Body Fossil Vs. Ichnofossil: Oldhamia Recta Revisited
A slab from the Floyd Church Formation in Stanley County (North Carolina) shows a wrinkled microbial mat, meandering trails with levees, grape-like clusters, and abundant rod-shaped fossils, 0.7-2.7 mm wide and 1.7-20.5 mm long (n = 249). Fossils originally identified as Oldhamia recta lack: levees; internal structures indicating backfilling or fecal material; scratch marks (denoting a recumbent lifestyle); and evidence of peristaltic movement. Crossovers are not uncommon, indicating some rigidity to the rod-shaped fossils; these also show alignment, interpreted as a result of currents. Similarly, YPM specimens lack evidence favoring origins from undermat burrowers or mat grazers. YPM specimens also show preferred alignments, with rose diagrams having consistency ratios of 0.92-0.96. This ratio is lower in Floyd Church specimens (0.74), but a bimodal grouping is also consistent with one group flattened by currents and another group fragmented and transported.
Oldhamia was described as bunches of fine rills, radiating from joints of sympodial axis, representing a grazing pattern. In contrast, tubes of the Floyd Church Oldhamia recta seldom join at the base, differing from other species of the ichnogenus. We thus conclude that the evidence favors a body fossil origin for these specimens, behavioral inferences are unwarranted, and its body fossil identity requires more study.