Abstract
Heterochronic development has been proposed to have played an important role in the evolution of echinoderms. In the class Ophiuroidea, paedomorphosis (retention of juvenile characters into adulthood) has been documented in the families Ophiuridae and Ophiolepididae but not been investigated on a broader taxonomic scale. Historical errors, confusing juvenile stages with paedomorphic species, show the difficulties in correctly identifying the effects of heterochrony on development and evolution. This study presents a detailed analysis of 40 species with morphologies showing various degrees of juvenile appearance in late ontogeny. They are compared to a range of early ontogenetic stages from paedomorphic and non-paedomorphic species. Both quantitative and qualitative measurements are taken and analysed. The results suggest that strongly paedomorphic species are usually larger than other species at comparable developmental stage. The findings support recent notions of polyphyletic origin of the families Ophiuridae and Ophiolepididae. The importance of paedomorphosis and its correct recognition for the practice of taxonomy and phylogeny are emphasized.
Highlights
A link between individual development and evolution has been discussed many times since Haeckel's [1] seminal work [2,3,4,5,6,7]
The morphology of the small postlarvae of Ophiura sarsii, Amphipholis squamata and Ophiomusium lymani concurs with previously published observations [51,41]
Stage, the disc is formed by six primary plates, no radial shields are evident, the arms are formed by joined pairs of lateral plates, lacking dorsal and ventral plates and they bear two spines (Fig 3A–3C)
Summary
A link between individual development (ontogeny) and evolution has been discussed many times since Haeckel's [1] seminal work [2,3,4,5,6,7]. Heterochrony has been widely considered as a basic process which is responsible for the evolutionary modifications of ontogeny [11,12] and as a key term to place embryology in a comparative phylogenetic framework [13,14]. Any consideration of heterochrony is basically absent or insignificant [21], as an intensive search through published literature showed (Table 1). This clearly contradicts the widespread claims that ontogeny is an integral part of evolutionary studies and phylogenetics and taxonomy. We present an extensive example of heterochronic changes via paedomorphosis among brittle stars and directly link this phenomenon to taxonomic practice
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