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


Oryctocephalid trilobites also show paedomorphic heterochrony. Here the paedomorhocline results in a decrease in the number of thoracic segments; an increase in the size of the pygidium; a decrease in spinosity; a decrease in the width of free cheeks and a lateral displacement of the eye lobe. Selection targeting the onset of maturity or the rate of development (perhaps as a result of a less stable environmental regime?) causes a number of non-adaptive, non-selective morphological consequences.


From McNamara (1986).