•Rocks are the solid physical record of past environments. Our classification of rocks is based on the processes that produced them.
•Igneous (chilled from a molten state) and metamorphic (recrystallized by intense heat &/or pressure) do not contain fossils.
•In contrast, sedimentary rocks (those made by fragments of previously existing rocks transported and redeposited) often contain fossils.
•Sedimentary structures (such mud cracks, raindrop marks, ripple marks, crossbeds, and the like), and other features such as the size, sorting, and roundness of clasts, record the environments on Earth's surface (where living things live and die) at the time the rocks formed.
•Because sedimentary rocks form by deposition of particles that were being transported, they naturally form layers (strata).
•Relative time represents the sequence of events; numerical time is the statement of dates or durations in terms of actual measured units (years, etc.).
•Geologic time is an example of "deep time": the history of the Earth is incredibly long compared to our personal experience, being measured in millions and billions of years.
•Because they naturally form strata, the relative sequence of time in sedimentary rocks are relatively straight forward to work out.
•The physical stratigraphy (position of different strata of a given spot) allows one to figure out the sequence of oldest to youngest event at this spot.
•Correlation from one spot to another can be done by tracing out particular beds (formations), assuming the two spots are physically close; for correlation over longer distances, methods such as biostratigraphy are needed.
•Biostratigraphy uses the sequence of index fossils through different strata as markers of time.
•The geologic time scale was initially developed using index fossils. It divides up the history of the Earth into Eons, which are subdivided into Eras, which are broken up into Periods, which are divided into Epochs, which are spit into Ages (or Stages).
•The best estimates of numerical time come from radiometric decay. Some naturally occurring isotopes change from the parent material into the daughter product at a constant rate of decay; comparing the ratio of daughter to parent alows you to calculate the age of the rock.
•However, radiometric dating works only for igneous rocks. So you can use volcanic ash beds, lava flows, and igneous intrusions to bracket the age of the fossil bearing strata, but rarely directly date the fossils themselves.
•Other methods of determining the age of rocks include using marker beds (that document single widespread events) or tracing the flip-flop of the Earth's magnetic field over time.
•Fossils are the physical remains or traces of their behavior preserved in the rock record.
•Trace fossils (such as footprints, burrows, nests, and coprolites [fossilized feces] are the record of behaviors of extinct animals.
•Body fossils (such as teeth, bones, shells, wood, leaves, pollen, etc.) were once part of a living thing.
•Taphonomy is the process by which parts of a living thing are buried and preserved as fossils.
•Different environments of deposition are better at preserving different types and sizes of fossils.
•Depending on the taphonomic history, a fossil might be complete, or only fragmentary, or anything in between.
•After burial, different diagenetic processes may alter the composition of the original hard parts of the body.
•Normally the soft tissue (flesh and so forth) decays after death, but in some diagenetic conditions they might be preserved (either unaltered, as carbonized stains, as permineralized material, or as impressions.)
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.
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):
Formed by the cooling of molten material
Classified by whether they cooled on the surface (volcanic) or while still underground (plutonic) and
Because initial conditions are hundreds or more degrees C, no fossils will be found
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
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 dissolvable 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 percolates 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)
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
Of course, another main bit of information that sedimentary rocks contain are fossils.
Deep Time: Ruins of a Former World
"Deep Time": analogy to "deep space"; the vast expanse of time in the
(geologically ancient) past.
Two different aspects of time to consider:
Relative Time: sequence of events without consideration of amount of time
Numerical Time: (sometimes called "absolute time"), dates or durations of events in terms of seconds, years,
millions of years, etc.
Relative time was determined LONG before absolute time.
Sedimentary rocks naturally form horizontal layers (strata, singular stratum).
Strata allow geologists to determine relative time (that is, sequence of deposition of
each layer, and thus the relative age of the fossils in each layer):
Principle of Superposition:
unless they have been disturbed, the strata at the
bottom of a stack were deposited first, the ones on top of that are next oldest, and so
on, with the youngest strata being the ones on top.
