PALEOECOLOGICAL RECONSTRUCTION OF CYLINDRICHNUS CONCENTRICUS IN THE CRETACEOUS WESTERN INTERIOR SEAWAY OF THE BOOK CLIFFS, UTAH

Cylindrichnus concentricus was named as a long, tapering, cylindrical shaft surrounded by a concentrically laminated muddy lining. Although it has been reported by many authors, there seems to be no common understanding of its formative process, namely, the ethology of the trace maker. In the type locality in the Upper Cretaceous Book Cliffs of Utah, C. concentricus are elongated structures more than 70 cm in maximum length and 1.5 cm in maximum diameter. Specimens are oriented subvertical at the top, and they show a bow-like bend in a vertical plane. The sand-filled cylindrical shaft is concentrically laminated with a slightly muddy lining. This lining consists of a tightly-packed array of megaphone-shaped laminae, and it becomes thinner toward the upper and lower ends. The overall shape is like an elongate, curving spindle. Branching is seen in some specimens. Numerous lengthwise striations often are seen on the outer surface of the lining.

The basic components of this trace fossil (its cylindrical shaft and concentrically laminated lining) closely resemble those of Rosselia socialis (commonly interpreted as the dwelling tube of a terebellid annelid), although their overall shapes are significantly different. This suggests that C. concentricus also may have formed through successive lining of muddy materials from inside of the shaft, like R. socialis. The striations on the outer surface of C. concentricus, which have been interpreted by some workers as bioglyphs of a crustacean, are probably just impressions of microfaults formed through the lengthwise splitting of the lining during its expansion.

C. concentricus appears similar to stacked occurrences of R. socialis, a morphotype that resulted from the trace maker's response to sedimentation, but the former differs from the latter in its detailed morphology and laminal arrangement. Thus, C. concentricus probably is not simply an ecophenotypic variant of R. socialis.