Rocky Mountain low: A non-linear Neuroptida (Insecta) longitude-landform age relationship in North America

Neuroptida, particularly Raphidiioptera and Megaloptera are Mesozoic or earlier lineages with 400 species detected north of Mexico. We hypothesized that selected and overall Neuroptida taxon richness is related to geologic province age across North America. We used Penny et al.’s (1997) data on Neuroptida distribution among U.S. states and Canadian provinces to model Raphidioptera, Megaloptera, overall Neuroptera, and Hemerobiidae, Chrysopidae, and Myrmeleontidae species richness in geologic provinces using GIS range analyses. The age of North American geologic provinces is generally related to longitude, with the Appalachian and Ozark Mountains the oldest terrestrial regions at 300 my, the Rocky Mountains 80 my in age, and the Sierra Nevada Range 20 my in age. Contrary to our expectations that Neuroptida species richness also should be related to longitude, Raphidioptera and Megaloptera species richness is greater in the Appalachians, Ozarks, and along the Pacific coast, and are anomalously depauperate in the Rocky Mountains and in the Interior Plains (P = 0.028). This pattern is consistent across the latitudinal species richness gradient, across the longitudinal aridity gradient, across elevation variation, and despite differences in sampling intensity among states and provinces. The depauperate Rocky Mountains and Interior Plains species richness may be related to slow colonization since those landscapes emerged from the Cretaceous seaway. The greater-than-expected richness of all Neuroptida along the West Coast suggests a far longer terrestrial history for the Pacific coast, or rafting of taxa on accreted terrains. Range data of other evolutionarily ancient taxa (e.g., Odonata, Amphibia) corroborate these patterns.