The Preservation of the Cambrian Murero Biota in the Mesones Group, Cadenas Ibéricas, Spain  

The upper-Lower–mid-Middle Cambrian Mesones Group of the Cadenas Ibéricas (NE Spain) yields diverse exceptionally-preserved fossils, including algae, intact sponges, palaeoscolecid and other cephalorhynch worms, xenusians, trilobites and other arthropods, chancelloriid scleritomes, pedunculate linguliformeans, and very diverse fully-articulated echinoderms (the Murero biota). It includes a significant part of the entire Cambrian soft-bodied taphonomic window of about 10 Ma.  Of those fossils, seventeen species representing eight phyla were analyzed with dispersive x-ray spectrometry, allowing semi-quantitative chemical detection, and some were studied under a petrographic microscope.  The integuments, skeletons, and soft tissues of the Murero fossils are replicated in chlorite, illite, and some accessory minerals.  This raises the question of whether these minerals formed: (1) as death masks; (2) during early diagenesis, after some mineral precursors; or (3) by replacement of former organic and mineral fabrics during metamorphic alteration.   At first glance, the third possibility seems most plausible because the entire Mesones Group underwent anchimetamorphism.  However, the chlorite that replicated the skeletons and cuticles, irrespective of their pristine composition (carbon, calcite, apatite), contains high Mg and Fe, and low Al, Si, and K, relative to the detrital chlorite and illite in the surrounding rock matrix.  When present, Ca is confined to replicating euhedral chlorite and fibrous illite, but only in the fossils.  Both chlorite and illite selectively replaced the primary microstructures.  These features, as well as the aggregation of chlorite flakes as tangentially oriented euhedral and subhedral blades, and high Fe/(Fe+Mg) and Al/Fe ratios in the mineral composition, are indicative of secondary formation of chlorite and illite after a Fe- and Mg-rich verdine-type clay.  In modern marine environments within a temperature range of 20–40 °C, the authigenic verdine-type clay can be  transformed into either euhedral chlorite or fibrous illite crystals via the catalytic actions of bacteria.  In some cases, if organic shell compounds are acidic enough, as in a linguliformean shell, they mediate the growth of clay minerals by themselves. 

In general, the Cambrian soft-bodied taphonomic window was open for a large variety of mineralogies preserving exceptional fossils (carbonaceous, phosphatic, pyritic, and even quartzose replicas are well known). This window coincided with the mass appearance in the fossil record of diverse ecdysozoans (arthropods, xenusians, and cephalorhynchs), which compose the bulk biovolume and species diversity of any Cambrian lagerstätte.  The closure of this window occurred during the late Middle Cambrian.  This interval is signified by several important events pointing to the end of a icehouse epoch and the beginning of a greenhouse epoch.