Northcutt R G
Neurobiology Unit, Scripps Institution of Oceanography, La Jolla, Calif., USA.
Brain Behav Evol. 1997 Jul;50(1):25-37. doi: 10.1159/000113319.
An outgroup analysis of multiple ontogenies provides the most robust approach to understanding phylogeny. Such an analysis of the lateral line system among extinct and extant gnathostomes reveals that lateral line placodes constitute the basic ontogenetic unit responsible for the development of this system. Six pairs of lateral line placodes appear to have existed in the earliest gnathostomes, and eight stages (stages A-H) can be recognized in their differentiation. Terminal truncation (heterochronic changes) in the primitive sequence of placodal development has occurred in one or more placodes in each gnathostome radiation, with the most extensive truncations occurring in arthrodire placoderms, lepidosirenid lungfishes and extant amphibians. The most extensive nonterminal changes in the primitive sequence of placodal development involve the failure of electroreceptors to form within the lateral zones of the elongatiang sensory ridges of the placodes. This nonterminal change appears to have occurred independently in ancestral neopterygian bony fishes, in many amphibians and, possibly, in the extinct acanthodians. At least two teleost radiations, osteoglossomorphs and ostariophysines, have re-evolved electroreceptors which may represent additional nonterminal changes in placodal patterning or, possibly, a change in the embryonic source of these receptors.
对多种个体发育过程进行外类群分析，是理解系统发育最可靠的方法。对已灭绝和现存的有颌类动物的侧线系统进行这样的分析表明，侧线基板是负责该系统发育的基本个体发育单位。最早的有颌类动物似乎存在六对侧线基板，在其分化过程中可识别出八个阶段（A - H阶段）。在每个有颌类动物类群中，一个或多个基板在基板发育的原始序列中发生了末端截短（异时变化），其中最广泛的截短发生在节甲目盾皮鱼、美洲肺鱼和现存两栖动物中。基板发育原始序列中最广泛的非末端变化涉及电感受器未能在基板延长的感觉嵴的外侧区域形成。这种非末端变化似乎在原始新鳍鱼类、许多两栖动物以及可能已灭绝的棘鱼中独立发生。至少两个硬骨鱼辐射类群，骨舌鱼目和骨鳔总目，重新进化出了电感受器，这可能代表了基板模式形成中的额外非末端变化，或者可能是这些感受器胚胎来源的变化。
