Re-evaluating Late Triassic Global Biostratigraphic Correlations

Irmis, R. B., Martz, J. W., Parker, W. G., and S. J. Nesbitt. 2010. Re-evaluating the correlation between Late Triassic terrestrial vertebrate biostratigraphy and the GSSP-defined marine stages. Albertiana 38:40-52.


A series of four land-vertebrate faunachrons have been proposed for biostratigraphical correlation of Upper Triassic non-marine strata, particularly in western North America but purportedly of global utility (Lucas, 1998). As these faunachrons are supposed to represent units of time it is necessary to tie them to the various stages of the Late Triassic, which are defined by marine fossils. Fortunately metoposaurs, phytosaurs, and aetosaurs have been found in both marine and non-marine strata and these have been used to provide ties to the marine scale (Lucas and Heckert, 2000).  To aid in this attempt, these often fragmentary specimens have been identified to the genus level and synonymy of numerous taxa have been proposed to "smooth" global correlations (e.g., Lucas, 1998; Lucas and Heckert, 2000).  Furthermore, first appearances of index taxa have been assumed to be synchronous across Pangea, allowing for hypothetical timing of major events in the Late Triassic such as the first appearance of groups such as the dinosaurs (Heckert and Lucas, 1999). 

In this paper we critically examine these taxonomic assignments and conclude that they, and thus the correlations they provide, are unsupported.  Furthermore, we argue that assuming the synchronous appearance of taxa globally masks potentially diachronous patterns of vertebrate dispersal.  This has already been supported by comparison of dates from the Ischigualasto (Argentina) and Chinle (western North America)Formations, which biostratigraphy considered to be time equivalents, but may be separated by as much as 10 million years (Furin et al., 2006; Irmis and Mundil, 2008).   Finally we argue that based on this detailed biostratigraphic frameworks must be constructed for particular regions and then correlated using independent methods such as radioisotopic dates. 

 To quote from the conclusions section "it is clear to us that the veracity of vertebrate biochronology as a means of correlating Upper Triassic strata has reached the present limits of its resolution, and has been compromised by controversial taxonomic practices and circular reasoning....To advance Late Triassic vertebrate biochronology, which has assumed an importance in chronostratigraphic correlation far in excess of its actual substance, must yield to other methods for understanding of Late Triassic vertebrate faunal change".

Albertiana is the official newsletter of the Subcommision on Triassic Stratigraphy and is published once a year.  More information and past issues can be downloaded from here.  Unfortunately, the current issue is not available on-line yet, but a PDF of this article can be obtained by e-mailing me or Randall Irmis.  Be advised that if we get many requests it may take a bit to respond to them all so please be patient.

The full abstract for the article is below:

Abstract - One of the main methods for correlating Late Triassic terrestrial strata is through the use of land-vertebrate faunachrons (LVFs). Use of LVFs is widespread because of their supposed global application and ability to be correlated with the marine stages of the timescale. New magnetostratigraphic and radioisotopic data indicate that the traditional correlation of Late Triassic LVFs requires revision, although some authors maintain that these original correlations are sound, and that the new correlations of the marine stages to the numerical timescale are in error. Here, we examine the available evidence for cross-correlation of Late Triassic LVFs with the marine stages and numerical timescale. We conclude that the biostratigraphic links between the LVFs and marine stages are not robust; they are based on nondiagnostic specimens and/or taxonomically controversial specimens, endemic taxa, and/or ambiguously correlated assemblage zones. Given the available data, new correlations of the LVFs, marine stages, and the numerical timescale using magnetostratigraphy and radioisotopic ages that support a “long Norian” are preferential to those using largely vertebrate biostratigraphy that support a “long Tuvalian.” We also outline a framework for improving the accuracy and relevance of Late Triassic vertebrate biostratigraphy going forward in the near future.

REFERENCES

Furin, S., Preto, N., Rigo, M., Roghi, G., Gianolla, P., Crowley, J. L., and S. A. Bowring. 2006. High-precision U-Pb zircon age from the Triassic of Italy: implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology 34: 1009-1012.

Heckert, A. B., and S. G. Lucas. 1999. Global correlation and chronology of Triassic theropods (Archosauria: Dinosauria). Albertiana 23:22-35. [Note: this article is identical to the one published by the same authors in the Gaia volume in 2000].

Irmis, R. and Mundil, R. 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. Journal of Vertebrate Paleontology, 28 (3 Supplement): 95A.

Lucas, S. G. 1998. Global Triassic tetrapod biostratigraphy and biochronology. Palaeogeogeography, Palaeoclimatology, Palaeoecology143:345-382.

Lucas, S. G. and A. B. Heckert. 2000. Biochronological significance of Triassic nonmarine tetrapod records from marine strata. Albertiana 24: 30-36.