Wednesday, June 18, 2014

Ensnared in the Paris Basin

Everything in the world
has a name
if you know it.
You know that.
~ from the poem Mesh by Maureen N. McLane

This meandering tale of being caught up in the Paris Basin begins with two fossil snail shells, segues to some fossil foraminifera shells, and ends up with me dropping down a rabbit hole with the English geologist Charles Lyell (1797 – 1875).  I think what links these various elements (if anything does) is the struggle to name and understand the basis of names.

The Paris Basin, occupying much of north central France, is a 43,000 square mile, stable, low area in the Earth’s crust - literally, a basin.  For much of its geological history, reaching back to the Mesozoic, this cratonic basin lay under water, and, over time, filled with sedimentary rock, hence its value for the study of stratigraphy and fossils, as well as its attractiveness to those in pursuit of fossil fuels.

The map below of the Basin comes from The Eocene & Oligocene Beds of the Paris Basin, by George F. Harris and Henry W. Burrows, published in 1891.  Though I really like this map, it reproduces rather poorly.  I’ve circled some of the key cities to give a sense of its dimensions.  The circle a bit off center marks Paris; to the northwest of Paris is Rouen.  Le Mans is at the bottom of the map, well south and west of Paris, and Orleans is south and a bit west of Paris.  Finally, Reims is the marked city northeast of Paris.

My interest in the Basin was prompted by the gift of a couple of beautiful, small fossil shells.  These shells from the gastropod Rimella fissurella were found at a site in the Basin near the village of Damery (some 85 miles northeast of Paris, its nearest large city is Reims).  They are dated from the Eocene Epoch’s Lutetian Age (47.8 to 41.3 million years ago), an age with a name honoring Paris.  Lutetia is what the Romans called the town that ultimately became Paris.

Some matrix still clings to the exterior of these shells.  The tiny amount which I extracted from the shells’ interiors contained a staggering number of fossil shells from foraminifera, those ubiquitous, marine, single-celled protists.  A majority of these forams come from a group of distinctive benthic forams, the miliolids.  Their shells frequently consist of elongate, entwined chambers, the so-called mioline arrangement.  Here are a few examples of the multitude that appeared in a trickle of that matrix.

I tentatively place each of these specimens in the following genera:  the first image is Triloculina, the second is Miliola, and the third is perhaps Quinqueloculina.  Be warned, I'm no expert at identifying genera among the foraminifera.

The taxonomic reality of the foraminifera is in flux which compounds my difficulties.  Names and placement in the taxonomic hierarchy matter and some of these for the forams seem particularly fluid.  For some experts, the miliolids are members of the Miliolida Order.  But that would mean that the forams in general belong to some taxonomic level above an order, a placement that others contest because they consider the entire taxon of foraminifera to be itself an order.  As a result, they might place the miliolids in the Suborder Miliolina.  It does get complicated.

The Eocene, according to paleontologists Alfred R. Loeblich, Jr., and Helen Tappan, was “marked by a great expansion of foraminifers.”  The miliolids, among forams, hold a particular place in the Paris Basin.  Loeblich and Tappan noted that the “miliolid limestones were deposited in the Eocene of France.”  (Treatise on Invertebrate Paleontology, Part C, Protista 2, Sarcodina, Chiefly “Thecamoebians” and Foraminiferida, Volume 1, 1964, p. C139.)

To my great surprise and pleasure, I discovered that, at one point in his seminal work, Principles of Geology, Charles Lyell focused on the microscopic shells found in the Paris Basin, specifically those showing up in great numbers in the Basin’s Calcaire Grossier (coarse limestone), an Eocene rock formation (volume 4, third edition, 1834).  He included a plate which illustrated several.

He identified the different views in the top row of this plate (1-3) as those of Triloculina inflata, while those in the second row (5-8) were labeled Quinqueloculina striata.  (The image above of the plate is included in the New York Public Library’s Digital Gallery - Pictures of Science:  700 Years of Scientific and Medical Illustration.)

