Short shoots from the Upper Jurassic Morrison Formation, Utah, Wyoming, and Colorado, USA

Abstract

Casts and anatomically preserved specimens of short or spur shoots occur in the Upper Jurassic Morrison Formation at several localities in Utah, Wyoming and Colorado, USA. Most of these shoots are assigned to Behuninia, which is emended to accept them. Some are also placed in the proposed new genus Steinerocaulis. Many of these structures were originally reported as fruiting structures of cycadophytes, but are here demonstrated to be short shoots probably of extinct conifers. The eroded short shoots are the remains of either external surfaces or remnants of internal structures, usually the secondary xylem and pith with only rare specimens retaining their cortices. The fossils occur either detached or attached to an axis in a spiral to subspiral, opposite to subopposite or whorled to subwhorled manner. One new combination (B. provoensis) and two new species of Behuninia (B. bassii and B. scotti) and one new combination for Steineroaulis (S. radiatus) are proposed. They are characterized by morphological rather than anatomical features in which they are all essentially the same. Based upon the morphology of leaf scars on these shoots and the fact that large-leaved forms of conifers lack short shoots, it is proposed these shoots bore needle-like leaves. Possible paleoecological conditions under which these shoots and associated fossil plants lived are also discussed.