INITIAL REPORT ON THE TRACE FOSSILS IN THE PENNSYLVANIAN-PERMIAN(?) CUTLER FORMATION, OURAY, COLORADO

Initial fieldwork in the Upper Pennsylvanian (to Lower Permian?) Cutler Formation at the type section near Ouray, Colorado, revealed several interesting associations of continental trace fossils in fluvial deposits. There were several types of large-diameter trace fossils. The first type were vertical cylindrical burrows ~15 cm in diameter and ~75 cm long that were subcircular in cross section. A discontinuous and variably thick (~0–2 cm) wall was present over most of the burrow length. The outside burrow wall (lining) was relatively smooth but contains discontinuous small ridges ~0.4 cm wide and 20–30 cm long that parallel the burrow length. The inner burrow wall (inside of lining) contains small grooves ~0.1–0.3 cm wide and ~0.5–2 cm long that were transverse to the length of the burrow. One of the burrows had a basal expansion at the base of the burrow. These cylindrical structures are interpreted as lungfish burrows that were used for estivation during periods when water was not present in channels and overbank ponds. The expanded base was part of the estivation chamber in which the lungfish settled into tailward. The second type of large-diameter burrows were subhorizontal (~10–20 degrees from horizontal), over 40 cm long, and strongly elliptical in cross section, ~10–15 cm wide and ~3–4 cm tall. Burrow walls contained longitudinal grooves ~0.2–0.4 cm wide and ~2–5 cm long. These cylindrical structures are interpreted as reptile burrows that were used as permanent dwellings. The third type were small diameter burrows (~0.2–0.5 cm) that occurred in associations among themselves and in all orientations. Internally, some appear to be backfilled with menisci; however, preservation of nearly all these burrow types did not allow their determination. These traces were abundant in interbedded sandstone and siltstones comprised of rippled and planar beds, and likely represent postdepositional bioturbation in subaerial conditions.