Department of Earth Sciences, University of California, Riverside, California 92521.
Integr Comp Biol. 2003 Feb;43(1):185-206. doi: 10.1093/icb/43.1.185.
The Trilobita were characterized by a cephalic region in which the biomineralized exoskeleton showed relatively high morphological differentiation among a taxonomically stable set of well defined segments, and an ontogenetically and taxonomically dynamic trunk region in which both exoskeletal segments and ventral appendages were similar in overall form. Ventral appendages were homonomous biramous limbs throughout both the cephalon and trunk, except for the most anterior appendage pair that was antenniform, preoral, and uniramous, and a posteriormost pair of antenniform cerci, known only in one species. In some clades trunk exoskeletal segments were divided into two batches. In some, but not all, of these clades the boundary between batches coincided with the boundary between the thorax and the adult pygidium. The repeated differentiation of the trunk into two batches of segments from the homonomous trunk condition indicates an evolutionary trend in aspects of body patterning regulation that was achieved independently in several trilobite clades. The phylogenetic placement of trilobites and congruence of broad patterns of tagmosis with those seen among extant arthropods suggest that the expression domains of trilobite cephalic Hox genes may have overlapped in a manner similar to that seen among extant arachnates. This, coupled with the fact that trilobites likely possessed ten Hox genes, presents one alternative to a recent model in which Hox gene distribution in trilobites was equated to eight putative divisions of the trilobite body plan.
三叶虫的特征是头部区域，其中生物矿化的外骨骼在一组分类稳定的定义明确的节段中表现出相对较高的形态分化，以及在个体发生和分类上具有动态性的躯干区域，其中外骨骼节段和腹肢在整体形式上相似。腹肢在头甲和躯干中都是同源的双枝附肢，除了最前面的一对附肢是触角状、口前的和单枝的，以及最后面的一对触角状尾须，仅在一个物种中出现。在一些进化枝中，躯干外骨骼节段分为两批。在这些进化枝中，有一些（但不是全部），批次之间的边界与胸节和成虫尾甲之间的边界重合。从同源的躯干状态重复地将躯干分化为两批节段，表明在身体模式调节方面存在进化趋势，这种趋势在几个三叶虫进化枝中独立地实现了。三叶虫的系统发育位置以及分类单元形成的广泛模式与现存节肢动物中的模式一致，表明三叶虫头部 Hox 基因的表达域可能以与现存蛛形纲动物相似的方式重叠。这一点，再加上三叶虫可能拥有十个 Hox 基因，提供了一个替代最近的模型的选择，即在该模型中，三叶虫的 Hox 基因分布被等同于三叶虫身体计划的八个假定划分。
