Trilobite Taphonomy of the Latham Shale (Lower Cambrian), California: A Burgess Shale-Type Deposit of Western Laurentia  

The Early Cambrian Latham Shale (Mojave Desert, California) has yielded a diverse fauna including abundant olenelloid trilobites, and rare unmineralized taxa, such as algae and anomalocaridids.  A >500-specimen sample from the formation shows that the most abundant trilobite species include Olenellus clarki, Mesonacis fremonti, and Bristolia bristolensis, with the abundance rank of species varying greatly between localities.  O. nevadensis, B. mohavensis, B. harringtoni, B. insolens, and B. anteros were also present in lower quantities.  The frequency distribution of the olenelloid trilobite fragments indicates the absence of size-based sorting.  Of the total sample, 97.8% were isolated complete or partial cephala.  Only 1.4% were complete articulated thoraces (with articulated cephala in most cases). Orientation data indicate that the cephala show no statistically significant up-down trend, and azimuth orientations of the long axis of the glabellas show no obvious preferred trend.  Taphonomic results suggest that the Latham Shale trilobites were deposited in a quiet-water setting in the inner detrital belt with little current activity and generally long bottom exposure times for moults and carcasses. 

In the up-down orientation of the sclerites; mean, mode, and range of cephalon widths; and percentage of complete specimens, trilobites of the Latham Shale differ markedly from other abundant trilobite deposits at certain sites in the Spence Shale of Idaho and the Bright Angel Shale of Arizona.  In contrast to the Latham, the trilobites in these latter deposits are more frequently articulated, and show a moderate to strong preference in up-down orientation.  These taphonomic differences likely result largely from relatively faster burial in higher-energy environments.  Possible differences in decomposition patterns between the Early Cambrian trilobite taxa of the Latham and those of the Middle Cambrian taxa of the Spence and Bright Angel may also have an impact.