Abstract

A unique aquatic arthropod Konservat Lagerstätte occurs in lacustrine-derived carbonate concretions within the Miocene Barstow Formation of southern California. Faunal elements are primarily aquatic, salinity-tolerant, autochthonous predaceous diving beetle larvae, fly larvae and pupae, mosquito and thrip larvae, and fairy shrimp. Three-dimensional preservation of soft tissues is via replacement by silica-based minerals as well as calcite, celestite, apatite, and gypsum.

The fauna occurs in three beds (each 1–3 m thick) within an approximately 100 m sequence of predominantly microlaminated mudstone deposited in a saline-alkaline lacustrine environment. Faunal composition is consistent between sites and concretion beds, but faunal composition and diversity changed significantly during lake history, corresponding to differences in brine chemistry as well as preservational facies, resulting from the shift toward a more nearshore regime. The changes in lake chemistry and subsequent changes in fauna indicate lake shallowing as well as regional and global climate change towards greater aridity during the Miocene that also is coincident with regional uplift.

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