Techniques

Abstract

Databases used for the analysis of past biotas should be as internally consistent as possible taking into account the incompleteness of the fossil record and the taxonomic distortions due to the history of their knowledge. A comprehensive and critically updated database of Southern Hemisphere bivalve occurrences through the Triassic and Jurassic was built. Most of paleobiogeographic analyses were performed within time slices to obtain comparable results in a time succession. Analytical methods were used for both (a) the analysis of latitudinal ranges along the South American paleo-coasts, and (b) the recognition of paleobiogeographic units for the Southern Hemisphere. a) The first approach to the study of species latitudinal ranges was cluster analysis, but this method, although useful, imposes a hierarchical structure on the data. Thus, to check for faunal changes along latitude, the distribution limits of species were explored using a technique similar to that considered for origination/extinction analysis, substituting first and last appearances by northernmost and southernmost geographical occurrences. Generalized linear models were used to look for changes on the proportional values of different species categories related to systematic and paleobiogeographic kinships. b) For the recognition of biochoremas, the incomplete and uneven nature of the data precludes the application of methods which may group areas according to the common absence of data, and we followed a traditional approach based on endemicity. In order to check the biogeographic structures without assuming a hierarchical or gradational arrangement, a Bootstrapped Spanning Network was calculated.