Rosa's Rule

Rosa's Rule, also known as Rosa's law of progressive reduction of variability, is the tendency to go from character variation in more primitive representatives of a taxonomic group or clade to a fixed character state in more advanced members. An example of Rosa's rule is that the number of thoracic segments in adults (or holaspids) may vary in Cambrian trilobite species, while from the Ordovician the number of thorax segments is constant in entire genera, families, and even suborders.

It is named for Italian paleontologist Daniele Rosa.