Constraints on the Morphological Evolution of Marsupial Houlder Girdles

Abstract

Throughout their evolutionary histories, marsupial mammals have been taxonomically and morphologically less diverse than their sister taxa the placentals. Because of this, it has been proposed that the evolution of marsupials has been constrained by the functional requirements of their mode of reproduction. Marsupials give birth after short gestation times to immature neonates that immediately crawl, under the power of their precociously developed shoulder girdles, to the teat where they attach and complete their early development. Using a novel approach incorporating adult and embryological morphological data, this study is the first to both: (1) statistically support adult patterns of morphological divergence consistent with the constraint hypothesis, and (2) identify ontogenetic patterns of morphological change that demonstrate that the constraint was responsible, at least in part, for their formation. As predicted by the marsupial constraint, the shoulder girdles of adult marsupials are less diverse than those of adult placentals, and adult marsupial scapulae are less morphologically diverse than adult marsupial pelves. Furthermore, marsupials that complete an extensive crawl to the teat are restricted to a common pattern of ontogenetic scapular shape change, strongly supporting the hypothesis that the morphological development of the marsupial scapula has been limited evolutionarily by its obligate role in the crawl to the teat. Because this study establishes that ontogenetic and evolutionary morphological change is correlated within mammalian scapulae, it is probable that the marsupial constraint also restricted the morphological divergence of the scapula over evolutionary time by limiting ontogenetic change in the scapula. These findings, coupled with the importance of the shoulder girdle in mammalian locomotor specialization, support the conclusion that the low morphological diversity of marsupial forms over evolutionary time could be directly due to the constraint on marsupial morphological evolution caused by the functional requirements of the crawl to the teat.

Rate of evolutionary change in cranial morphology of the marsupial genus Monodelphis is constrained by the availability of additive genetic variation

“…This link is particularly important given the recent proliferation of studies showing that occupation of morphospace by marsupials is more restricted than by eutherians, even when fossils are taken into account (e.g., (Sears, 2004; Cooper & Steppan, 2010; Kelly & Sears, 2011; Bennett & Goswami, 2013). In particular, our results suggest that the occupation of morphospace in Monodelphis is the result of a complex interplay between the limited dimensionality of available genetic variation in multivariate space and strong stabilizing selection on two major axes of genetic variation.…”

“…If genetic in origin, this strong covariation among traits within populations has the potential to produce strong microevolutionary constraints on evolution along any dimension other than overall size, the only dimension with abundantly available variation in marsupials (Porto et al , 2009). These microevolutionary constraints, in turn, would provide us with a proximate cause for the long standing observation that the occupation of morphospace by marsupials is much more restricted than by eutherians, even when fossils are taken into account (e.g., Sears, 2004; Cooper & Steppan, 2010; Kelly & Sears, 2011; Bennett & Goswami, 2013). …”

The predatory behaviour of the thylacine: Tasmanian tiger or marsupial wolf?

“…However, it is possible that all marsupials are constrained to retain the capacity to pronate/supinate the elbow owing to constraints on the forelimb morphology necessary for the neonate to crawl to the pouch (e.g. [20]). Thus, the marsupial reproductive strategy might actually limit the capacity of marsupials to become cursorially-adapted pursuit predators.…”

Biting through constraints: cranial morphology, disparity and convergence across living and fossil carnivorous mammals

“…Marsupials are born after a short period of gestation, in some cases as little as a few weeks after conception, and the neonate is equipped with only well-developed oral apparatus, to suckle, and forelimbs, to perform the crawl to the pouch [3][4][5][6][7][8][9][10][11][12][13][14][15]. This early ossification has been demonstrated to have constrained the morphological evolution of the forelimb across marsupials [10], but has not been explicitly tested in the cranium. Specifically, it can be hypothesized that marsupial cranial morphology is limited in its evolutionary 'potential', relative to that of placental mammals, by the well-established early development of cranial structures (e.g.…”