AN UNUSUAL TAPHONOMIC SCENARIO FOR THE BUCK MOUNTAIN (PERMIAN) CEPHALOPOD FAUNA IN NEVADA

The Buck Mountain Lagerstätte in Nevada (Artinskian - Arcturus Fm.) has an unusual taphonomic history. Cephalopods (n=262) from in situ carbonate concretions dominate the fauna (ammonoids: 82% prolecanitids, 16 % goniatites; 1% orthoconic and coiled nautiloids), with other fauna constituting fractions of a percent each.

In some concretions body chamber fragments are common, in others phragmocones dominate and some are barren. Many phragmocones (18%) exhibit pre-burial shell damage; some (~2 %) bear round and/or oval punctures. Forty-two percent of the ammonoid phragmocones retain partly complete body chambers and 53 % are without body chambers, while 5 % of the specimens are body chamber fragments. Many (> 50%) prolecanitids contain phosphatized membranes and/or siphuncular tissues. A few prolecanitid body chambers (n=7) contain mandibles. Numbers of cephalopod mandibles exceed the number of phragmocones by nearly fourfold, meaning that if all of these mandibles belong to the ammonoids, about half of the animals are missing.

The Buck Mountain Lagerstätte taphonomy is atypical of most Paleozoic cephalopod occurrences because: 1) the lack of benthic faunal diversity and phosphate deposition suggests a highly stressed and low oxygen bottom conditions. 2) Extreme shell damage indicates many specimens could not have floated into the locality postmortem. 3) Punctures in phragmocones, missing body chambers, and partly missing phragmocones suggest predation as a cause of damage. 4) Phosphate coatings of membranes and siphuncular tissues occurred very quickly after death. 5) The relative proportions of mandibles to phragmocones suggest that more than 50% of the shells became buoyant and floated postmortem to other sites. In summary, we suspect the Buck Mountain preservation is a product of very early phosphate and carbonate precipitation acting as a diagenetic sealant.