6533b831fe1ef96bd1298d58

RESEARCH PRODUCT

L'ontogenèse complexe du spatangue Echinocardium cordatum: Un test des standards des trajectoires hétérochroniques

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description

The growth of the spatangoid echinoid Echinocardium cordatum is described by using the three standards (size,shape and age) that are classicaly involved in analyses of heterochronies. A population from Normandy, including all the growth stages, allows an ontogenetic reference to be established, while comparisons are performed with adult samples from other localities (Brittany, Aquitaine, and New-Zealand). The morphology of E. cordatum is characterized by 23 measured parameters and 13 calculated indexes. The ontogenetic trajectory of the species is depicted from the measurements of the size increments, from the analysis of the shape changes, and from an estimate of age. An estimate of the age-standard is deduced from the recognition of size-classes in relation to the time of sampling. The close equivalence of size-standard and total length of the test is discussed. Statistical analysis of ontogenetic trajectories are carried out for the 13 indexes. A multivariate analysis provides a synthetic picture of these trajectories and leads to define the shape-standard. These standards offer a frame to describe the succeeding events of the ontogeny of E. cordatum (shape changes, growth crisis) and to describe this ontogeny as the succession of four main stages. Comparisons with samples from other localities are performed: they show the originality of the population of New-Zealand and suggest that the species is polytypic. The palaeontological definition and use of heterochronic standards are discussed according to the described growth model.