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GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2020
Biostratigraphy


Glen Rose Formation with characteristic fauna

Key Points:
•The formation is the fundamental unit of stratigraphy.
•The Principle of Faunal Succession facilitates the correlation of rock units across wide distances.
•Biostratigraphy depends on index fossils.
•Biostratigraphic units are rock units, not time units.
•Biozones are defined based on first and last occurrence data.
•Biases of the rock record and quirks of evolution impart difficulties to the interpretation of biostratigraphy.
•Biochrons are time units.
•Quantitative biostratigraphy allows fossils to address questions about sedimentation and depositional processes.

"Organized Fossils are to the naturalist as coins to the antiquary; they are the antiquities of the earth; and very distinctly show its gradual regular formation, with the various changes inhabitants in the watery element. (William Smith, 1817. Stratigraphic System or Organized Fossils.)

Stratigraphy without fossils:

Paleontology can illuminate important issues in both biology and geology. Today's topic addresses one of paleontology's greatest applications in the service of geology.

Stratigraphy: The detailed history of Earth's surface revealed by the order and relative position of superposed rock units. Using the stratigraphic principles of Nicholas Steno: (1668) and the uniformitarian principle of James Hutton (1795), Geologists of the early 19th century could establish the relative ages of rock units on the formation scale.


Contact of the Coconino (above) and Hermit Shale formations - Grand Canyon
Formations: the smallest mappable rock units. E.G. the Chinle Formation. Remember, formations must:

Give us Time Lines!

:

If we want a global history of Earth's depositional environments, we ideally want to identify time lines in the rock record: geographically widespread surfaces that formed at the same time as a result of the same events, like: In the early 19th century, looking strictly at the rocks, themselves, these were not easy to find, and stratigraphers had to address confusing aspects of the rock record like:

But as it happens, many fossils provide decent, widespread approximations of time-lines.

Index Fossils, Correlation, and the birth of Biostratigraphy

  • In 1796 William Smith, a British civil engineer, addressed this, adding the principle of Faunal succession to those of Steno and Hutton. Essentially, Smith noted that:

    In essence, changes in fossil faunas function as stratigraphic time lines.


    William Smith's geologic map from
    University of Nebraska Omaha - GEOL1170
    By noting the fossils present, it became possible to: In 1815, Smith published the first geologic map of England and Wales (or anything - right).


    The great thing: These correlations made the association of formation scale units into larger ones (the stages and systems on which the Geologic Time Scale is based) possible. Thus, essentially all stratigraphy above the formation scale depends on biostratigraphy.


    The ammonite Sphenodiscus, restricted to Maastrichthian Stage of Cretaceous.
    Which fossils do we use?

    Rock units are not time units!

    With Steno's and Smith's principles as a basis, geologists define a hierarchy of higher order rock units, including:

    Larger units need not be contiguous in space but are assumed to be contiguous in time. Their upper and lower boundaries must be instantaneous and isochronous. From these, we derive the Geologic Time scale, in which geochronologic Periods correspond to lithostratigraphic Systems.

    In GEOL 331 we assume that all students, regardless of background, are have memorized the The Geologic Time Scale - simplified version, and are able to interpret a detailed version.

    Remember, when we talk about a formation, stage, series, or system, we are talking about a bunch of rock, not an interval of time. The numerical dates that we place on their upper and lower boundaries are secondary to the identity of the rock units. For example, the formal base of the Jurassic is a stratotype - a specific contact in the Kuhjoch section of the Karwendel mountains in Austria, between rock units dated to 201.3 Ma. If we were to discover that Triassic rocks below that contact were actually only 200 Ma, we would not say, "Oops. I guess the rocks of the stratotype are Jurassic after all." Rather, we would revise the age of the boundary between the Triassic and Jurassic.

    Subsequent to Smith:

    So let's learn more about bio zones.

    Biozones - Rock units!

    Primary data of biostratigraphy: presence or absence of a fossil taxon in a geologic horizon

    Last Appearance Datum (LAD): either local or global

    First Appearance Datum (FAD): either local or global

    Biozone (often just "zone"): Rock unit characterized by one or more taxa that permit it to be distinguished from adjacent rocks.

    The following examples are drawn from the hypothetical microcosm at right. As we explore these, note that the definition of most biozones requires some element of uncertainly or inferrence.


    Types of Biostratigraphic units (and thus rock units):


    Abundance Zone of taxon B
  • Abundance Zone (also called Peak Zone, Acme Zone): Subset of teilzone where index species reaches some higher level of abundance: useful locally, but almost certainly environmental rather than time-related. That isn't to say that there is no time signal necessarily - Note examples of global changes in abundance due to global environmental changes. E.g.:

    Reasons for caution

    Biostratigraphy opened the door to global correlation of strata, but is, nevertheless subject to biases and filters that make it more reliable on a local scale than a global one.

    Biases of the Rock Record

    There are no perfect index fossils.

  • The rock record is incomplete and no rock unit preserves ancient environments on a global scale. Thus: impose their own non-biological signal. Consider:


    Henodus chelyops from Wikipedia

    Lesson - Evolution (without which biostratigraphy would be impossible) can be difficult to interpret.

    Mass Extinctions: Abrupt simultaneous extinctions of numerous taxa, if real, are powerful biostratigaphic markers and are interesting ecological phenomena in their own right. Alas, identifying them isn't straightforward. It requires continuous deposition and a reasonably dense fossil record.

