TAPHONOMY, CONTINUED ENVIRONMENTAL STRESS OR PROLONGED MASS EXTINCTION RECOVERY PROCESSES: MYSTERY OF THE EARLY TRIASSIC MARINE FOSSIL RECORD RESOLVED

Traditionally the body fossil record has served as the primary source of information on faunal trends through the Phanerozic. From such data paleobiologists have documented that the greatest biodiversity crisis suffered by eukaryotes occurred at the end of the Permian. Data from the marine body fossil record also shows that for 5-6 million years after this mass extinction, encompassing the entire Early Triassic, biodiversity continued to remain low. As attention has increasingly focused on the end-Permian mass extinction, a debate has developed on the nature of the Early Triassic fossil record. Does it reflect a primary or a secondary (taphonomic) signal? If the signal is primary, does it reflect continued environmental stress, or prolonged biotic recovery processes, after the end-Permian mass extinction?

Other aspects of the stratigraphic record, particularly biosedimentologic data, as well as evidence from stable isotopes, help provide resolution of this problem. Trace fossils represent in situ paleobiologic data, and studies show that bioturbation was reduced in a variety of ways throughout the Early Triassic. Sedimentologic features, including microbial carbonates and large seafloor calcium carbonate precipitates, occur through the Early Triassic, also indicating that environmental conditions which would cause biotic stress existed during this time. Significant fluctuations in the record of several stable isotope systems have also been documented through this interval.

Thus environmental stress, most likely of the kind which caused the end-Permian mass extinction, continued through the Early Triassic. Because much of the Early Triassic body fossil record is dominated by bivalves and gastropods, with a significant original aragonite shell composition, there is undoubtedly some taphonomic component affecting the quality of the fossil record for this interval. And analyses, to date, show that environmental stress varied globally through the Early Triassic, which led to differential biotic recovery in space and time during and after this time. Effectively, however, a variety of data demonstrates that continued environmental stress for eukaryotes during the Early Triassic was the main contributor to the development of the sparse fossil record exhibited by this 5-6 million year interval.