(A note: there are often gaps in the rock record, caused by either periods of non-deposition or periods of erosion. These will fall
in superposition order, but mean that
any given section of rock won't have the complete record of all geologic time. These gaps are reflected as weathering surfaces or
erosional surfaces (technically called unconformities)
Principle of Cross-cutting Relationships:
any structure (fold, fault, weathering surface, igneous rock intrusion, etc.) which cuts
across or otherwise deforms strata is necessarily younger than the rocks and structures it
cuts across or deforms.
In some cases, the particular rock type, color, sedimentary structures, and so on
were the same in strata in nearby sections. These groups of strata were named
Represent units of rock produced by the same conditions (environment) and having the
same history (produced over a particular sequence of time)
Given formal names (e.g., the Morrison Formation, the Hell Creek Formation, the
Solnhofen Limestone, etc.)
Sometimes groups of formations which lie directly on top of or next to each other are
catalogued together as formal Groups, and sometimes groups which lie directly on
top of or next to each other are placed into formal Supergroups
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.
Fossils, however, were useful:
Principle of Fossil Succession: there is a unique, non-repeating pattern
(history) of fossils through stratigraphic time. All rocks containing fossils of the
same species were deposited during the duration of that species on Earth.
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
The Mesozoic Era is divided into three periods:
The oldest (furthest from us in time) is the Triassic Period
("three-fold period"), comprised of the Early Triassic, Middle Triassic,
and Late Triassic Epochs
The middle one is the Jurassic Period ("Jura mountain period"), comprised of
the Early Jurassic, Middle Jurassic, and Late Jurassic Epochs
The youngest (closest to us in time) is the Cretaceous Period ("chalk period"),
comprised of only the Early Cretaceous and Late Cretaceous Epochs.
Although the Geologic Column was developed as a relative time scale, geologists wanted
to figure out the numerical age dates for Era-Era boundaries and other events.
Discovered various techniques:
Main one: Radiometric dating
Radioactive materials decay
at predictable rate, known as the half-life
Atoms decay from one form (parent) to another (daughter product), releasing energy
After one half-life has passed,
half the original parents in the material will have decayed into the daughter product;
after two half-lives, only one-quarter of the parent material remains, with three quarters
daughter product; after three half-lives, 1/8 to 7/8; after four half-lives, 1/16 to 15/16;
and so on.
Can thus date rocks:
Compare the ratio of parent product to daughter product
Radiometric dates will only be effective for igneous rocks, since those are the ones
that form by cooling and locking atoms into place
In sedimentary rocks, can date the individual grains of sediment: tells you age of
source rock, but not deposition
In metamorphic rock, recrystallization redistributes atoms and obscures signal
Since only igneous can best be dated radiometrically, use principles of superposition,
cross-cutting relationships, etc., to determine ages of sedimentary rocks (and their
fossils) relative to numerical dates, and tie dates into
Geologic Column by correlating
with index fossils
Note: radiocarbon (carbon 14) dating cannot be used for Mesozoic fossils!
Half-life is WAY too short; only useful for tens-of-thousands-of-years scale.
Some large geologic events (major volcanic eruptions, asteroid impacts, etc) leave a characteristic thin layer of rock across wide regions (sometimes globally)
The magnetic (but NOT the geographic) poles have
"flip-flopped" throughout geologic time, so that sometimes a magnet's north pole points towards geographic North, and sometimes toward geographic South.
Magnetic polarity can be recovered by some iron-bearing rocks (sedimentary and igneous).
Because based on the Earth's magnetic field, the changes occur everywhere on the
planet at the same time.
Radiometric dates reveal the Paleozoic-Mesozoic boundary is 252.17±0.06 Ma (million years ago); the Triassic-Jurassic boundary is 201.3±0.2 Ma, the Jurassic-Cretaceous boundary is 145.0 Ma, and the Mesozoic-Cenozoic boundary is 66.0 Ma. (These represent recent recalibrations; many texts and figures show slightly different numbers for these based on pre-2013 calculations.)
Most effective approach in getting age dates for a fossil bed is to combine multipletechniques: get relative age relationships between local units, find index fossil ages for the sedimentary rocks, and radiometric and magnetic dates where possible.
Here is a nice set of graphics to put the scale of geologic time in perspective.
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.
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:
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.
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.
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)