MACHAERIDIANS OF THE AL ROSE FORMATION, INYO MOUNTAINS, CALIFORNIA

Newly collected material of the scleritome bearing metazoan Machaeridia from the Lower Ordovician Al Rose Formation (Inyo Mountains, California) constitutes one of the older occurrences of the group as well as the first report from the Great Basin.

The Al Rose Formation represents a deep-water shelfal environment containing a trilobite-dominated fauna typical of lower Ordovician mudstones (Nileid biofacies). Whereas the trilobite and graptolite diversity is relatively high, the machaeridian diversity is low with probably only one species belonging to Plumulites sp. present. Machaeridian sclerites in the Al Rose are preserved as impressions in the mudstone and as yet only isolated material is known. Three sclerite morphologies occur, which all with certainty can be placed in one scleritome, these are modified outer sclerites, non-modified outer sclerites and inner ones. General similarities are large to other plumulitid machaeridians where three sclerite morphologies occur relatively frequent. In more detail the morphology of these sclerites differ somewhat from other Ordovician plumulitids, especially the flat inner sclerites give a "primitive" appearance and may represent a more ancestral condition than plumulitids with inner sclerites folded around the longitudinal angle.

The Al Rose machaeridians are of striking low diversity when compared with Ordovician faunas in the northeast that may contain several species and two genera. Interestingly, these northeastern machaeridian faunas are quite similar to those of Europe, and get progressively more so towards the end of the Ordovician and the closure of the Iapetus. This difference between Great Basin and northeastern faunas may partly owe to lack of understanding of the Great Basin machaeridian distribution but also to environmental differences where the Al Rose represents a deep shelf of a passive margin without a corresponding environmental type in the northeast. Further control of machaeridians in the Great Basin and a more precise knowledge of their stratigraphic distribution in North America are needed to fully evaluate the apparent differences in morphology and composition of machaeridian faunas.