LAND-VERTEBRATE FAUNACHRONS, GIS AND CLADOTAXONOMY: TOWARDS A GLOBAL TRIASSIC TETRAPOD BIOCHRONOLOGY

The boundaries of land-vertebrate faunachrons (lvfs) are defined on the first appearances of key index taxa that form the basis of a global Triassic tetrapod biochronology: FAD Lystrosaurus – Lootsbergian (Induan); FAD Cynognathus – Nonesian (Olenekian); FAD Eocyclotosaurus – Perovkan (Anisian-Ladinian); FAD Mastodonsaurus – Berdyankian (Ladinian-Carnian); FAD Parasuchus – Otischalkian (Carnian); FAD Rutiodon – Adamanian (Carnian); FAD Typothorax coccinarum – Revueltian (Norian); and FAD Redondasaurus – Apachean (Norian-Rhaetian). Recent attempts to test the utility of the lvf concept using GIS have raised concerns about the taxonomy of index taxa and ability to correlate lvfs globally, specifically within the Middle and Late Triassic. However, GIS is an insufficient tool to assess biostratigraphic utility; it is a way of organizing spatial, two-dimensional data, not a means to test data arranged in a temporal dimension. The taxonomy used in the lvf methodology is not cladotaxonomic, so it does not use hypothetical ghost lineages to constrain biostratigraphic correlation. Additional concerns proposed by some critics include the taxonomic “lumping” vs. “splitting” of key taxa, such as the parasuchid (phytosaur) Pseudopalatus (an index taxon of the Revueltian lvf) being distinct from the superposed Redondasaurus (its first appearance defines the beginning of the Apachean lvf) and the stagonolepid (aetosaur) T. coccinarum (its first appearance defines the beginning of the Revueltian lvf) being distinct from the superposed Redondasuchus (an index taxon of the Apachean lvf). Regardless of these taxonomic disagreements, the lvfs are based on temporally-successive first appearances of key taxa, and are independently substantiated by superposed, temporally-successive tetrapod assemblages and lithostratigraphic correlations.. Thus, disagreements about taxonomy or correlation of specific tetrapod assemblages do not invalidate the overall biochronologic framework of the lvfs. The Triassic lvfs remain a strong biochronologic framework and are robust tools for global biostratigraphic correlation of Triassic tetrapod assemblages.