Heterochrony in the Silurian radiation of encrinurine trilobites

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Abstract
Divergence of Silurian encrinurine trilobite clades from common ancestry may be modelled as heterochronic pattern. Comparing ontogenies of ancestral late Ordovician Encrinuroides and the descendant punctatus and variolaris plexi of Silurian Encrinurus provides a test of this hypothesis. Heterochrony in the punctatus plexus is dissociated with respect to the entire organisms, but regionally global throughout cranidia (paedomorphic states) and pygidia (peramorphic states). Regulatory dissociation may explain the apomorphous enrollment strategy of this group. The variolaris plexus, for which a growth series of Balizoma dimitrovi (Perry and Chatterton) provides ontogenetic data, shows a complex of peramorphic cephalic character states. Changes in developmental timing thus provide a mechanism by which morphologic and ecologic divergence of closely-related clades was catalyzed in the Llandovery encrinurine radiation. Paedomorphic and dissociated lineages of the odontopleurid Leonaspis provide a comparable example of divergent heterochronic pattern associated with coexisting (in the same biofacies) species of comparatively recent common ancestry.
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