CPSP118G Fall Semester: Earth, Life & Time Colloquium
Fossils and the Worlds of the Past
Thomas R. Holtz, Jr.
Fossils: The physical traces of past life.
Or, more fully, a fossil is any remain of an ancient organism or its behavior
preserved in the rock record.
(Derived from the Latin word "fossilium": that which is dug up. Originally used for anything found
in the ground, but by the 19th Century had come to mean traces of past life.)
Fossils are the only direct evidence of past life, although indirect evidence exists in
the form of the evolutionary and biogeographic distribution of modern organisms.
Two major types of fossils:
- Trace fossils: the record of organisms' behavior preserved in rock.
- Body fossils: the physical remains of an organism preserved in rock.
Trace fossils are, essentially, biologically-generated sedimentary structures.
Preservation of trace fossils is just like other sedimentary structures: must have rapid
burial, and preserved by lithification of the rock itself.
Body fossils: can be preserved in a variety of ways.
In general, only organisms with hard parts can be preserved: shells, bones & teeth,
For vertebrates (such as dinosaurs), body fossils are primarily bones and teeth
- A structural unit of vertebrate anatomy: humerus, scapula, metatarsal, etc.
- A composite material: calcium phosphate (hydroxylapatite) grains in a protein (collagen) matrix
- A living tissue: store for various nutrients (calcium, phosphate, etc.), modified and reused throughout life
But the rest of the vertebrate is soft tissue (and in many organisms there are NO hard parts),
and so these are only preserved in rare instances.
Bone (like shell and wood) is not solid material, but porous. Pore space is occupied by organic
material in life. Upon death, organic material begins to decay.
In order for bones and teeth to become fossilized (turned into a fossil):
The study of burial and fossilization is called taphonomy. There are various modes
of preservation after the bone is buried:
- Animal must die (in the case of bones) or lose teeth
- Body must be buried by sediment before decay, weathering, scavengers, etc.,
destroy the remains
- The vast majority of living things wind up inside other living things (i.e., are eaten or
decayed). Only a tiny fraction are buried.
- Environment of deposition becomes important. High energy environments bury quickly, but are
likely to destroy smaller bodies. Low energy environments might preserve small corpses, but are not
quick enough to bury large animals before they decay/are scavenged.
- VAST majority of fossils are broken up bones or teeth. A small fraction are complete isolated bones or teeth. A smaller
fraction still are a few bones in articulation (still connected). A very small fraction are
nearly complete skeletons.
- Unaltered bone:
simple burial, some weathering. Organic material may be lost (but see below), but original hard parts
are all still present with nothing added. Relative rare in dinosaur fossils, especially as one gets further
back in time.
- Permineralized: most common mode of preservation of dinosaur body fossils!
- Pore space is filled in with ground water: some dissolved minerals
precipitate in pores (probably some contribution by bacterial activity)
- Is the same process as going on in cementation of the sediment around it
- Original hard parts remain, but extra material added to pores
- Because the new material is added, fossil will break like rock and be colored like the mineral
that filled in the poor space
- "Petrified" wood is actually
- In some cases, soft tissues can be permineralized, but this
seems to be very rare
- Recrystallization: very common in calcitic fossils, but not so common in vertebrate bone.
After burial, calcite crystals reorder and grow into each other. Original mineralogy
remains, but structure is lost.
- Replacement: grades from permineralization.
- Partial to complete replacement of crystals of one mineralogy with another, controlled
by hard part material and by dissolved material in ground water
- Carbonization: organic material is "distilled" under pressure.
- Many material is lost, but carbon film left behind
- Mode of preservation of coal
- Also preserves soft tissues of some animals
(like the feathers
of some dinosaurs or the body outline
of ichthyosaurs) and plants
- Bacterially controlled
- Impressions of dinosaur skin
can form if the body was pressed into the mud before either decay set in or the mud hardened
Different organisms have different potential for fossilization:
- Hard parts vs. no hard parts
- Single hard parts (e.g., gastropods & cephalopods) vs. two hard parts (e.g.,
brachiopods & bivalves) vs. many well-connected parts (e.g., arthropods & echinoderms)
vs. many parts connected only by soft tissue (e.g., vertebrates)
- Microscopic to sediment-sized to immense
- Lived in erosive environments (e.g., mountains) vs. depositional environments
- Lived in accessible vs. inaccessible environments (e.g., lowlands and continental
shelves vs. deep oceanic basins)
Geologists could use physical rock relationships to figure out sequence of events for any given spot. But how
to tell when event in one part of the world happened relative to events in some other part of the world?
Needed a new method of correlation. Rock type doesn't work, because the same
environment will produce the same rock type regardless of relative or absolute time. William "Strata" Smith, however,
discovered thatFossils, however, were useful:
Fossils allowed correlation from continent to continent. Only certain types of fossils
(called index fossils)
were useful for correlation. To be a good index fossil, the species should:
- Have been VERY common, so chances of individuals being buried is good
- Have hard parts, so chances of fossilization are good
- Have a wide geographic range, so that correlation over wide region is possible
- Lived in (or could be deposited in) different environments, so can be found in
- Have some distinctive features, so it can be recognized from closely related
- Have a short geological duration
(a few million years at most), so finding a fossil of the species in a rock means it had
to be deposited in those few million years
Using index fossils, geologists were able to correlate across Europe, and then to other
continents. Created a global sequence of events (based on the sequence of (mostly
European) formations and the succession of fossils) termed the
Geologic Time Scale.
Became a "calendar" for events in the ancient past: used to divide up time as well
Geologic Column divided into a series of units: from largest to smallest Eons, Eras,
Periods, Epochs, Ages:
Animal and plant fossils are mostly restricted to the last (most recent) Phanerozoic
Eon ("visible life eon"). The Phanerozoic Eon is comprised of three Eras:
- The Paleozoic Era ("ancient life era")
- The Mesozoic Era ("middle life era"): the "Age of Dinosaurs"
- The Cenozoic Era ("recent life era"): the "Age of Mammals". We are still in the
So fossils show us:
- That living things in the past were different than those at present
- That environments of the past were different than those at present
- That both environments and living things have changed through time
- That we can use that changing pattern of living things to reconstruct the history of the Earth (and, as we'll
see next semester, the History of Life!)
In other words, fossils help us understand Earth, Life & Time!
But WHY did those enivornments change? And HOW can we get at numerical time? We'll see that next week.
Last modified: 10 August 2007