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
  • Metamorphic
    • Formed by recrystallization of previously existing rocks due to intense heat and/or pressure
      • Rocks are "baked" and/or "squashed", but not melted (or they would become igneous...)
    • Classified by the direction and intensity of pressure or degree of heat, and the resulting changes from the original rock to the new type
    • Any fossils previously existing in the rocks will likely be obliterated by metamorphism
  • Sedimentary
    • Formed by accumulation and lithification of bits of previously existing rock and/or organic matter
      • Bits of previous rock and/or organic matter are called sediment
      • Because sedimentary rocks form by deposition, they naturally create horizontal beds called strata (singular stratum)
      • Major divisions of sedimentary rock reflect the type of sediment:
        • Biogenic sedimentary rocks: sediment made of solid bits of organic material (whole or broken up) that gets deposited
          • Coal: buried remains of plant life
          • 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
          • Evaporites (rock salt, gypsum, and related rocks): form when salty water evaporates leaving a residue of salt
        • 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 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)
            • 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.

"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 Original Horizontality: because strata are deposited under gravity, they form horizontal layers. If the strata are no longer horizontal, something has disturbed the sediments AFTER they became rocks.
• 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.

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:

• 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

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:

• 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 different formations
• Have some distinctive features, so it can be recognized from closely related forms
• 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 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 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 Cenozoic Era.

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.

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:

• 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 and particles
    • 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.
• Marker Beds
  • Some large geologic events (major volcanic eruptions, asteroid impacts, etc) leave a characteristic thin layer of rock across wide regions (sometimes globally)

• Magnetostratigraphy
  • 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.
  • Can use the particular "bar code"-like pattern of flip-flops to match any section to known global pattern (based on continuous record of lava on ocean floor)

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 multiple techniques: 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.

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:

• Footprints and trackways
• Burrows
• Feeding marks
• Coprolites (fossilized feces)
• Nests
• And more
• Represent activities of the animal while alive, rather than part of the dead creature

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:

• 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):

• 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
  • "Petrified" wood is actually permineralized wood
  • 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)

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Last modified: 30 July 2020