Erth 305 - Sep 7th

Process of Fossilization

• Definition of Fossilization:

  • The process of fossilization is the transformation of a dead organism into a fossil.

  • The fossilization process is inherently highly destructive.

  • Not all living organisms or organisms that once lived on Earth were or will be transformed into fossils.

Environmental Conditions Required for Fossilization

• Burial Rate Requirements:

  • The dead organism must undergo rapid burial.

  • Rapid burial occurs via sudden catastrophic events (e.g., produced by a landslide) or through a high rate of sedimentation within the sedimentary basin.

• Geochemical Basin Environment:

  • Anoxic conditions at the bottom of a sedimentary basin increase the fossilization potential.

  • Essential chemical components characteristic of anoxic environments include:

  • Dissolved hydrogen sulfide ($H_2S$)

  • Carbon dioxide ($CO_2$)

  • Methane ($CH_4$)

  • Ammonia ($NH_3$)

Modes of Casual Fossilization

Casual fossilization encompasses several taphonomic pathways: petrification, lithification, permineralization, recrystallization, carbonization, moldic fossilization, replacement, impressions, and impregnation.

Petrification, Lithification, and Permineralization

• Mechanisms and Definitions:

  • These processes occur in the case of fossils that present a porous internal structure (e.g., vertebrate bones, tree trunks, etc.).

  • Lithification: The transformation of the hard body parts into stony material.

  • Permineralization: The pore filling with different minerals.

  • Petrification: Involves the concurrent occurrence of both lithification and permineralization.

• Documented Examples:

  • Psaronius: A fern tree from the Permian of Germany (housed at Naturkundemuseum, Berlin).     

    

  • Dinosaur Bone Thin-Section: A section through a dinosaur bone from the Upper Cretaceous of Canada (Alberta); photograph courtesy of Dr. L. Bloom (University of Calgary). Mineralization components include oxides, quartz ($Q$), sulfates ($[SO_4]s$), and carbonates ($[CO_3]s$).     

    

  • Futabasaurus: An aquatic reptile from the Upper Cretaceous of Japan (housed at Museum of Natural Sciences, Tokyo; photograph courtesy of Dr. K. Tanaka).     

    

  • Cladoxylon: An early land plant that evolved woody tissue from the Middle Devonian of Germany (housed at Naturkundemuseum, Berlin).     

    

Recrystallization

• Mechanisms and Pathways:

  • Involves the transformation of one mineral of the hard body parts into another.

  • The most frequent recrystallization pathway is that in which aragonite is transformed into calcite; this process is also referred to as calcification.

• Documented Examples:

  • Globotruncanella: A foraminifer with planktic (planktonic) habitat from the Upper Cretaceous of the Central Pacific Ocean.     

    

  • Uintacrinus: Sea-lily; recrystallized specimens from a marine sea-lily meadow of the Upper Cretaceous of Kansas (USA) (housed at Senckenberg Museum, Frankfurt auf Main).     

    

Carbonization

• Chemical Composition and Expulsion Process:

  • Life forms on Earth have an elemental composition dominated by the CHON elements ($C$, $H$, $O$, $N$).

  • Sulfur ($S$) and phosphorus ($P$) occur in smaller proportions, while the rest of the naturally occurring elements are mostly encountered as traces.

  • Elements are expelled during the burial and maturation processes of the organic matter.

• Documented Examples:

  • Pecopteris: Fossilized fern fronds from the Upper Carboniferous of Poland (EU) (housed at Naturkundemuseum, Berlin).     

    

Moldic Fossilization

• Mechanisms and Characteristics:

  • Involves the complete or partial removal of the original material of the hard body parts.

  • An empty space is formed within the sedimentary rock, which basically is a lithified sediment.

  • The driving process is that of dissolution.

• Documented Examples:

  • Fossilized Pine Cones: Quaternary of UAR (Egypt) (housed at Naturkundemuseum, Berlin).     

    

  • Fossilized Trilobites: In a slab of rock from the Cambrian of Japan (housed at Museum of Natural Sciences, Tokyo; photograph courtesy of Dr. K. Tanaka).     

    

Replacement

• Mechanisms and Pathways:

  • The empty space resulting from moldic fossilization is filled with a newly precipitated mineral, frequently resulting in spectacular fossils.

  • Pyritization: One case of replacement in which the empty space is filled with pyrite.

• Documented Examples:

  • Stropheodonta: A brachiopod with a pyritized specimen from the Devonian of Canada (housed in the collections of the University of Calgary).     

    

Impressions

• Mechanisms and Characteristics:

  • Fossils that are produced by a dead organism in soft sediment due to its weight and that of the sediment added on top of it during the process of organic matter decay.

• Documented Examples:

  • Pecopteris: Fossilized leave from the Upper Carboniferous of Germany (EU) (housed at Naturkundemuseum, Berlin).     

    

Modes of High-Quality Fossilization (HQF)

High-quality fossilization modes provide exceptional preservation pathways that safeguard detailed morphology.

Fossilization in Amber

• Characteristics and Preservation:

  • Amber is a natural resin with high viscosity.

• Documented Example:

  • Plesiomyrex: Insect specimen from the Eocene of Germany (EU) (from Dlussky and Radchenko 2009).     

    

Fossilization in Tar Pits

• Environmental Settings:

  • Fossilization happened in tar swamps, where hydrocarbons from the Earth's interior reach the surface.

• Documented Example:

  • Cybister: Insect specimen from the Pleistocene of the USA (California) (Collections of the University of Calgary).     

    

Additional High-Quality Modes

• Congealment: Preservation achieved via freezing.

• Dehydration (Desiccation): Preservation achieved via extreme drying.

Fossil Lagerstätten

• Definition and Significance:

  • Geological sites providing high-quality preservation through causal fossilization processes in which the soft body parts are often preserved.

  • These 'fossil ores' are paramount to reconstructing the life history and evolution on Earth.

  • About 100 fossil lagerstätten are known.

• Burgess Shale:

  • Geological Setting: Middle Cambrian Stephen Formation of Canada (British Columbia).

  • Representative Specimen: Amiskwia (flat worm) (NMNH, Smithsonian Institution, USNM PAL 57644), www.si.edu.     

    

• Chengjiang Fauna:

  • Geological Setting: Lower Cambrian of China.

  • Evolutionary Value: Represents an excellent record of the early chordates and vertebrates.

  • Representative Specimen: Haikouichthys (earliest known vertebrate); photograph courtesy of Dr. D.-G. Shu (Northwest University, Xi'an Shaanxi).