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


Studies on Cambrian trilobites (esp. McNamara [1986]) and other Cambrian groups show a widespread occurance of heterochrony in both "directions" (i.e. paedomorphosis and peramorphosis). Heterochrony occurs as a result of mutations affecting regulatory as opposed to structural genes. The high incidence of heterochrony, then, resulted from the fact that control of developmental timing and other gene regulation functions was poorly established before the Ordovician: Cambrian groups show a high degree of morphological plasticity and very fast rates of evolution.


Paedomorphosis in Olenellus Armatus - glabellar development used as metric for determining developmental stage. Scale line segment is 1mm. Note that size decreases along the lineage and the striking resemblance of the adult O. armatus to the embronic O. lapworthi .

By correlating successively more paedomorphic Olenellidae trilobites with shallower and warmer environments, McNamara (1986) concluded that changes in environmental conditions directly triggered early onset of maturation in this group of Cambrian trilobites. Many invertebrates can have precocious maturation induced by changing ambient temperature (e.g. nautiloids). Selection does not seem to play a dominant role in driving evolution here. Instead, changing environments induced early maturation (paedomorphosis), which in turn caused the profound morphological changes observed in this lineage.