comparing them to an outgroup that possesses a great deal of variation (think cow, then bat), may then give a deeper understanding of the evolutionary mechanisms that acted upon either, one, or both taxa.
Something you may not know about marsupials is that at the completion of birth the neonate makes its way unaided to the pouch (marsupium), where it then develops further. Keeping this in mind, the question of ossification (bone formation) and chondrification (cartilage formation) rates becomes even more interesting. If these very undeveloped marsupial neonates are expected to make their way unaided to the pouch, there must be something going on during development providing the forelimb necessary for this sort of movement. In fact, the forelimb of the marsupial neonate has been found to develop and appear specifically adapted for the climbing motion necessary to get to the pouch. It has been suggested that the constraints in phenotypic variation observed in the marsupial forelimb (which I mentioned above) is a result of these functional demands and altriciality of the postnatal.
So, then, what is this ‘heterochrony’ thing I see in the title of the paper? Well, heterochrony (changes in the timing of events during development that result in changes in size or shape), it seems, is the culprit. It has been suggested by authors previous to this article that the diversity constraints on the marsupial forelimb are due to the acceleration of its development in marsupials, as compared to placentals. Some studies have even demonstrated that bone and cartilage formation timings is indeed heterochronic, but it is not known in which direction (have placentals slowed down, have marsupials sped up, or both?). This is the question the authors wish to truly get at.
Using postcranial ossification data from 11 marsupial and 13 placental species the authors take advantage of micro-CT scanning technology to assess the presence of bone throughout the development of each species. The raw ossification data was then ranked according to their sequence, to determine whether differences between marsupials and placentals also reflects variation in ossification sequence of single bones. Using some fun little programs, PAUP among them, the authors then use this developmental data to construct a phylogeny. This is an interesting approch, which I disagree with. Parsimony analysis is already one of the laziest strategies in constructing phylogeny available (parsimony does not an evolution make), to then take such a strategy and put data into it that is not compatible with reconstructing phylogeny (at least at the level of complexity the authors use) is laughable*. The authors, thankfully, recognize its inneffectiveness (kinda…), and don’t make it the focus of the paper.
The results of the study show, interestingly, that there was no (NONE!) intraspecific variation in ossification sequence in any species. More pertinent to the question of sequence, there are two areas of the postcranium that reveal a major difference between placentals and marsupials:
First, forelimb and tarsal/metatarsal rank variation is much lower in marsupials compared to placentals, and second, hind limb and posterior element (except for tarsal/metatarsals) rank variation is lower in placentals compared to marsupials.
This ossification pattern of the hind limb long bones when compared to the forelimb and shoulder girdle in marsupials is most obviously heterochronic. Results from this study, however, show that the difference in this ontogeny between marsupials and placentals is more complex than a mere acceleration of forelimb development. The results show that, rather than merely being an acceleration, the heterochrony of ossification sequence in marsupials is life-history related. It it hard to say which factors influence the timing of postcranial development also contribute to the conservation of forelimb diversity in marsupials, and future research in this direction is clearly necessary (because I say so!).
This study provides the largest comparative dataset to date based on which hypotheses of mammalian postcranial heterochrony could be assessed, with particular focus on heterochrony between marsupials and placentals. Heterochronically delayed ossification of the hind limb and early ossification of axial and shoulder girdle elements was recorded in marsupials, contradicting previous hypothesis of heterochronic forelimb acceleration in marsupials. This heterochrony suggests that processes such as mechanic stresses or energy allocation “trade-offs” play a major role in shaping mammalian skeletal ontogeny.
* It is nice to see that late ossification of the hind limb long bones and earlier ossification of the anterior axial segments place (nearly) all of the marsupials into a monophyletic group (PAUP got lucky…).
(Obligate Image)
Diprotodon, the largest marsupial known. Think hippo sized koala. Scary, huh?
(References)
Weisbecker, V, Goswami, A, Wroe, S & Sánchez-Villagra, M, 2008, 'OSSIFICATION HETEROCHRONY IN THE THERIAN POSTCRANIAL SKELETON AND THE MARSUPIAL-PLACENTAL DICHOTOMY', Evolution, vol. 62, no. 8, pp. 2027-2041. 10.1111/j.1558-5646.2008.00424.x
