Bennettitaleans (cycadeoids) of the Early Jurassic Navajo Sandstone

The Pliensbachian to Toarcian Navajo Sandstone represents one of the largest eolian units in the geologic record, cropping out over an area of 366,000 square kilometers. Eolian deposition was interrupted at two stratigraphic levels by widespread freshwater interdune lacustrine deposition along the eastern edge of the Navajo sand sea. Previously described paleobotanical remains are limited to Equisetum and a conifer of uncertain affinities identified as Araucariaceae or Xenoxylon. Several localities near Moab, Utah preserve a greater diversity of paleobotanical remains, dominated by multiple specimens of bennettitalean seed plants.

The extinct gymnosperm group, Bennettitales (cycadeoids) is mostly commonly represented in the Moab, Utah area by the genus Cycadeoidea, found as large flattened chert disks (up to 1.5 meters diameter). The fossils are restricted to a widespread shale bed. The flattened disks are stems resembling the ‘crow's nest' or ‘beehive' structures common to cycadeoids. These form by collapse of the pith during decay. The chert disks have upper surfaces with cusp-shaped ledges, several centimeters across that represent leaf scars pushed outward after the pith collapsed. Some disks have a series of triangular indentions circling the margin of the chert disk that represent cone attachment scars. The chert disks have either sand filled cores, or have a narrow ring of manoxylic wood composed of scalariform tracheids, a simple type of gymnosperm wood found in cycadeoids.

These cycadeoid fossils are found mostly in ‘dry' interdune deposits, (i.e., first order bounding surfaces within eolian sands), with rarer occurrences along the distal margins of interdune lacustrine carbonates, suggesting growth under conditions of low soil moisture. Lacustrine carbonates are dominated by taxodiaceous conifers, suggesting ecological succession by more mesic plant species as soil moisture levels increased.