    Confidence intervals: Until now we have concentrated on FADs and LADs, but actually every horizon in which a given taxon occurs is a datum that can be used to constrain its confidence interval statistically. A very sparse record yields wide 95% confidence intervals above and below observed FADs and LADs. A dense record yields narrow 95% confidence intervals. In a case of a single occurrance, the confidence interval is infinite.


    The coelacanth Latimeria from BBC Earth
    Lazarus Taxa: Taxa that temporarily "disappear" and then reappear in fossil record. This might be because of environmental changes, or local extirpation and re-immigration. E.g. North American horses, coelacanths (right).


    Miocene tiger shark fossil in Holocene sediments from Polar Trec
    Zombie effect: Post-extinction reworking of specimens into younger sediments (right). As in Fasstt et al., 2000, these can be significantly misleading.

    Bias of Evolution


    Flabellum, a modern scleractinian coral from Wikipedia
    Elvis Taxa: Taxa that converge on extinct forms, giving false impression of Lazarus taxa. (A particularly common problem with planktonic forams, whose morphology is strongly biomechanically constrained. Also reef forming organisms, consider Cambrian archeocyathid sponges, late Paleozoic rugose corals and Cretaceous rudistid clams.)


  • In a biological sense, FADs are arbitrary. Assuming that evolution is gradual, (no consensus on that!) how do we identify the first occurrence of a new species? Indeed, how do we distinguish an evolving lineage from a branching phylogeny (right)?

    Good News and Bad:

    The bad news: Constraints on biostratigraphy


    The good news:


    Biostratigraphic Conventions:

    Mindful of these considerations, we see why biostratigraphers employ a variety of zone definitions despite their invocation of conjecture and assumptions: In many circumstances, the ability to bring lots of data to bear on a problem is simply more important than the avoidance of the fuzziness that follows from inference and conjecture. The biostratigrapher seeks the optimal tradeoff for the specific situation.


    Lower boundary of Guadalupian Stage of Permian: Global Standard Stratotype and Point
    from ICS Subcommission on Permian Stratigraphy
    Biostratigraphic nomenclature and golden spikes:

    Biostratigraphy is the principal determinant of such important things as period boundaries. Boundaries between periods are arbitrarily decided, but usually involve biostratigraphic markers. Some conventions:

    E.G. the Silurian-Devonian boundary (right) was decided to be the base of the Monograptus uniformis (a graptolite) zone. Over forty such golden spikes have been established.

    But, how does this help us with the stratigraphy of environments in which fossil preservation is spotty and intermittent?

    Biochronology - Time Units:

    When biostratigraphic data is combined with numeric age information we can use biozones as the basis for biochrons - time units! (as opposed to biozones which are rock units)).


    Central Alaska during the Rancholabrean NALMA from Geo Currents
    One famous application: Land Vertebrate Ages. Originally developed during the early and mid-20th century as North American Land Mammal Ages (NALMAs), these were based on the recognition of the regular co-occurrance of distinct mammalian assemblages. NALMAs answered the need for some kind of biochronological tool for North American Cenozoic terrestrial rock units, but were limited by the fact that Cenozoic mammal fossils tended to occur in isolated pockets, often in places with little precise stratigraphic context. (Cave deposits, for example.) As a result, they could not be used as the basis for proper rigorous biozones based on the rocks in which they were found. Instead, they are tied to the time intervals in which the assemblages are inferred to have lived. For regions like the continental deposits of North America, however, NALMAs are much better than nothing, and more frequently mentioned than proper biozones. The NALMAs of the Pleistocene Epoch, for example are good examples and often cited. E.G.:

    Over a century of development, competing criteria were used in definitions of ages. Today, biostratigraphers must:

    Nevertheless, as a quick-and-dirty, NALMAs/Land Vertebrate Ages are sufficiently useful that they have been expanded globally and pushed confidently into the Late Cretaceous as Land Vertebrate Ages. But note: The use of Land Vertebrate Ages is a tradition among North American paleontologists and stratigraphers originally born of necessity but now, perhaps, just a tradition. Prothero, 1995 reviews the application of new data from improved radiometric dating and magnetostratigraphy, and concludes that the time is ripe for incorporation of NALMAs into proper biozone-based biostratigraphy.

    All rest on the assumption that biostratigraphic units are good proxies for time. As indicated above, this seems to be a good first-order approximation.

    Final thoughts

    Quantitative Biostratigraphy

    Besides hopefully constraining their age and sequence, does biostratigraphy add to our kowledge of the deposition of sediments? Actually, yes.

    Graphic correlation: method for stratigraphic correlation based on statistical correlation of first and last appearances, but not biozone terminology. Facilitates comparison of locality sections containing local FADs and LADs of the same taxa. Used to:

    Composite standards: The examples above correlate teilzones from pairs of localities. On a larger scale, the data used to achieve this can be combined into substantial composite standard databases that: Comparison of information from a particular locality with the composite standard facilitates robust identification of range extensions, hiatuses, and outliers.

    Moral: No method of biostratigraphy lacks significant biases and limitations, however biostratigraphy compensates by enabling stratigraphers to bring an overwhelming quantity of data to bear, making it cumulatively very powerful.

    Additional reading: