Modularity and developmental patterning in the evolution of early jawed vertebrates

大约5.4亿年前，所有颅类动物的祖先，也就是今天的盲鳗、七鳃鳗、软骨鱼（鼠鱼、鳐鱼和鲨鱼）和硬骨鱼（鳐鱼、腔棘鱼、肺鱼和四足动物）的祖先，获得了独特的胚胎学特征——神经嵴和表皮基板（在器官发生早期形成的特殊胚胎细胞群）。这两种发育上的新奇之处解释了颅骨的许多解剖特征，如大脑和颅面软骨和骨骼。没有其他发育上的新奇现象与有颌脊椎动物或颌口动物（棘目动物、placoderms动物、软骨动物和硬骨鱼类）的早期进化有关。因此，基底齿槽骨的相互关系很难解决。最有可能的是，认识到新的发育特征，如发育模块（一个相互作用的元素网络，作为相对独立的发育单位）和模式（这些元素的形成顺序）可以帮助解决颌口动物的系统发育问题。所有先前提出的解剖模块将被编译以重建颌口模块的理论模型。这些基本模块的存在将通过观察(1)它们的内部变异模式和(2)它们的形成模式在活的颌口动物中进行测试。这些模块和模式将在化石分类群中进行评估，并随后用于重建系统发育。换句话说，我们将解决关于颌类脊椎动物历史中模块化和模式进化的三个问题：(1)颌类动物起源时的模块化组织是什么？(2)在现存和化石颌口动物中，我们是否还能识别出这些模块及其相关的形成模式？(3)基于新的模式化特征能否重构颌口动物的生命树？这个研究项目将有助于理解颌骨脊椎动物进化的发育机制，我们属于这个分类类群，并从它那里继承了基本的解剖学和发育特征。

Approximately 540 million years ago, the ancestor of all craniates, which gave rise to today's hagfishes, lampreys, cartilaginous (ratfishes, skates and sharks) and bony fishes (ray-fined fishes, coelacanths, lungfishes and tetrapods), acquired distinctive embryological features - the neural crest and the epidermal placodes (specialized groups of embryonic cells forming early during organogenesis). These two developmental novelties are accountable for numerous anatomical characteristics of craniates, such as the brain and craniofacial cartilages and bones. No other developmental novelties have been associated to the early evolution of jawed vertebrates, or gnathostomes (acanthodians, placoderms, cartilaginous and bony fishes). As a result, basal gnathostome interrelationships are poorly resolved. Most likely, the recognition of new developmental features such as developmental modules (a network of interacting elements behaving as relatively independent units of development) and patterning (the order of formation of these elements) can help solving gnathostome phylogeny. All anatomical modules that have been previously proposed will be compiled to reconstruct a theoretical model of gnathostome modularity. The presence of these fundamental modules will be tested in living gnathostomes by looking at (1) their internal pattern of variation and (2) their pattern of formation. These modules and patterning will be evaluated in fossil taxa and subsequently used to reconstruct phylogeny. In other words, we will be addressing three questions about the evolution of modularity and patterning in jawed vertebrate history: (1) What was the modular organization of gnathostomes at their origin? (2) Can we still recognize these modules and their associated patterns of formation in living and fossil gnathostomes? (3) Can we reconstruct the gnathostomes' tree of life based on new modular and patterning characters? This research program will help understanding developmental mechanisms that have been responsible for the evolution of jawed vertebrates, the taxonomic group we belong to and from which we inherited fundamental anatomical and developmental characteristics.