NEAREST-NEIGHBOR ANALYSIS OF TRILOBITE MORPHOSPACE

Previous observations indicate that morphologic distinctness of suprageneric taxa (superfamilies, suborders, and orders) of trilobites increased during their Cambro-Ordovician diversification. Analysis of nearest-neighbor distances (NNDs) among trilobites in morphospace is used to test the robustness of this pattern while avoiding the overprint of higher-level taxonomy. The properties of the uniform distribution are used to determine the bounds of the morphospace, and computer simulation of random points is used to circumvent edge effects in the analysis of NNDs. Observed NNDs became significantly smaller relative to expected distances, supporting the hypothesis that morphospace became more discontinuously occupied from the Cambrian to the Ordovician. However, absolute NNDs (not considered relative to expected NNDs) actually increased. Therefore, the increase in clustering intensity reflects the expansion of morphospace that accompanied the diversification, because the expected NND is proportional to the morphologic range. The increases in clustering and in absolute NND hold even if Cambrian genera are treated as equivalent to Ordovician species. © 1990 Society of Systematic Zoology.