Tetrapod Footprint Biostratigraphy and Biochronology

Abstract

Tetrapod footprints have a fossil record in rocks of Devonian-Neogene age. Three principal factors limit their use in biostratigraphy and biochronology (palichnostratigraphy): invalid ichnotaxa based on extramorphological variants, slow apparent evolutionary turnover rates and facies restrictions. The ichnotaxonomy of tetrapod footprints has generally been oversplit, largely due to a failure to appreciate extramorphological variation. Thus, many tetrapod footprint ichnogenera and most ichnospecies are useless phantom taxa that confound biostratigraphic correlation and biochronological subdivision. Tracks rarely allow identification of a genus or species known from the body fossil record. Indeed, almost all tetrapod footprint ichnogenera are equivalent to a family or a higher taxon (order, superorder, etc.) based on body fossils. This means that ichnogenera necessarily have much longer temporal ranges and therefore slower apparent evolutionary turnover rates than do body fossil genera. Because of this, footprints cannot provide as refined a subdivision of geological time as do body fossils. The tetrapod footprint record is much more facies controlled than the tetrapod body fossil record. The relatively narrow facies window for track preservation, and the fact that tracks are almost never transported, redeposited or reworked, limits the facies that can be correlated with any track-based biostratigraphy.

A Devonian-Neogene global biochronology based on tetrapod footprints generally resolves geologic time about 20 to 50 percent as well as does the tetrapod body fossil record. The following globally recognizable time intervals can be based on the track record: (1) Late Devonian; (2) Mississippian; (3) Early-Middle Pennsylvanian; (4) Late Pennsylvanian; (5) Early Permian; (6) Late Permian; (7) Early-Middle Triassic; (8) late Middle Triassic; (9) Late Triassic; (10) Early Jurassic; (11) Middle-Late Jurassic; (12) Early Cretaceous; (13) Late Cretaceous; (14) Paleogene; (15) Neogene. Tetrapod footprints are most valuable in establishing biostratigraphic datum points, and this is their primary value to understanding the stratigraphic (temporal) dimension of tetrapod evolution.

Keywords:

• Biostratigraphy
• biochronology
• palichnostratigraphy
• tetrapod footprints
• Phanerozoic

ACKNOWLEDGEMENTS

I owe a heavy debt to Hartmut Haubold, Adrian Hunt and Martin Lockley for teaching me much about tetrapod footprints over the past decade, and to Jerry MacDonald for opening the world of Permian footprints to me. Yami Lucas helped with manuscript preparation. Emma Rainforth generously provided me with a copy of her (then) unpublished article on Otozoum. Susan Harris, Adrian Hunt, Joe Monks, Rich McCrea and Kate Zeigler made helpful comments on an earlier version of this article. John Foster and Martin Lockley provided constructive reviews. I dedicate this paper to the late John Ostrom, who many years ago, when I was a student, advised me not to study footprints too much lest I fall prey to overinterpreting them. I believe this article proves that I took at least part of his advice!