The Role of Heterochrony in the Evolution of Cambrian Trilobites

Abstract

Summary 1. Renewed interest in the role of changes to developmental regulation in organisms has highlighted the importance of heterochrony in the evolution of the Metazoa. 2. Beecher's interpretation of the evolution of the Trilobita as having been principally by peramorphosis is examined, as is the view of later workers, principally Stubblefield and Hupé, that paedomorphosis was a dominant factor in trilobite evolution. 3. Both peramorphosis and paedomorphosis are considered to have been important in trilobite evolution. 4. The role of paedomorphosis in the evolution of major morphological novelties is critically examined. Its importance in changes to the structure of the glabella is discussed and new terms proposed to describe the ontogenetic and phylogenetic state of the glabella. 5. The highly variable nature of early Cambrian trilobites, in particular the large degree of ontogenetic change, is considered, along with possible poor developmental control of the growth and moulting hormonal systems, to have been significant in providing a high degree of intrapopulational morphological variability. Selection of these heterochronic variants was responsible for the rapid diversification of the Trilobita during the Cambrian.

Heterochrony: the Evolution of Development

“…This is thought to have been due to their developmental systems having been poorly constrained when they first evolved in the Early Cambrian (McNamara 1986b). This is shown by the high variability in segment number, both within and between species in these earliest trilobites.…”

Ontogeny and intraspecific variation of the early Cambrian trilobiteOlenellus gilberti, with implications for olenelline phylogeny and macroevolutionary trends in phenotypic canalization

“…Study of the degree and pattern of variation (including ontogenetic change) within trilobites is therefore crucial to our understanding of particular evolutionary trends or events and to phylogenetic reconstruction, as well as to issues such as species diagnosis, diversity estimation, and biostratigraphical correlation (e.g. McNamara 1978McNamara , 1986Hughes 1991Hughes , 1994Webster et al 2001;Webster & Zelditch 2005, 2011aAdrain & Westrop 2006;Webster 2007aWebster , b, 2009aWebster , 2011aHopkins & Webster 2009). The need for detailed characterization of the nature and magnitude of intraspecific variation is magnified for olenellines given their basal phylogenetic placement within the Trilobita (above) and the well-known difficulty in establishing robust diagnoses of, and hypotheses of relationship amongst, olenelline species and higher taxa (Palmer & Halley 1979;Palmer & Repina 1993;Palmer & Repina in Whittington et al 1997;Lieberman 1998Lieberman , 1999.…”

“…Some workers have claimed that olenellines exhibited a high degree of intraspecific variation relative to more derived trilobites, based on non-quantitative observations of variation in the shape and size of various cephalic features (Palmer & Halley 1979), meristic variation in thoracic segment number (McNamara 1986), and the frequency of intraspecific polymorphisms coded in cladistic analyses (Webster 2007a). These observations have been levied as support for a hypothesis that developmental processes were less buffered against internal and/or external sources of variation, and the resulting phenotypes thus less tightly canalized, in early trilobites (McNamara 1986; see also Hughes 1991Hughes , 1994 although alternative or complementary ecological explanations are equally plausible; (see Webster 2007a). However, quantitative morphometric data have been used to discriminate some olenelline species (e.g.…”

Strength in numbers: High phenotypic variance in early Cambrian trilobites and its evolutionary implications

“…This interval, however, does appear to be the acme of variation in the overall number of trunk segments (13) but, in the early Cambrian, aspects of head shape show markedly less morphological variety than at later times. (14) It has long been have argued that, while derived clades were dominated by peramorphic heterochrony, (15) early trilobite evolution was characterised by the dominance of paedomorphosis, (16) hinting at a difference in evolutionary mode among earlier trilobites and those clades that rose to dominance later, in the Ordovician and thereafter. Although the progressive development of trilobite segmentation invites heterochronic interpretation, recent studies have questioned whether such evolutionary transitions among trilobites are strictly heterochronic in origin.…”