PALEONTOLOGICAL ASSESSMENT OF THE MISSISSIPPIAN (CHESTERIAN) LOWER REST SPRING SHALE, COTTONWOOD MOUNTAINS, DEATH VALLEY NATIONAL PARK

Mid-Carboniferous formations in the Cottonwood Mountains of Death Valley National Park include the local succession of the Mississippian Perdido Formation, the overlying Rest Spring Shale, and the Pennsylvanian Tihvipah Limestone. Although most previous regional, paleontological, and stratigraphic studies, including the Death Valley National Park Paleontological Survey (Nyborg and Santucci, 1999), refer to the Rest Spring Shale as characteristically unfossiliferous, several pertinent taxonomically restrictive studies have commented primarily on components of its ammonoid fauna (Titus, 2000; Gordon, 1964). We found that a diverse and abundant, well-preserved fossil invertebrate and plant assemblage occurs in the lower Rest Spring Shale at the type section in the Cottonwood Mountains. Most of the fossils appear to be associated with an apparently early diagenetic concretionary unit near the base of the formation.

The paleobiodiversity of the megafossils is fairly high in the lower Rest Spring, including over 22 species of megafauna/flora. The fauna is predominantly molluscan: including goniatite ammonoids; the nautiloid Rayonnoceras; several small orthoconic cephalopods (e.g. Mitorthoceras? and possible bactritoids); at least three bivalve taxa; and the archeogastropod Glabrocingulum.

Additionally, at least two taxa of hapsiphyllid solitary rugosans; two strophomenid and one spiriferid brachiopod; two bryozoan taxa; including a reticulate fenestellid; a couple of echinoid plates; and abundant crinoid ossicles give the fauna a typical Carboniferous "flavor". Plant fragments include both leaves and wood. Overall the preservation of the fauna appears exceptional, and is reminiscent of other faunas recovered from the Antler Foreland Basin "Chainman Shale" exposed in central Nevada and western Utah.

Two groups of goniatite ammonoids dominate the fauna, comprising more than half of all fossils observed. One group (almost one of every two ammonoids observed) includes taxonomically indeterminate, taphonomically flattened and broken discs, probably smashed during post depositional, pre-lithification of the shale. A second group consisting of several taxa of well preserved, uncrushed goniatites is also present, probably preserved through early diagenetic cementation associated with the concretions.