UPRIGHT CRINOIDS OF THE THORNTON REEF, WENLOCK (SILURIAN) OF ILLINOIS, USA

The rare preservation of columns perpendicular to bedding may provide convincing evidence that ancient crinoids adopted an upright attitude during life. However, taphonomic and sedimentological analyses are imp ortant in determining whether such occurrences are truly in situ or re present unusual allochthonous accumulations. The Crinoid Biosome of th e Thornton Reef Complex, Silurian of Illinois, USA, includes pluricolu mnals preserved perpendicular to bedding, in association with more com mon specimens parallel or angled to bedding. Upright pluricolumnals ar e relatively shorter (<150 mm) than the longest non-upright specimens; they sometimes occur as imbricate accumulations and lack attachment s tructures (although these are preserved separately). Brachiopod valves and colonial corals may also be preserved perpendicular to bedding. T hese features suggest that the Thornton beds were formed as viscous ma ss flow deposits and that the upright crinoid columns are allochthonou s or, at best, parautochthonous. (C) 1996 by John Wiley & Sons, Ltd.