Course Introduction: What Good is the Fossil Record?


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Fossils: physical remains of organisms or traces of their behavior preserved in the rock record. Fossils can be roughly divided into:

Paleontology is the scientific study of fossils. More broadly, it is the study of ancient organisms.

One traditional way of dividing up paleontology is along taxonomic lines (i.e., what part of the Tree of Life is being studied). Indeed, professional societies and publications tend to fall along these categories. These general categories are:

However, this is not the only way to divide up the field. Another approach is to consider the types of questions being asked about the fossils:

Paleontology is a science at the cusp of two other larger fields: biology and geology. Because of this, essentially no university has a "Department of Paleontology". Instead, paleontologists might be in either a geology department, a biology department, or both. (A note: in contrast, many natural history museums actually do have full "Departments of Paleontology".).

Paleontologists are expected to know material from both their parent disciplines. From Biology they need information about (and help inform our understanding of) general organismal biology (zoology, botany, etc.), evolutionary biology, ecology, etc. From Geology they need to learn sedimentology, stratigraphy, geochronology, geochemisty, paleonenvironmental analysis, and paleoclimatology. But any given researcher is likely to concentrate on just a few of these for their particular work.


What Good is the Fossil Record?
Why should we study fossils? What sort of information do we get from the remains of ancient organisms?

First and foremost, fossils show that life in the past was different than today. If it were not for fossils, there would be no reason to assume that living things had a history (and that said history involved species different than living ones).

Additionally, fossils are our only indication of a succession of ancient forms of life through time. In other words, there was not simply a "prehistoric world", but a succession of "prehistoric worlds".

Fossils document the origins and evolution of the various branches of the Tree of Life. Although we can use the morphology and genomes of extant animals to reconstruct the basic relationships among organisms, living forms do not show the earlier stages of anatomical transitions from their common ancestors. In fact, there are cases where simply looking at modern taxa might be positively misleading (e.g., the transition to a single functional jaw bone in the lower jaw of modern mammals would seem to have come from a single ancestor using only living forms, but is demonstrably convergent between monotremes and the marsupial-placental group when fossils are added.)

Fossils are our only evidence of entirely extinct branches of the Tree of Life. One might be able to predict what the common ancestor of birds and crocodiles was like, or of arachnids and crustaceans, and so forth using living animals (maybe...), but that approach would never reveal the existence of (for instance) sauropod dinosaurs and pterosaurs on the one hand, or trilobites and eurypterids in the second.

The fossil record is the only evidence of mass extinctions. While extinctions of individual species, or even limited extinctions (for example, on islands), have happened within human experience, we would never know that the entire biosphere can undergo tremendous crashes requiring millions of years to recover without fossils. And from that we would never know HOW severe rapid and/or intense environmental changes can be to ecosystems.

The fossil records provides some of the best evidence of environmental change and the only evidence of how organisms respond to environmental change on the long time scale. There are many different methods to reconstruct the climates of the ancient past. At least some of them use the presence/absence, distribution, or isotopic composition of various sorts of fossils.

For a pragmatic issue, fossils are the source of a considerable amount of our energy supply. Coal is comprised of the body fossils of plants; petroleum and natural gas are the decay products of buried organic material (and thus body fossils on at least the chemical level).

And fossils represent aesthetically pleasing objects. We have national and state/provincial parks which highlight beautiful and/or wonderful fossils, and many people collect and display fossils because they are intriguing. In fact, people have been doing this since before we were Homo sapiens.


The Pull of the Recent
The fossil record is thus our window onto the history of the life. But how clear is this window? That is, how accurately does the preserved fossil record document the real history of living things.

The basic pattern of the diversity of marine fossils shows an increasing number of taxa, punctuated by mass extinctions. One of the biggest (and unresolved) issues in paleontology is: are there really more species in the modern world? Or is there simply a better record for younger fossils? The latter idea has been termed "The Pull of the Recent" (note: The Recent (capitalized) was a traditional name for the present epoch of geologic time, now formally called the Holocene.)

There are a lot of merits to the concept of the Pull of the Recent:

In fact, some attempts to take these (and other) factors into account suggest that the increase is less profound than previously thought.

However, other studies show that there is a qualitative distinction between ancient (e.g., Ordovician) vs. younger (e.g., Neogene) fossil communities. Examination of well-sampled faunas show that there really does seem to be a distinction in the single-site diversity [number of species] of lower-diversity ancient and higher-diversity modern communities.

We will re-examine these issues at the end of the course. But first, we need to look at how fossils form.


Geologic Timescale
Just as a reminder, all GEOL331 students should be familiar with the geologic time scale, and most especially the Phanerozoic:

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Last modified: 19 August 2016