Exceptional fossil preservation: implications for palaeobiology and taphonomy

Although fossils are relatively common in some sedimentary deposits, the majority consists of fragmented or distorted skeletal hard parts (such as shell and bones). However, in rare cases, special burial and diagenetic conditions have allowed a greater portion of the original anatomy to be incorporated into the fossil record. This exceptional type of preservation is characterised by the retention of labile soft to sclerotized tissues and/or delicate, three-dimensional, articulated skeletal units. Exceptionally preserved fossils contribute with important data that greatly enhance our understanding of the evolution of life on Earth. In particular, when analysed using sophisticated and sensitive techniques, these unique fossils have the potential to reveal new insights into both biological and ecological aspects of ancient life.

In this thesis, a selection of fossils representing different types of exceptional preservation is analysed using a multi-proxy approach that includes microscopic, tomographic and molecular techniques. The material encompasses: (1) pigmented internal and integumentary soft tissues, (2) a virtually intact bird skull, and (3) arthropod cuticle (occurring in both sclerotized and naturally biomineralised form). The greater part of the material has been collected from the earliest Eocene Ølst and Fur formations of north-western Jutland, Denmark, although complementary fossils from the stratigraphically older (Cretaceous) Weno and Pawpaw formations are also included. This work is presented as a series of cases studies, and aims at inferring novel aspects of palaeobiology and taphonomy from these rare fossils.

Detailed microscopic and molecular analyses of remnant internal and integumentary soft tissues in fossil fish larvae revealed that these organic remains are dominated by residues of the biochrome eumelanin. Similarly, sclerotized cuticle in Eocene insects consists chiefly of eumelanic pigments, whereas most other organic components (such as chitin and proteins) have been lost during the course of time. In addition to eumelanic pigments, the investigated insect exocuticle also exhibited a finely laminated organisation comparable to those biophotonic nanostructures that produce metallic colours in modern insects.

Analyses of Cretaceous crab cuticle showed that all originally chitinous components were replaced by secondary minerals. This finding has implications for interpretations of the original composition and function of fossilised cuticle also in other extinct arthropods.

Finally, computed tomography was employed on a pristine, three-dimensionally preserved Eocene bird skull. The excellent preservation of the fossil bird allowed the production of a detailed digital 3D version of its skull, from which the brain and inner ear could be faithfully reconstructed. This imaging analysis revealed that virtual renderings are useful not only for phylogenetic studies but also for providing clues about potential sensory and behavioural capabilities.

Collectively, the findings from these studies highlight the diversity of information that can be gained from exceptionally preserved fossils, including novel aspects of phylogeny, colour-mechanisms and their functions, as well as insights into the taphonomy of animal fossils.