Paedomorphosis in Middle Cambrian xystridurine trilobites from northern Australia

Heterochrony, that is phyletic change in the onset or timing of morphological development, is considered to have been the principal factor affecting the evolution of a number of xystridurine trilobite species during the Middle Cambrian. Galahetes shows morphological and size characteristics which indicate that precocious maturation, that is progenesis, was the process responsible for its paedomorphic evolution during the Templetonian Stage. The morphological characteristics of adult Galahetes parallel those found in juvenile stages of Xystridura templetonensis, whose morphology is essentially that of the conservative, structurally ancestral xystridurine stem form which persisted through the Ordian and Templetonian Stages. Both X. altera and X. dunstani are also considered to have evolved from this stem form by paedomorphosis during the Templetonian Stage. In these species, however, the process is thought to have been neoteny, whereby retardation of somatic development resulted in retention of characters w...