Raindrop Imprints and Their Use in the Retrodeformation of Carboniferous Trace Fossils

The Narragansett Basin of Massachusetts and Rhode Island is a Carboniferous terrestrial basin and is the only location within New England that preserves fossil evidence of early amphibians and reptilomorphs. Footprints and trackways constitute the only record of these early tetrapods, but deformation during the Alleghanian orogeny has distorted the traces. To improve identification, description, and comparison with non-distorted material, tracks from Plainville, Massachusetts were retrodeformed using raindrop imprints as strain markers. The axial ratios of 354 raindrop imprints found on 12 slabs were analyzed using the Rf/Φ technique. Strain ratios for slabs ranged from 1.33 to 1.93, with individual raindrop imprints ranging from 1.14 to 2.51. Although weakly expressed, cleavage exhibits an anastomosing characteristic and is visible on bedding surfaces in the form of a bedding-cleavage intersection lineation. The fluctuation of the bedding-cleavage intersection lineation varies between 10° and 33° among the raindrop-imprint—bearing slabs. This fluctuation is strongly correlated to the strain ratio of a slab, and this correlation was used to obtain strain ratios for slabs lacking raindrop imprints. Vertebrate tracks appeared less distorted following retrodeformation, and the accuracy of the methods was successfully tested by retrodeforming insect traces that were originally bilaterally symmetrical. Deformed raindrop imprints and bedding-cleavage intersection lineation provide a useful new way of retrodeforming fossils, as well as a more complete understanding of the fossil record of early tetrapods in New England.