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Geology of the National Parks - Death Valley

 

click for timescale Death Valley National Park through time

The Earliest Animal: Latest Precambrian and Early Cambrian time

The dullness of that late Precambrian mudflat, however, was deceiving, for during those quiet times the first animals were developing in the oxygen- rich shelter of marine plant communities. Bacterial and algal plant-life had already prospered for nearly three billion years, and with the appearance of cell nuclei (certainly by Pahrump times) the stage was set for more complex life forms. On Death Valley's tidal flats, algae with its filaments and slime had long been binding mud into wavy laminated mats (stromatolites) which may have provided havens for the newcomers. The very earliest animals are exceedingly rare, occurring well west of Death Valley in limy offshore muds contemporary to the Stirling Quartzite. The developmental pace increased in Wood Canyon times, for this sandy formation preserves a host of worm tubes and enigmatic trails. Ultimately, in late Wood Canyon sediments the first animals with durable shells emerge to open the earliest copiously fossiliferous period, the Cambrian.

The base of the Cambrian marks a great leap forward for the animal kingdom, but doubt lingers whether the evolutionary burst represented a major expansion of animal species themselves or simply the origin of readily-fossilized protective shells. In either case, what environmental change might have prompted the animal revolution? The late Precambrian ice-age? The melting of its glaciers with consequent flooding of continental margins? Attainment of an oxygen level in the atmosphere finally sufficient for animal respiration? The last idea reasons that adequate oxygen (about 1% of the modern amount) liberated the primitive naked animals from dependence on their host plants, permitting abrupt wholesale invasion of well-aerated shallow surface waters and tidal flats. In view of the thin air of the time, hard shells may then have been a shallow-water protection against ultraviolet radiation. Or were they simply a response to predation, this planet's first evidence of aggression?

In Death Valley, the animal revolution is marked in the upper Wood Canyon Formation by appearance of trilobites, archaeocyathids, and primitive echinoderms. Many lineages arose at the time, but the world was truly won by trilobites, the now-extinct inch-long mud-grovellers (a bit like horseshoe crabs), who ruled the Earth for 100 times the period of man's recent dominance.

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