Developmental variations in neodevonian phacopide trilobites.

Heterochrony refers to changes in the timing and the rate of development of ancestral morphological features. This phenomenon has been observed in some neodevonian phacopide trilobites. Their evolution is marked by progressive displacement of the visual complex towards the external margins, accompanied by displacement of the facial suture. This evolutionary pathway is a result of paedomorphosis. These features seem to be adaptative, and may occur in all lineages, leading to convergences between several taxa. A paedomorphic phenomenon also concerns the change of moulting behaviour. This particular feature may not be related to environment but represents a feature inherited from the ancestral forms. (C) 2003 Academie des sciences. Publie par Editions scientifiques et medicales Elsevier SAS. Tous droits reserves.