Heterochrony and Evolutionary Processes

Cambrian Trilobites





Historical Antecedants

Recognising Heterochrony

Modern examples: Sexual Dimorphism

Cambrian trilobites

Cope's Rule

K- and r- selection: Tertiary echinoids

Consequences for debates on adaptation, constraints and evolutionary dynamics



Peramorphosis is notably less common in Cambrian trilobites, but it does occur, notably in Redlichidia. Peramorphosis here is mostly limited to frontal lobe development and caused:

  1. An increase in the area occupied by the frontal lobe (lobe development is extended), which in turn causes
  2. an increase in the area occupied by the stomach, which
  3. allowed descendants to exploit a different ecological niche (in this case an increased sedimentation regime).

The crucial question for sorting out the mechanisms that drove Redlichid evolution, then, is whether a change in sedimentation regimes created a selection pressure that induced heterochronic changes or whether peramorphosis occurred for altogether different reasons, and Redlichids sought out different niches as a result.