RETRODEFORMATION OF CARBONIFEROUS TRACE FOSSILS FROM THE NARRAGANSETT BASIN, UNITED STATES, USING RAINDROP IMPRINTS AND BEDDING-CLEAVAGE INTERSECTION LINEATION AS STRAIN MARKERS

摘要

Retrodeformation of fossils can improve taxonomic and paleobiologic interpretations, but fossils that lack symmetry or other regular geometric properties are difficult to retrodeform unless strain markers are preserved within the fossil-bearing strata. Strain markers, however, are generally distributed unevenly, and their use can be problematic if strain varies among specimens and the markers do not occur in close association with the fossils. We use two approaches to retrodeform Carboniferous tetrapod tracks from the Narragansett Basin of southern New England: (1) a direct method in which the strain ratio (R-s) is obtained from deformed raindrop imprints preserved on the same specimens as the tracks and (2) an indirect method in which the range in orientation of the bedding-cleavage intersection lineation is used as a proxy for R-s. The latter follows from a strong correlation between the range in orientation of the bedding-cleavage intersection lineation and R-s values obtained from the deformed raindrop imprints, and allows the retrodeformation of tetrapod tracks not preserved in close association with deformed raindrop imprints. Our methodology was applied to both anamniote and amniote traces and was tested by retrodeforming originally bilaterally symmetric insect traces from the same location. In all cases, the trace fossils return to their expected morphology. This work shows that deformed raindrop imprints and bedding-cleavage intersection lineation are useful strain markers for the retrodeformation of trace fossils and expands our understanding of the fossil record of early tetrapods in southern New England.