Fröbisch Nadia B
Redpath Museum, McGill University, 859 Sherbrooke Street West, Montreal H3A2K6, Canada.
Biol Rev Camb Philos Soc. 2008 Nov;83(4):571-600. doi: 10.1111/j.1469-185X.2008.00055.x. Epub 2008 Oct 20.
Two different patterns of the condensation and chondrification of the limbs of tetrapods are known from extensive studies on their early skeletal development. These are on the one hand postaxial dominance in the sequential formation of skeletal elements in amniotes and anurans, and on the other, preaxial dominance in urodeles. The present study investigates the relative sequence of ossification in the fore- and hindlimbs of selected tetrapod taxa based on a literature survey in comparison to the patterns of early skeletal development, i.e. mesenchymal condensation and chondrification, representing essential steps in the late stages of tetrapod limb development. This reveals the degree of conservation and divergence of the ossification sequence from early morphogenetic events in the tetrapod limb skeleton. A step-by-step recapitulation of condensation and chondrification during the ossification of limbs can clearly be refuted. However, some of the deeper aspects of early skeletal patterning in the limbs, i.e. the general direction of development and sequence of digit formation are conserved, particularly in anamniotes. Amniotes show a weaker coupling of the ossification sequence in the limb skeleton with earlier condensation and chondrification events. The stronger correlation between the sequence of condensation/chondrification and ossification in the limbs of anamniotes may represent a plesiomorphic trait of tetrapods. The pattern of limb ossification across tetrapods also shows that some trends in the sequence of ossification of their limb skeleton are shared by major clades possibly representing phylogenetic signals. This review furthermore concerns the ossification sequence of the limbs of the Palaeozoic temnospondyl amphibian Apateon sp. For the first time this is described in detail and its patterns are compared with those observed in extant taxa. Apateon sp. shares preaxial dominance in limb development with extant salamanders and the specific order of ossification events in the fore- and hindlimb of this fossil dissorophoid is almost identical to that of some modern urodeles.
通过对四足动物早期骨骼发育的广泛研究，已知四足动物肢体的软骨凝聚和软骨化存在两种不同模式。一方面，羊膜动物和无尾两栖动物骨骼元素的顺序形成中存在轴后优势；另一方面，有尾两栖动物中存在轴前优势。本研究基于文献调查，将选定的四足动物类群的前肢和后肢骨化相对顺序与早期骨骼发育模式（即间充质凝聚和软骨化，这是四足动物肢体发育后期的关键步骤）进行比较。这揭示了四足动物肢体骨骼骨化顺序与早期形态发生事件的保守程度和差异程度。肢体骨化过程中凝聚和软骨化的逐步重演显然是不成立的。然而，肢体早期骨骼模式的一些深层次方面，即发育的总体方向和指形成顺序是保守的，特别是在无羊膜动物中。羊膜动物肢体骨骼的骨化顺序与早期凝聚和软骨化事件的耦合较弱。无羊膜动物肢体中凝聚/软骨化顺序与骨化之间更强的相关性可能代表了四足动物的一个原始特征。四足动物肢体骨化模式还表明，其肢体骨骼骨化顺序的一些趋势为主要类群所共有，这可能代表了系统发育信号。本综述还涉及古生代离片椎两栖动物Apateon sp.的肢体骨化顺序。这是首次对其进行详细描述，并将其模式与现存类群中观察到的模式进行比较。Apateon sp.在肢体发育中与现存蝾螈一样具有轴前优势，这种化石离片椎类动物前肢和后肢骨化事件的特定顺序几乎与一些现代有尾两栖动物相同。
