Ambient inclusion trails in Palaeozoic crustaceans (Phosphatocopina and Ostracoda)

Hollow microtubular structures with pyrite grains at their termini have been found in phosphatocopines (Hesslandona toreborgensis and Hesslandona cf. angustata) from the late Cambrian of northern Poland, and in the metacopine ostracod Cytherellina submagna from the Early Devonian of Podolia, Ukraine. The Cambrian phosphatocopines are secondarily phosphatised and microtubes (5–20 μm in diameter) are preserved inside the phosphatized bivalved shield, visible on the shield surface as half-open microstructures with pyrite grains at their terminal end. They are visible only where an external coating layer has peeled off. In the Devonian ostracod C. submagna, microtubes (c. 1–8 μm in diameter) occur within the phosphatized internal mould/steinkern and are visible on the surface of the mould due to the dissolution of the calcite carapace during extraction of the limestone samples. These microtubular structures are here interpreted as ambient inclusion trails (AITs) due to the presence of terminal pyrite crystals of equivalent diameter to the microtube, polygonal microtube cross-sections, plus longitudinal striations on the microtube walls that record the movement of the migrating angular pyrite crystal. AITs are thought to form when mineral crystals, typically pyrite, are impelled to migrate through a fine-grained mineral matrix under increased fluid/gas pressure. Our new materials provide the first record of exceptionally preserved AITs occurring in crustaceans and only the third reported occurrence of AITs in the Palaeozoic. The intimate occurrence of AITs with phosphatized crustacean body fossils supports the hypothesis that AITs are likely driven by increases in gas pressure due the decomposition of soft tissue. In the case of Cytherellina submagna, our calculations suggest that as little as 20 μg of organic matter decaying within a volume of 0.06 mm3, encased by an impermeable outer carapace, may have been sufficient to produce a large number (> 100) of AITs.