WEBSTER, M.: Allometric patterning in trilobite ontogeny: testing for heterochrony in Nephrolenellus

Heterochrony, a decoupling between morphological development and ontogenetic time, has been touted as the commonest mode of evolutionary change. Evolutionary change involving modification to the pattern of morphological development cannot be described as a decoupling between morphological development and time. Conservation of the ancestral pattern of ontogenetic shape change (locally or globally) therefore serves as a test by which pure heterochrony can be rigorously distinguished from alternative modes of ontogenetic modification. The Early Cambrian olenelloid trilobite Nephrolenellus geniculatus differs from its ancestral sister species N. multinodus in retention of fewer glabellar axial nodes, development of a stronger adgenal angle, and impingement of glabellar lobe L4 into the anterior border. In a simple ontogenetic comparison, all these features can be interpreted as extrapolations of ontogenetic trends present in the ancestral species. Under the standards usually imposed, this putative case of peramorphosis is one of the most strongly supported examples of heterochrony in trilobites. However, detailed ontogenetic comparisons using thin-plate spline analyses show that N. geniculatus and N. multinodus differ significantly in local and global patterns of shape change from the earliest preserved ontogenetic stage. The descendant ontogeny was predominantly modified in spatial, rather than temporal, aspects. The hypothesis of evolution by pure heterochrony in Nephrolenellus is rejected in favor of allometric repatterning of shape. That even this case fails to be interpretable as pure heterochrony suggests that oversimplified ontogenetic studies may have considerably overestimated the prevalence of heterochrony in trilobite evolution.