POST PERMO-TRIASSIC TERRESTRIAL VERTEBRATE RECOVERY: SOUTHWESTERN UNITED STATES

Recovery of marine biodiversity following the Permo-Triassic extinction was delayed as much as a full order of magnitude (ca. 10 myr) longer than other mass extinctions. Terrestrial vertebrate biodiversity is thought to have taken ~20 myr longer. The present study tests, at the scale of an individual fossil community, whether a disparity in biodiversity existed in the American Southwest, between the Moenkopi Formation, containing an early Middle Triassic (Anisian) terrestrial vertebrate fauna, and the Chinle Formation, containing a successor Late Triassic (Norian) vertebrate fauna. Taking Chinle faunal biodiversity to represent full biotic recovery, comparison of taxonomic and guild diversity of faunas from similar depositional and taphonomic environments in these two formations allowed us to assess the possibility of incipient terrestrial recovery of biodiversity in the Anisian.

Comparisons were made between the Holbrook Member fauna of the Moenkopi, a unit best characterized as a low-sinuosity medium- to coarse-grained fluvial deposit, and each of four Chinle stratigraphic units, representing fluvial settings from sandy low-sinuosity to muddy high-sinuosity. Three metrics were applied: generic and familial taxonomic diversity and guild diversity, these were compared by rarefaction. Units of extraordinary preservation in the Chinle – the so-called “blue layers” – were removed from the analysis. Generic-level and guild rarefaction show similar trends, with two biostratigraphically younger Chinle faunas appearing slightly less diverse than two older Chinle faunas and the Holbrook Member Moenkopi fauna. At the familial-level the same trend is seen among the Chinle faunas, but the Holbrook fauna plots nearer to the younger Chinle faunas. In all tests the Holbrook Member fauna is within the variation seen in Chinle faunas.

The results of our study contradict those of earlier studies which claim that biodiversity in Triassic communities was not fully recovered for 15-30 million years after the end of the Permian. Instead, our analysis suggests that by the early Anisian (6 Ma after the P-T extinction) biodiversity had reached levels comparable to those seen in the Late Triassic. This brings terrestrial vertebrate recovery more in line with the 4-10 million years it took for recovery in the marine realm.