Taphonomy: The realm of paleontology dedicated to the study of the processes by which an organism becomes part of the fossil record. Yields an understanding of the filters and biases of the fossil record.

Becoming a fossil: From the birth of the organism to discovery by a paleontologist, fossils go through four general stages.

Biotic stage: Birth to death. The organism grows whatever tissue is capable of being preserved then dies. It has done its part. Whether it will become part of the fossil record depends on whether its carcass can remain intact long enough to be buried. The quicker the better because.....

Interment stage: Death to final burial. The carcass is exposed to: Decomposition: destruction of soft tissues (mostly by bacteria) Disassociation: hard parts become separated Abrasion: surface details of hard parts lost Breakage: hard parts degraded into fragments Winnowing: fragments sorted by size due to moving water

Diagenetic stage: Final burial to discovery. Once buried, the remains are officially fossils, however their existence is still perilous. Diagenesis results in: Dissolution: hard parts chemically altered into soluble substances Compaction: remains crushed by overlying sediment Recrystallization: ground water & minerals enter, form crystals & disrupt remains

Investigative stage: Discovery to ultimate destruction. Every day, fossils are unearthed by erosion, only quickly to be destroyed by it. To enter the fossil record, as scholars understand it, a fossil must be exhumed (usually by natural processes), discovered, and described.

Not all clades of organisms produce preservable hard parts. Soft bodied organisms CAN produce fossils under special (konservat-lagersätten) situations to be considered later.

The compositional roster of these skeletons is non-random. Main hard substances found in organisms:

Calcite and/or aragonite - CaCO3 (calcareous hard parts dominate the fossil record). Calcareous skeletons are thought to have originated at least twenty times in eukaryote evolution. Also the first biomineral skeletons known, dating to 750 m.a. Typically secreted extracellularly. The abundance of independent origins suggests the exaptation (coopting) of an underlying biochemical mechanism present in soft-bodied precursors such as: Biochemistry for transport of Ca2+ ions. Biochemistry for prevention of "accidental" external calcification, a danger in times of high CaCO3 saturation.

Silica - SiO2 (mostly in the form of cryptocrystalline quartz or Opal - hydrated silica - SiO2- nH2O . Less widely distributed, with at least eight independent origins. Silica and opal are secreted intracellularly, in vesicles possibly derived from golgi bodies. Thus almost all silica secreting organisms are unicellular.

Calcium phosphates such as apatite - Ca5(PO4)3 In stark contrast to silica, appears exclusively in animals, mostly brachiopods (left) and vertebrates. Not surprising since PO3 ion is a relatively scarce nutrient that one might prefer not to "waste" on skeleton construction. Accordingly, vertebrates, who make the most extensive use, have skeletons that are constantly being remodeled by resorbtion, allowing the metabolism frequent access to the PO3. The biochemistry of phosphate precipitation seems to be linked with that of calcium carbonate. Indeed, carbonate substrates have been used surgically as bases for bone regeneration.

Complex organic molecules forming the framework for robust soft tissues: While not exactly "hard parts" some soft tissues have reasonably good fossilization potentials. These include: Chitin (a sugar) - the basis of arthropod cuticle. Cellulose (a sugar) - the foundation of vascular plant skeletons Spongin (a protein) - the basis of the structural tissue of most sponges

Phylogeny of skeleton-forming eukaryotes from Knoll, 2003.

Organisms with calcareous skeletons

Foraminfera: (Cambrian - rec.) Include benthic and (derived) planktonic forms Barely macroscopic (must use microscope to see details) Basal members with agglutinated skeletons, more derived with calcareous tests Extraordinarily abundant in marine record

Chlorophyta green algae: Dasycladales: (Ediacaran - rec.) Include Receptaculitids, (Cambrian - Permian) a strictly Paleozoic group of dasyclades and important reef former "Calcareous algae" Dead Benthic marine green algae with skeleton of calcite or aragonite Broken down dasyclade skeletal fragments are a major component of limestones Volumetrically significant. Also as binders of sediment.

Coccolithophorida: (Jurassic - rec.) Planktonic, extraordinarily small forms Skeleton comprised of many individual clacareous plates called coccoliths Accumulations of coccoliths form the rock chalk. Thus volumetrically as well as biostratigraphically significant.

Dinoflagellata: (Neoproterozoic - rec.) Planktonic, some parasitic (including zooxanthellae) Microscopic Normal test is cellulose; have a calcareous cyst stage

Organisms with silicious skeletons

Radiolaria: (Cambrian - rec.) Planktonic, microscopic Opaline (i.e.hydrated silica) skeletons Biostratigraphically significant.

Bacillariophyta, better known as diatoms: (Jurassic - rec.) Planktonic, microscopic Shells are two-pieced frustules (like little pillboxes), made of opal Accumulations produce sediment called diatomaceous earth Biostratigraphically significant.

Organisms with diverse skeletal materials