Smith M M, Coates M I
Craniofacial Development, UMDS-London University, UK.
Eur J Oral Sci. 1998 Jan;106 Suppl 1:482-500. doi: 10.1111/j.1600-0722.1998.tb02212.x.
The theory that teeth evolved from dermal denticles linked with the origin of jaws no longer accounts for the diversity of new data emerging from the fossil record. We have reviewed oropharyngeal dental patterns in all fossil groups of early vertebrates to establish the primitive condition, in order to understand the polarity of change. The evolutionary precedence of dermal denticles before teeth now seems less likely; both may be alternative manifestations of a common morphogenetic system. This developmental system involves regulatory changes affecting the odontode, a fundamental exoskeletal unit, and can explain skeletal diversity. However, tooth and denticle differences may have diverged at loci deep within vertebrate phylogeny, as real differences exist between them. Teeth were conceived as evolving from non-growing odontodes with regulation of precise increase in size, position, sequence of time of development, and polarity of shape. A characteristic feature of teeth is the ability to replace from a developing sequence, programmed with these parameters, prior to demand. Tooth whorls, a feature of denticles in the oropharyngeal region, may be regarded as a preadaptation of this tooth replacement mechanism. The new fossil evidence suggests that teeth may have evolved from these more specialised oropharyngeal denticles in agnathan vertebrates.
牙齿从与颌骨起源相关的真皮小齿进化而来的理论，已无法解释化石记录中不断出现的新数据的多样性。我们回顾了早期脊椎动物所有化石类群的口咽齿模式，以确定原始状态，从而了解变化的极性。现在看来，真皮小齿在牙齿之前出现的进化先后顺序不太可能成立；两者可能是共同形态发生系统的不同表现形式。这个发育系统涉及影响牙质（一种基本的外骨骼单位）的调控变化，并且可以解释骨骼的多样性。然而，牙齿和小齿的差异可能在脊椎动物系统发育的深处就已分化，因为它们之间存在实际差异。牙齿被认为是从不生长的牙质进化而来，其大小、位置、发育时间顺序和形状极性都受到精确调控。牙齿的一个特征是能够按照这些参数在发育序列中进行替换，且是在需求之前就已编程。牙轮，即口咽区域小齿的一个特征，可被视为这种牙齿替换机制的一种预适应。新的化石证据表明，牙齿可能是从无颌脊椎动物中这些更特化的口咽小齿进化而来的。