Lyell wrote that, for this plate, conchologist Paul Gérard Deshayes (1797 – 1875) had "carefully selected the most remarkable types of form.”  Of the various taxa depicted, Lyell observed that they are
common in the Paris basin and may be considered as characteristic of the Eocene period generally.  They appear as yet to be exclusively confined to deposits of that period, and are for the most part abundant in them wherever they have been attentively studied.  (Volume 4, p. 123.)
It’s at this point that I found myself slipping down the rabbit hole with Lyell.  When I first read the passage just quoted, I noticed nothing exceptional about it – here Lyell asserted that the Eocene was marked in particular by the presence of specific fossils.  Having barely cracked the several volumes of the Principles of Geology, I assumed this passage reflected a key element of Lyell’s process for identifying and naming divisions of the Tertiary Period – linking specific fossils to different strata.  But that, apparently, would be to misread him quite seriously.

In the Principles of Geology, Lyell named the Pliocene, Miocene, and Eocene as divisions of the Tertiary Period.  This period label (Tertiary) is no longer an officially accepted one and, indeed, the period has been divided and the new divisions named the Paleogene Period, 66.0 to 23.03 million years ago, and the Neogene Period, 23.03 to 2.58 million years ago.

Lyell relied on a mathematical analysis of an inventory of species whose fossils were found in the Paris Basin.  The key to identifying these different segments of the Tertiary was the percentage of fossil shellfish species found in each that was still around (extant).  As he put it, he was “classing the different tertiary groups by reference to the proportional number of recent species found fossil in each.”  (Principles of Geology, Volume 3, Fourth Edition, 1835, p. 385-386.)  For instance, based on his data, in the strata he named Miocene, roughly 19 percent of fossil mollusc species were extant, as were some 3 percent in the Eocene.  (Percentages as cited by Donald R. Prothero in The Eocene-Oligocene Transition:  Paradise Lost, 1994, p. 41.)

According to historian Martin J. S. Rudwick, Lyell was actually engaged in promoting a “statistical paleontology” where the percentage of a fossil faunal assemblage that had survived to the present would function as a “chronometer.”  (M.J.S. Rudwick, Charles Lyell’s Dream of a Statistical Palaeontology, Palaeontology, Volume 21, Part 2, 1978.)  How close a strata’s assemblage was to completely turning over (that is, containing no extinct species) was a gauge of how near the strata was to the present.  Presumably, strata in different locations could be correlated by this method and, in theory, the “chronometer” could be reset to use a different, earlier point in time as the baseline and so be applied still further back in time.

Significantly, with these new divisions of the Tertiary (Pliocene, Miocene, and Eocene), Lyell wasn’t trying to describe the entire expanse of the time of the Tertiary.  Rather, as depicted by Rudwick, Lyell believed he was identifying specific “moments” or “points in time” in the Tertiary and that vast stretches of time separated those individual moments.

If Rudwick’s analysis of what Lyell was actually about is correct, I suspect that it has been largely missed by many of the Principles’ readers.

Lyell himself identified one potential (but evidently not, in his eyes, fatal) flaw with this method of correlating such moments across geological formations:
The reader will already perceive that the systemic arrangement of strata, so far as it rests on organic remains, must depend essentially on the accurate determination of species; and the geologist must therefore have recourse to the ablest naturalists, devoted to the study of certain departments of organic nature.  (Principles of Geology, Volume 3, p. 383.)
Recently, paleontologist Donald Prothero identified several of the fundamental problems with Lyell’s approach.  He noted that throughout the rest of the geologic time scale, specific rocks and specific fossils are used to identify geologic units.  "A geologist can actually stand on the outcrop and collect samples of fossils from the internationally agreed 'standard.'  Lyell's conception was not based on a rock unit per se, but on an abstract concept of molluscan change."  (The Eocene-Oligocene Transition:  Paradise Lost, p. 41.)

As for that "molluscan change," the rate at which mollusc species die off is not consistent or linear, making it inappropriate for the basis of a “chronometer.”  Also, the identifications of individual species upon which the system rests are, themselves, subject to change as different scientists try their hand at the taxonomy and past designations are rethought (in this last criticism, Prothero expanded on the issue of the identification accuracy that Lyell had acknowledged).  Prothero went further:
Finally, the most serious problem is that most of the Paris Basin molluscs are restricted to that region and cannot be recognized elsewhere, so it is impossible to correlate [using this method] with any other region.  (p. 42.)
Perhaps most telling, Rudwick observed that Lyell never sought to apply this method to earlier time periods (apparently with good reason).

And, so, the method failed but the names remained.  You know that.

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