APPLICATION OF CLADISTIC ANALYSIS TOWARDS RESOLVING BIOSTRATIGRAPHIC UNCERTAINTY: AN EXAMPLE FROM THE UPPER TRIASSIC

A common belief among paleontologists and biostratigraphers of the past century was that morphological complexity increased over time, leading to the rational that plesiomorphic taxa came from older strata. A prime example of the application of this generality toward resolving chronostratigraphic uncertainty is the use of continental vertebrates (phytosaurs, aetosaurs, rauisuchians, and metoposaurs) to provide time control for Upper Triassic strata globally. Most of these studies utilized genera rather than species as the lowest taxonomic unit and were therefore of limited resolution to begin with. Cladistic analysis of two taxa, Rauisuchia and Parasuchia (phytosaurs), using new morphological observations, indicates that the utility of their component genera as chronological indicators has been overstated. Until now, the monophyly of key index taxa within Parasuchia has not been rigorously tested. Features previously considered diagnostic of Paleorhinus (e.g., tubular rostrum, post-temporal arcade not ventrally deflected, homodont dentition) are all plesiomorphic for Parasuchia and therefore offer no taxonomic or systematic resolution. At least two taxa of basal parasuchians ("Paleorhinus" scurriensis and Francosuchus) have been lumped with Paleorhinus, but were recovered as distinct taxa in this analysis. The purported occurrence of Paleorhinus in marine strata in Austria is based upon a non-diagnostic specimen of basal parasuchian, rendering the use of Paleorhinus as an indicator of late Carnian age dubious. Other traditionally recognized parasuchian genera are in need of taxonomic revision, necessitating a re-evaluation of their chronological significance. Rauisuchians have a notoriously incomplete fossil record, with an abundance of ghost taxa, but a new phylogenetic analysis indicates that derived rauisuchians similar to Postosuchus (a supposed index taxon for the later Carnian and early Norian) existed as early as the Anisian. In both examples, rigorous cladistic analysis of specimens has recovered phylogenetic trees that offer better resolution for the evolutionary history of each clade, while clarifying the taxonomic and biostratigraphic utility of component taxa.