Hox genes are known from a wide variety of organisms. In arthropods, these genes control segment characteristics. Trilobites, being arthropods, probably contained eight major Hox genes that controlled their segment types. The trilobite Bauplan contains eight regions that are most likely under the influence of one or more of these Hox genes. The cephalon contains the frontal lobe, glabellar, and occipital ring regions; the thorax contains the anterior thoracic and posterior thoracic regions; and the pygidium contains the articulating ring, axial, and terminal piece regions. Changes in character distribution within or between these regions represent homeotic evolution, which may have resulted from the modification of Hox transcription or of downstream regulatory genes. A phylogenetic analysis is used to recognize homeotic evolution in trilobites, leading to the conclusion that homeotic evolution is common among Cambrian trilobites.

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