| Abstrakt: |
The Green River Formation, Wyoming, contains such an abundance of well-preserved flora and fauna that this late early Eocene Lagerstätte is one of the best known from North America. Despite having being studied since the mid-nineteenth century, little is known about the taphonomic processes that resulted in a diverse suite of organisms, especially abundant fossil fish, being preserved in exquisite detail. Two distinct patterns of completeness and articulation recur among the fossil fish: complete and fully articulated, or extensive disarticulation of the anterior part of the fish only ('half and half' preservation). To decipher the processes involved in the preservation of Fossil Lake fish, specimens of the extant taxon Carassius auratus were decayed experimentally over a 6-month period. Various scenarios approximating conditions in the Eocene lake were replicated in the laboratory and each monitored to record the rate of degradation and how different starting conditions impacted on the taphonomy of the fish. Varying salinity or oxygen level did not induce any discernable differences in the pattern of decay; degradation occurred faster at higher temperatures. Unexpectedly, decay rates varied greatly between replicate samples. Soft tissues decomposed rapidly and extensively, and, even in the absence of any disturbance, the skeletal disarticulation that followed was repeatedly much greater than that exhibited by the vast majority of fossil fish from the Green River Formation. The results indicate clearly that decay in a quiet-water environment is, on its own, insufficient to explain the consistently high fidelity preservation of Green River fish: some additional factor is implicated. We propose that microbial mats at the sediment-water interface were key; fish carcasses became adhered to the sediment surface inhibiting floating, disarticulation and loss of bones, with 'half and half' specimens curving laterally and not fully adhering, thus only that part (typically the posterior) in contact with the substrate retained a high degree of skeletal fidelity. [ABSTRACT FROM AUTHOR] |
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