Colorado & Utah, USA

Introduction

The Morrison formations of Colorado and Utah preserve Jurassic aged conifers, cycads, ferns. In the Henry Mountains of Utah collectors find conifers and cycads with great cell detail and fantastic coloration. The Yellow Cat area in Utah produces conifer casts with carnelian agate. Perhaps the most interesting gymnosperm from the Morrison deposits of Colorado and Utah is the gymnosperm Hermanophyton.

Hermanophyton represents the genus of an extinct plant stem found in Jurassic aged deposits of Colorado and Utah. It is the striking anatomy of this stem in cross-section that first attracted collectors to this rare fossil. The xylem or water conducting tissue of this stem is arranged into 9 to 15 large wedge-shaped segments.

This unusual arrangement of the xylem is not unlike some present-day lianes (vines); however, the diameter and straightness of this genus indicates they were not vine-like, but more columnar-like trees (Tidwell & Ash, 1990). Primary rays are found between the woody wedges of secondary xylem. At the leading edge of the secondary xylem vascular cambium produced xylem towards the center and phloem or food conducting tissue towards the outer edge. Faint growth rings can be seen in some specimens and are thought to be evidence of dry spells. Leaf traces can be found in the tissue surrounding the xylem cylinder. Leaf traces can also be found on the outer covering or ramentum on well-preserved specimens.

No roots, leaves, or reproductive structures of Hermanophyton have been identified. Some have speculated that this plant was a seed fern; however, with the physical evidence at hand we can only say that it is a gymnosperm stem of unknown affinity (Tidwell & Ash, 1990).

The occurrence of Hermanophyton species is well documented in the Salt Wash Member of the Morrison Formation near Hanson Creek Canyon in the Henry Mountains of Utah and in the Brushy Basin Member of the Morrison Formation near Cortez, Colorado. In the Salt Wash Member sandstones, conglomerates, and mudstones indicate terrestrial environments with fluviatile (river) and lacustrine (lake) deposits. The sandstones, conglomerates, and mudstones of the Brushy Basin Formation are interpreted as evidence for overbank deposits and meandering rivers. Radiometric dating and microfossil evidence suggest that these upper Jurassic aged deposits are 150 million years old (Selden & Nudds, 2004).

Hermanophyton is found in sandstone. Cycadeoid trunks, conifer wood, invertebrates, and dinosaur bones are also found in these Morrison rock units. In fact, the Morrison Formation of Colorado, Wyoming, and Utah have produced some of the best dinosaur specimens know, such as, large sauropods, stegosaurs, and theropods. Most dinosaur bone beds in the Morrison represent non-catastrophic death assemblages, see Selden and Nudds for a good discussion on catastrophic vs. non-catastrophic death assemblages. Insect galleries have also been identified in some specimens of Hermanophyton (Tidwell & Ash, 1990).

With scientific evidence we can imagine the Morrison Basin 150 million years ago. Herds of herbivorous dinosaurs roamed the planes in search for food in the vegetation that surrounded lakes and rivers. The vegetation that surrounded these areas contained cycadeoids, conifers, ferns, and our described Hermanophyton. These ecosystems supported fish, insects, amphibians, reptiles (such as crocodiles and pterosaurs), and small mammals. The herds of dinosaurs supported carnivorous dinosaurs, like Allasaurus. The rock deposits also speak of environments subjected to repeated episodes of drought and flood. Droughts followed by floods formed the lake and river deposits that act as windows into the past.

Back to Jurassic; Colorado & Utah, USA

Bibliography

Daniels, F.J. 1998. Petrified Wood: The World of Fossilized Wood, Cones, Ferns, and Cycads. Western Colorado Publishing Company: USA.

Daniels, F.J. and Dayvault, R.D. 2006. Ancient Forests: A Closer Look at Fossil Wood. Western Colorado Publishing Company: Canada.

Dernbach, U. and Tidwell, W.D. 2002. Secrets of Petrified Plants: Fascination from Millions of Years. D’ORO: Germany.

Selden, P. and Nudds, J. 2004. Evolution of Fossil Ecosystems. The University of Chicago Press: Chicago, pg 88-98.

Tidwell, W.D. and Ash, S.R. 1990. On the Upper Jurassic Stem Hermanophyton and its Species from Colorado and Utah, USA. Palaeontographica 218, 77-92.

Tidwell, W.D. 1998. Common Fossil Plants of Western North America. [2nd Edition]. Smithsonian Institution Press: Washington, pgs 214-215.