Basics of Geology: Every Rock is a Record of the Environment in Which It Formed
Fossils are contained in rocks, and therefore in order to understand dinosaurs one has to understand
how rocks came to be and what information they contain. Rocks are our key to understanding
environments of the past; how those environments (including position of the continents and
composition of the atmosphere!) change over time; and to uncovering time itself.
Rocks:
Naturally occurring cohesive solids comprised of one or more minerals or mineraloids
Are a record of the environment in which they formed
Are generated in one of three primary manners (basis of rock classification):
Igneous
Formed by the cooling of molten material
Classified by whether they cooled on the surface (volcanic) or while still underground (plutonic) and
by composition
Because initial conditions are hundreds or more degrees C, no fossils will be found
Many types of limestone
are made primarily of the shells of once-living things
Made of calcium carbonate
Typically form in salty water (and thus marine or brackish lakes)
Chalk is a type of limestone
formed by microscopic algal plates,
common in the warm shallow seas of the later part of the Age of Dinosaurs
Most of the carbonate mud in the shallow marine environment (and the white beaches of places like the Bahamas and the
Caribbean) is the broken down remains of the internal skeleton of marine algae
Chemical sedimentary rocks: sediment is in the form of dissolved bits (ions) that precipitate
out of water:
Some limestones form from crystals of calcium carbonate that precipitate directly out of warm salty water,
rather than being biogenic
Detrital (also called "siliciclastic" and "clastic") sedimentary rocks:
sediment is grains of various sizes weathered from previously existing rock, cemented
together by minerals in the ground water
Very commonly produced in terrestrial and near-shore environments
By far the most common in which dinosaur fossils are found
Basic detrital sedimentary cycle: Uplift to erosion to transport to
deposition to lithification (typically cementation):
A region experiences uplift, pushing up once-buried rocks and exposing them to the surface
elements
This source rock (or more likely, rocks) experiences erosion: weathered away and broken
up by wind, rain, water, plant roots, gravity, etc.
The broken fragments (sediment) are transported by water, wind, glacial ice, etc.
As sediment is transported from the host rock, it undergoes changes
As distance increases, roundness increases (edges get worn way)
As distance increases, sorting increases (different sized particles get winnowed out)
As distance increased, maturity increases (softer and more easily dissolveable minerals breakdown,
leaving only clay and silica (aka sand and silt) in the end)
Sediment is deposited at some location (stream bed, banks of a river, desert,
delta, lake bed, ocean, etc.)
These locations (deserts, flood plains, rivers, lakes, swamps,
coastlines, continental shelves, etc.) are called depositional environments
The particular environment of deposition will leave different types of sedimentary structures: see below
Sediment is lithified (turned to rock): sometimes simply by compression, but more
often by cementation:
Ground water perculates between the grains of sediment
Dissolved minerals in the ground water precipitate out, glueing (cementing) the grains together
The major types of detrital rocks are based on their sediment size, shape, and mixture:
Breccia: big angular chunks mixed in with
smaller sediment; deposited very close to the source rock (and thus not rounded or sorted)
Conglomerate:
large rounded chunks surrounded by smaller sediment; deposited further from source than breccia, commonly form in
channels of rivers
Sandstone: formed by relatively
well-sorted, well-rounded particles; deposited in many environments (deserts, beaches, river beds, nearshore marine, etc.)
Various sorts of mudstones: very well sorted with
very small particles; deposited in quiet water (lakes, floodplains, off shore, lagoons, etc.)
The Rock Cycle: any rock can be
transformed to any other major class of rock, because rocks are classified by the process in which they are formed. So
if you melt an igneous, metamorphic, or sedimentary rock, and it cools down, you form a new igneous rock; if you recrystallize
an ingneous, metamorphic, or sedimentary rock, you form a new metamorphic rock; and if you erode an igneous, metamorphic, or
sedimentary rock and deposit the sediment from it, you form a new sedimentary rock.
Because sedimentary rocks form where animals and plants lived and died, these are the rocks in which fossils are
common. One of the main categories of information sedimentary rock contain is the paleoenvironment (the conditions
that existed when that rock was formed). The different environments
of deposition represent different paleoenvironments. Some of the clues to discover
paleoenvironments:
The roundness, sorting, and maturity of sediment in a detrital sedimentary rock indicates the relative distance to the source rock
(i.e., breccias form right near their source, mudstones at a great distance)
The energy of the environment (how fast the water or wind was moving) is reflected in different sized
particles of sediment (and, since fossils are buried by the sediment, different types of fossils):
Quiet water (lagoons, lakes, deep ocean, etc): very fine grained sediments (mudstones, fine-grained limestone, etc.),
preserve small details, but unlikely to contain fossils of large animals (which would not be buried before decay sets in)
Faster moving water, wind, etc.: deposit large amounts of sediment (esp. sand) quickly, more likely to bury large objects
(such as large dinosaur bodies)
Environment of deposition often indicated by sedimentary structures: traces left in the
sediment by various processes before lithification. Some common sedimentary structures include:
Mud cracks: indicate
mud that was wet, then dried, then buried by later sediment
Ripple marks: indicate
moving wind or water (current in stream; wave action along shore; etc.)
Raindrop marks: indicates
a wet surface (but exposed to air) that was rained on, then buried
Coal beds: indicates abundant
plant life that was buried faster than it could decay
Putting the sedimentary structure and rock type (lithology) evidence together allows you to reconstruct the
paleoenvironment. For example, this set of dinosaur tracks
are found associated with the impressions of halite (rock salt), indicating that the dinosaur was walking in an
arid environment.
Of course, another main bit of information that sedimentary rocks contain are fossils.
Fossils and Fossilization 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.
They include:
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):
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 (like river channels) bury quickly, but are
likely to destroy smaller bodies. Low energy environments (lakes, lagoons, etc.) might preserve small corpses, but are not
quick enough to bury large animals before they decay/are scavenged.
Larger bodies can be covered by rivers at flood stage:
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.
The study of burial and fossilization is called taphonomy. There are various modes
of preservation after the bone is buried:
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.
Unaltered soft tissue preservation have been found inside some unaltered bone; seem to
require very particular groundwater conditions to preserve proteins, etc.
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
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.
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)
