The Abundance of Heterochrony in the Fossil Record. In: Mckinney ML, editor. Heterochrony in Evolution. Boston: Springer US; 1988. pp. 287-325. [DOI: 10.1007/978-1-4899-0795-0_15]

HETEROCHRONY AND ALLOMETRY: LESSONS FROM THE WATER STRIDER GENUS LIMNOPORUS

Heterochrony and allometry both deal with evolutionary modifications of ontogenies. Although data about both morphology and age are required to identify heterochronic processes, age data are not needed to study allometry. Using a simple graphical model, we show that allometric patterns cannot be used to infer the underlying heterochronic processes. We present a case study of the water strider genus Limnoporus Stål (Heteroptera: Gerridae) to illuminate the distinct roles that allometry and heterochrony play in integrated studies of the evolution of form. Multivariate analyses reveal several evolutionary modifications of growth trajectories (changes in direction, lateral transposition, and ontogenetic scaling), which are fairly consistent with the hypothesized phylogeny of the genus. Because there is no positive correlation between instar durations and size increments, size cannot be used as a proxy for age data in studies of heterochrony. In fact, a measure of overall size itself shows a remarkable variety of heterochronic changes among the six species. Mixtures of several heterochronic processes predominate over the more unitary reflections of "pure" processes. Heterochronic changes in different branches of the phylogeny, apparently independent of size scaling, suggest considerable potential for adaptive evolution. "Local" differentiation of ontogenetic traits within small clades may be at least as important as "global" evolutionary trends in large clades and will often be missed in "global" analyses.

Ontogeny of the trilobite Olenus wahlenbergi Westergård, 1922 from the upper Cambrian Alum Shales of Andrarum, Skåne, Sweden

The upper Cambrian trilobite Olenus wahlenbergi Westergård, 1922 occurs abun- dantly in stinkstone concretions in the Alum Shales at Andrarum, in Skåne, southern Sweden (Olenus/Homagnostus obesus Zone). All growth stages from anaprotaspis onwards are represented, though the early stages are comparatively uncommon and complete specimens are rare. Fine details of structure are preserved, suitable for scanning electron microscopic study.

The protaspis has five axial rings, which in the adult become divided transversely and reduced to three, and the preglabellar field expands markedly in the holaspis. The librigena broadens during development and the eye, with its widely separated lenses is retained on the librigena throughout meraspid development. The visual surface dehisces during ecdysis from the early holaspid onwards, due to emplacement of the ocular suture. The hypostome apparently changes from a conterminant to a natant condition in the early holaspis, which is consistent with the growth of the preglabellar field at this stage. The transitory pygidium has a slightly serrated margin throughout development, but becomes entire when all thoracic segments have been liberated.

Cuticular sculpture changes dramatically throughout ontogeny. Large (Type A) tubercles appear on the fixigena in early meraspides; these become prominent and coalesce by the end of meraspid development. They are abruptly replaced by smaller (Type B) tubercles in the early holaspid, which link to form a rough network in mature specimens. Comparable Type A tubercles in early meraspid librigenae give rise to a strong polygonal caecal sculpture in holaspids, while a radial sculpture on the broadening anterior border develops independently. The significance of these cuticular changes remains obscure. A median occipital organ is described for the first time in the Olenidae.

Spatiotemporal reorganization of growth rates in the evolution of ontogeny

Heterochrony, evolutionary changes in rate or timing of development producing parallelism between ontogeny and phylogeny, is viewed as the most common type of evolutionary change in development. Alternative hypotheses such as heterotopy, evolutionary change in the spatial patterning of development, are rarely entertained. We examine the evidence for heterochrony and heterotopy in the evolution of body shape in two clades of piranhas. One of these is the sole case of heterochrony previously reported in the group; the others were previously interpreted as cases of heterotopy. To compare ontogenies of shape, we computed ontogenetic trajectories of shape by multivariate regression of geometric shape variables (i.e., partial warp scores and shape coordinates) on centroid size. Rates of development relative to developmental age and angles between the trajectories were compared statistically. We found a significant difference in developmental rate between species of Serrasalmus, suggesting that heterochrony is a partial explanation for the evolution of body shape, but we also found a significant difference between their ontogenetic transformations; the direction of the difference between them suggests that heterotopy also plays a role in this group. In Pygocentrus we found no difference in developmental rate among species, but we did find a difference in the ontogenies, suggesting that heterotopy, but not heterochrony, is the developmental basis for shape diversification in this group. The prevalence of heterotopy as a source of evolutionary novelty remains largely unexplored and will not become clear until the search for developmental explanations looks beyond heterochrony.