FIRST REPORT OF LARGE-DIAMETER BURROWS FROM THE MORRISON FORMATION AT SKYLINE DRIVE, CAñON CITY, COLORADO

Large diameter burrows are reported for the first time from outcrops along Skyline Drive in Cañon City, Colorado, immediately below the Morrison Formation-Dakota Group contact. Three burrow morphotypes are found in the Brushy Basin Member of the Morrison Formation, whose depositional environments has been interpreted as fluvial and lacustrine. These burrows are 1) dominantly vertical shafts, Camborgyma; 2) horizontal burrows with terminal chambers, cf. Katarrhedrites; and 3) vertical, helical burrows, cf. Daimonelix. Burrows are found in two mudstone beds bounded by sandstone beds. The matrix around most of the burrow is mudstone of varying colors that lack slickensides; burrow apertures are often found in the overlying fine-grained sandstone beds. Burrow infill varies, most often filling with the surrounding mudstone, and occasionally the overlying sandstone. The first, and most common, morphotype is vertical to oblique cylindrical shafts (5–90 cm long) that may or may not terminate with a (≤ 8 cm diameter) chamber or a short (≤ 10 cm) obliquely horizontal tunnel. Many burrow walls show longitudinal striations and convex round to ovoid marks. Transverse striations were not apparent; this may be due to the platy nature of the muddy infill and weathering. Some of the burrows originate from conical apertures, whereas others do not. These burrows are consistent with Camborygma isp., previously interpreted as burrows of crayfish, and have been found in the Morrison Formation of Utah. The second most common morphotype begins with a short vertical to oblique shaft leading to a horizontal tunnel and terminal chamber. These burrows are significantly larger than the first morphotype, with shaft diameters ≤ 15.5 cm, tunnel lengths up to 1.5 m, and terminal chambers reaching 37 cm in diameter. This burrow morphotype is consistent with Katarrhedrites isp. The third burrow morphotype, the rarest, consists of complex vertical helical architecture for the upper portion of the burrow. These burrows are smaller than those reported from the Morrison in Utah, with shaft diameters beginning at 18 cm and reducing down shaft to 7 cm. The burrow helical axis trends vertically and the burrow depth reaches ~1.3 m. The complexity and morphology of this morphotype is similar to mammal or therapsid burrows assigned to cf. Daimonelix isp.