Article: Morphologic patterns of diversification: examples from trilobites
The morphologic diversification of the Trilobita is investigated using a Fourier description of the cranidia of Cambrian and Ordovician trilobites from North America. Morphologic diversity increases from the Early Cambrian to the Middle Ordovician, but does not correlate well with patterns of generic or family diversity. Suprageneric taxa of trilobites are shown objectively to represent morphotypes. Morpholog dispersion among suprageneric taxa and the distinctness of these taxa both increase from the Cambrian to the Ordovician. This result agrees with patterns based on hypostomal morphology (Whittington 1988a, 1988b) and therefore is not an artifact of using cranidial morphology. These patterns are caused by the originatic of new higher taxa, not evolution within established higher taxa. Higher taxa tend to retain the same morphology once established, rather than diverging gradually. In this respect, higher taxa may be said to have sudden origins. The origination of higher taxa may be linked to the opening of new adaptive zones, particular in the Early Ordovician, following widespread extinctions of trilobites.