Department of Genetics, Evolution and Environment, University College London, London, UK.
BMC Biol. 2013 Apr 26;11:52. doi: 10.1186/1741-7007-11-52.
In contrast to placental neonates, in which all cranial bones are ossified, marsupial young have only the bones of the oral region and the exoccipital ossified at birth, in order to facilitate suckling at an early stage of development. In this study, we investigated whether this heterochronic shift in the timing of cranial ossification constrains cranial disparity in marsupials relative to placentals.
We collected three-dimensional (3D) landmark data about the crania of a wide range of extant placentals and marsupials, and from six fossil metatherians (the clade including extant marsupials and their stem relatives), using a laser scanner and a 3D digitizer. Principal components analysis and delta variance tests were used to investigate the distribution and disparity of cranial morphology between different landmark sets (optimizing either number of landmarks or number of taxa) of the whole skull and of individual developmental or functional regions (neurocranium, viscerocranium, oral region) for extant placentals and marsupials. Marsupial and placental data was also compared based on shared ecological aspects including diet, habitat, and time of peak activity.
We found that the extant marsupial taxa investigated here occupy a much smaller area of morphospace than the placental taxa, with a significantly (P<0.01) smaller overall variance. Inclusion of fossil taxa did not significantly increase the variance of metatherian cranial shape. Fossil forms generally plotted close to or within the realm of their extant marsupial relatives. When the disparities of cranial regions were investigated separately, significant differences between placentals and marsupials were seen for the viscerocranial and oral regions, but not for the neurocranial region.
These results support the hypothesis of developmental constraint limiting the evolution of the marsupial skull, and further suggest that the marsupial viscerocranium as a whole, rather than just the early-ossifying oral region, is developmentally constrained.
与胎盘新生儿不同，所有颅骨都已骨化，有袋类幼仔在出生时仅具有口腔区域的骨骼和枕外骨化，以便在发育的早期阶段进行哺乳。在这项研究中，我们研究了颅骨骨化的这种时间变化是否限制了有袋类相对于胎盘类的颅骨差异。
我们使用激光扫描仪和 3D 数字化器收集了广泛的现生胎盘类和有袋类的颅骨的三维（3D）地标数据，以及来自六个化石有袋类（包括现生有袋类及其祖先的类群）的颅骨 3D 地标数据。我们使用主成分分析和 delta 方差检验来研究不同地标集（优化地标数量或分类群数量）的颅骨形态在整个颅骨和个体发育或功能区域（神经颅、内脏颅、口腔区域）之间的分布和差异现生胎盘类和有袋类。还根据饮食、栖息地和活动高峰期等共享生态方面比较了有袋类和胎盘类的数据。
我们发现，与胎盘类相比，本研究中调查的现生有袋类分类群占据的形态空间要小得多，整体方差明显（P<0.01）较小。包含化石分类群并没有显著增加有袋类颅骨形状的方差。化石形式通常靠近或在其现生有袋类亲属的范围内绘制。当分别研究颅骨区域的差异时，在内脏颅和口腔区域观察到胎盘类和有袋类之间存在显著差异，但在神经颅区域则没有。
这些结果支持了发育约束限制有袋类颅骨进化的假说，并进一步表明，有袋类的内脏颅整体受到发育约束，而不仅仅是早期骨化的口腔区域。
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