Morphogenesis in the genus <Emphasis Type="Italic">Peronopsis</Emphasis> Hawle et Corda (Agnostina)

The morphogenesis of the early members of the genus Peronopsis (Trilobita, Agnostina) is studied. The pygidium of Agnostina exhibits growth-related changes more clearly than the cephalon. In Peronopsis inarmata Hutchinson, 1962 and P. brighamensis Resser, 1938 the axial furrow develops around the anteroglabella in meraspid degree 0. The most significant changes occur in the shape of the pygidium axis and the postaxial field; posterolateral spines do not change throughout growth. Specific characters continue to develop until the advanced holaspid stage. Within the genus Peronopsis, three distinct growth patterns of the pygidium are recognized. Different relative growth rates of different parts of the pygidium were responsible for the evolutionary transitions between the growth patterns. The growth pattern when the pygidium lacks a postaxial furrow was the most primitive and gave rise to a number of species with a long axis reaching the border, a growth pattern that in turn gave rise to the species with the postaxial furrow (a character typical of Agnostina). The evolution of these growth patterns is inferred from ontogenetic observations and is supported by the stratigraphic succession of Peronopsis and their immediate descendants in different regional faunas.