EXCEPTIONALLY PRESERVED FROGS FROM THE MIOCENE OF LIBROS, NE SPAIN

The Libros basin of NE Spain contains a lacustrine-hosted exceptional fauna of early Late Miocene age. The exceptionally preserved fauna and flora, including amphibians, birds, snakes and leaves, occur in the deep-water laminated mudstone facies; frogs are particularly abundant. The frogs are characterised by high degrees of completeness and articulation, a function of deposition within laminated organic-rich muds below anoxic monimolimnetic waters of a deep, stratified lake. Soft tissues, consisting of a near-2D dark brown film and a white granular material, vary considerably in extent between specimens. The mode of preservation is independent of cm-scale variations in lithology, although it may have been controlled by finer scale variations in environmental conditions that are reflected in a mm-scale lamination. SEM investigation reveals that the soft tissues are defined by layers of bacteria and EPS (Extracellular Polymeric Substances) preserved as carbonaceous remains. The bacteria are differentiated into at least two distinct types and are partitioned into a number of size-specific layers within the fossilised soft tissues. In addition, but to a lesser extent, soft tissues are directly replicated in aggregates of calcium phosphate crystallites (white granular material). Dermis is replicated with highest fidelity and is the most extensively preserved tissue. A taphostratigraphy of the soft tissues reveals a central layer of carbonaceous bacteria surrounded by phosphatised dermis. Replication of bacteria by authigenic minerals is restricted to limited phosphatisation at, and towards, the external surfaces of the specimens; unlike superficially similar early Cenozoic lacustrine faunas e.g. Grube Messel, there is no evidence of extensive authigenesis of bacteria.