EARLY BIOMINERALIZERS: STRANGE ECHINODERM PLATES FROM THE LOWER CAMBRIAN OF THE SOUTHWESTERN UNITED STATES AND MEXICO

Among the first animals to begin building biomineralized skeletons were echinoderms. While the first echinoderm species were widely distributed and abundant, their initial success was short-lived, perhaps only spanning a million years in the Series 2 Early Cambrian. The taxonomic affinity of these echinoderms is unknown because no articulated specimens exist. They occur as disarticulated 1 - 3.5 mm diameter ovoid-circular plates in coquinas that are 10 cm - 3 m thick and extend laterally for 10s to 1000s of meters. Plates occur in interbedded carbonates and siliciclastics. These echinoderm beds are unusual because they are mostly monotaxic accumulations of echinoderm plates; rarely they contain trilobite and archeocyathid debris.

Echinoderm beds occur throughout the upper member Wood Canyon Formation in California and Nevada, and the Buelna Formation in Sonora. Four stratigraphic sections of these formations were logged, and samples of both echinoderm beds and other carbonates were collected at 1-2 m resolution. Stratigraphic sections reveal the sequence of depositional environments and sedimentology in which the echinoderm beds occur. Three levels of analysis were conducted on echinoderms, including outcrop, polished-slab, and thin-section examination. The lithologies, grain sizes, and grain types, packing, and fabric of each scale was noted. This three-tiered analysis allows us to see levels of replacement and other grains or intraclasts in the samples.

Echinoderm plates are concordantly to obliquely oriented and dispersed to well-packed within beds. The finest-grained, least-altered carbonates were petrographically screened and microdrilled to assess the isotopic (d13C, d18O) and trace element (Mn, Sr, Ca, Mg) chemistry of seawater before, during, and after the proliferation of these echinoderms. Microdrilling of echinoderm plates will allow us to assess their skeletal chemistry, and it records seawater values. These results will help determine if the onset of echinoderm biomineralization or the extinction of these early echinoderms correlates to changes in ocean chemistry or depositional environment.