PALEOECOLOGICAL CONTEXT OF EARLY BIOMINERALIZERS – EASTERN CALIFORNIA AND WESTERN NEVADA, USA

Sedimentary rocks typically show a variety of evidence indicating original physical and biological depositional conditions. By the latest Neoproterozoic all the types of typical Phanerozoic biological features of sedimentary rocks had appeared but one – the biomineralized skeletons of benthic organisms. These are first recorded at this time with the appearance of Cloudina, fossils of the earliest biomineralizing organisms.

The significance of this small (length up to 1 cm; 2-4 mm diameter) tube-dwelling organism as the first metazoan with a biomineralized semi-rigid skeleton makes understanding of its specific paleoecological and depositional context essential wherever it occurs. Cloudina from the Precambrian Reed Formation and overlying lower member of the Deep Spring Formation from eastern California and western Nevada were examined from four localities. At these study sites preservation of Cloudina varies significantly, from relatively complete tests to fractured debris. They occur primarily in shell beds, which show varying evidence for transportation before final deposition. At one locality (Mt. Dunfee, NV) many well-preserved specimens show distinct sediment infilling, which may indicate relatively deep storm scour in production of these beds.

In addition, the broader context of these cloudiniids is necessary for establishing a baseline towards understanding how early biomineralizers evolved in response to the ongoing “agronomic revolution” of increasing bioturbation that was occurring during the Neoproterozoic-Cambrian transition. These Cloudina occur before the Precambrian-Cambrian boundary, and hence before the appearance of significant vertical bioturbation. Their common existence in carbonate beds has led to the interpretation that they lived as mat stickers emplaced vertically within a mat-bound substrate. The thick Lower Cambrian sections in the study area provide numerous carbonate intervals from which to assess the context of succeeding early biomineralizers. This stratigraphic interval in eastern California and western Nevada thus provides a basis on which to determine how the first biomineralizing organisms adapted to non-actualisitc seafloors and subsequently evolved as part of the “Cambrian substrate revolution.”