Early Phanerozoic development of infaunal metazoans: trace fossil evidence from the Great Basin

Throughout the Phanerozoic a large percentage of marine organisms have lived infaunally. However, the history of Phanerozoic infaunal behavior has been the subject of controversy in recent years. Workers differ as to whether the depth and extent of bioturbation has been similar throughout the Phanerozoic or whether early Paleozoic bioturbation was much reduced in comparison with that of later times. In order to aid in resolution of this controversy, ichnofabric was examined from inner shelf carbonates of early to late Cambrian age in the Poleta, Bonanza King and Nopah Formations in the Great Basin sequence of California, Nevada and Utah. Sedimentary fabric was ranked as follows: O-no bioturbation, 1-discrete isolated traces, up to 10% of original bedding disturbed, 2-approximately 10 to 40% of original bedding disturbed, 3-last vestiges of bedding discernible, approximately 40 to 60% disturbed, 4-bedding completely disturbed but not mixed, 5-bedding nearly or totally homogenized. Data reveal relatively extensive bioturbation as early as the Atabanian. The most commonly represented sedimentary fabrics are types 2, 3 and 4. Depth of bioturbation was not observed to exceed 10 cm in any section and shows no significant trend through the Cambrian. Although these observations apply only to Great Basin strata, thismore » method can be used to compare ichnofabric in other Cambrian strata and indeed, strata throughout the Phanerozoic to further determine patterns of development of the infaunal biological benthic boundary layer.« less