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HONR 259C "Fearfully Great Lizards": Topics in Dinosaur Research

Fall Semester 2017
Doc Holtz's School of Rock: Making the Fossil Record


Weathered limb bone of a mid-sized tyrannosaur in the latest Cretaceous Hell Creek Formation, near Ekalaka, MT

Key Points:
•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.

Rocks:

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:

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 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):

Use these principles to figure out time sequence in any particular section of rock. BUT, how to extrapolate the sequence at one section with the sequence at another?

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 formations:

Mapping out formations, groups, and supergroups, geologists could connect sequences of rocks across regions. But what about across continents and oceans?

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:

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:

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 as rocks.

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 Mesozoic Era is divided into three periods:

No one region has a continuous sequence of time. Any given location has likely had periods of non-deposition or erosion, which would leave gaps in the geological and fossil record at any given spot.

An interactive project on geologic time, for those who want to explore in more detail.

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:

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 are, essentially, biologically-generated sedimentary structures. They include:

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, wood, etc.

For vertebrates (such as dinosaurs), body fossils are primarily bones and teeth

Bone:

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:

Different organisms have different potential for fossilization:


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Last modified: 5 September 2017

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Dinosaur footprints at the Mill Canyon Dinosaur Tracksite, Utah