Heterochrony, or change of developmental timing or growth rates, is an important parameter in the evolution of morphological diversity (Gould 1977; McKinney and McNamara 1991; Richardson 1999). It is a developmental mechanism through which the evolution of morphological traits can be altered (e.g., Gould 1977; Hall 2003; McNamara and McKinney 2005). Heterochrony often coincides with changes in life-history parameters because these expose the growing organism to differing selection pressures (Bernardo 1993; McKinney and Gittleman 1995; Ryan and Semlitsch 1998).
Mammalian postcranial evolution has been discussed as an example of how marked disparity in morphological diversity can result from differences in life history (Müller 1967; Lillegraven 1975; Sears 2004). The mammalian postcranial skeleton displays an impressive array of diverse phenotypes, including adaptation for flight in bats, for obligate swimming in cetaceans, pinnipeds, and sirenians, and for fossoriality in many clades. However, most of this diversity occurs in placentals. Marsupials, who are also less speciose (Nowak 1999), show comparatively little morphological diversity in their postcranial skeleton, particularly with respect to their forelimbs (Lillegraven 1975; Sears 2004). This contrast between marsupials and placentals coincides with extensive differences in life-history traits relating to neonatal maturity and gestation length (Lillegraven 1975, 1979). Whereas marsupials have a uniformly short gestation period varying less than fourfold—from 12.5 days reported in two peramelemorphs (Strahan 1997) to 45 days in Matschie's tree kangaroo (Flannery 1995)—placentals have longer gestation periods that vary more extensively (roughly 40-fold; 16 days in the hamster Mesocricetus auratus and over 660 days in elephants; Nowak 1999). Marsupial neonates are too immature to survive away from the mother; following birth, they immediately move toward the teat (Gemmell et al. 2002; Tyndale-Biscoe 2005). This movement is achieved by action of the shoulder girdle and/or forelimb, which are conspicuously developed and appear specially adapted for climbing motion (Klima 1987; Sánchez-Villagra and Maier 2003). It has been suggested that the combination of extreme altriciality and functional demands of the postnatal move places a constraint on the phenotypic variation of the marsupial forelimb (Lillegraven 1975; Gemmell et al. 2002; Sears 2004; but see Kirsch 1977). This hypothesis is supported by a study on marsupial scapular and pelvic diversity (Sears 2004), which demonstrated that marsupial scapular shape is less diverse and ontogenetically more homogenous than that of placentals. As a mediator for this diversity constraint, heterochronic acceleration of forelimb development in marsupials compared to placentals is often named (Gemmell et al. 1988; Frigo and Wooley 1996; Sears 2004). However, empirical support for this proposal has only recently emerged through comparisons of chondrification and ossification sequences in the two clades (Sánchez-Villagra 2002; Bininda-Emonds et al. 2007). Developmental sequences are suitable for investigating the influence of evolutionary change on ontogeny (Smith 1997; Bininda-Emonds et al. 2002; McNamara and McKinney 2005); therian postcranial ossification and chondrification patterns are of particular interest because they occur before (in the case of chondrification) or soon after (in the case of ossification) birth in marsupials (e.g., Hall and Hughes 1987; Gemmell et al. 1988; Frigo and Wooley 1996; de Oliveira et al. 1998). Using event-pair analysis (where the timing of each event is related to every other event), the studies by Sánchez-Villagra (2002) and Bininda-Emonds et al. (2007) demonstrated that chondrification and ossification onset timing of fore-and hind limbs is indeed heterochronic between placentals and marsupials. These results are congruent with the hypothesis that marsupial forelimb development is accelerated compared to that of placentals, but the direction and polarity of this heterochrony has not been established. Consequently, it is also possible that it is the placentals whose skeletal formation is heterochronic, suggesting that the developmental sequence of marsupials is plesiomorphic; alternatively, it could be the marsupial hind limb that develops late, implying that heterochrony between marsupials and placentals is more complex than previously thought. The latter scenario has already been proposed by Müller (1967), although this part of her work was never discussed elsewhere (perhaps because Müller's papers were written in German). In the absence of a close outgroup (monotremes have not been sampled due to lack of available material), the direction and character of sequence heterochrony in the limbs of marsupials and placentals can only be confirmed using the remaining postcranium as a reference. Although Sánchez-Villagra's (2002) study included ossification onset timing of the whole postcranium, the sample size was small and interpretation of heterochrony and its polarity in event-pair analysis was limited because of the complexities of summarizing the large datasets resulting from event-pair coding (Jeffery et al. 2005). Parsimov, a computer program developed by Jeffery et al. (2005), can improve this by implementing an algorithm that analyses all possible scenarios of heterochrony and returns the most parsimonious solution (i.e., that involving the least heterochrony).
The present study includes postcranial ossification data from 11 marsupial and 13 placental species, a tripling of the taxonomic sampling from Sánchez-Villagra's (2002) dataset. Improved taxonomic representation provides a more reliable basis on which hypotheses of ossification heterochrony between marsupials and placentals can be tested. Large-scale sampling also allows additional comparison between marsupials and placentals with respect to the variation in ossification sequence positions of single bones. We examine questions pertaining to the potential influence of the marsupial/placental life-history dichotomy on postcranial skeletal ontogeny: Is the forelimb of marsupials really accelerated compared to that of placentals? Are differences between marsupials and placentals also reflected in variations of ossification sequence ranks of single bones? Lastly, is heterochrony common within marsupials and placentals, and does it provide any phylogenetic signal?