Exceptional fossils can preserve diagenetically-altered biomolecules, and understanding the pathways to such preservation is vital to utilising fossil information in evolutionary and palaeoecological studies. Experimental taphonomy explores the stability of tissues during microbial/autolytic decay or their molecular stability through maturation under high pressure and temperature. Maturation experiments are often hampered by the fact that maturation occurs inside sealed containers, which does not allow for the loss of labile, mobile, or volatile molecules. On the other hand, maturation of tissues wrapped inside aluminium foil can sometimes be too open of a system, leading to loss of both labile and recalcitrant materials. Here we present a novel experimental procedure for maturing tissues under elevated pressure/temperature inside compacted sediment. In this procedure, porous sediment allows for labile maturation breakdown products to escape from the sample into the sediment and maturation chamber while recalcitrant, immobile components are contained, in a manner expected to be similar to the natural conditions of fossilisation. To test the efficacy of this novel procedure with respect to simulating fossil diagenesis in a simple case study, we investigate the differential survival of melanosomes relative to proteinaceous tissues through maturation of fresh lizard body parts and feathers. Macro- and ultrastructures are then compared to fossils. Similar to many carbonaceous exceptional fossils, the resulting organic components are thin, dark films composed mainly of exposed melanosomes resting on the sediment in association darkened bones. Keratinous, muscle, collagenous, and adipose tissues appear to be lost. Such results are consistent with predictions derived from non-sediment-encased maturation experiments and our understanding of biomolecular stability. These experiments might also suggest that organic preservation is largely driven by the original molecular composition of the tissue and the diagenetic stability of those molecules, rather than the tissue’s decay resistance alone, and this should be experimentally explored in the future.