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Color Patterns in Fossil Gastropods ~ This Citizen Scientist Comes Up Mostly Empty
Fossil mollusk shells can be compared to their modern day counterparts in a wide variety of ways, but, with rare exceptions,
when it comes to color on the shells or the patterns of those colors. For the most part, fossils shells offer little visible trace of the color characteristics they sported in life, robbing paleontologists of a tool used by zoologists as they distinguish among living species.
This post first explores a method being used to draw out color patterns in fossil shells, a method that has a fairly long history. My efforts to duplicate that method are then described. Finally, as is usually the case for my explorations of natural history, I give play to several unanswered questions. In this case, ones that are quite fundamental in nature.
In 2015, paleontologist Jonathan R. Hendricks published the results of an analysis of the color patterns revealed in 359 fossil specimens of conus shells from the late Miocene and early Pliocene. (Glowing Seashells: Diversity of Fossilized Coloration Patterns on Coral Reef-Associated Cone Snail (gastropods: Conidae) Shells From the Neogene of the Dominican Republic, PLOS One, April 1, 2015.) He chose cone shells in part because of the richness of the color patterns that often mark them, and he used his results to regroup certain cone shells and identify several new species. The study was widely cited in the popular science press and in the general press.
To expose color patterns, Hendricks soaked shells in a solution of sodium hypochlorite (bleach), exposed them to UV light, and photographed the specimens as they fluoresced under the UV light. He then adjusted each UV photograph using a
function (“invert”) which, essentially, produces the negative of the image. As a result, the areas that fluoresced white took on color while the areas that had been dark in the original image had their color washed out. This last step is important. As Katherine Kamp Krueger, then The Field Museum’s custodian of paleontology collections, wrote in her seminal article of 1974 on the basic methodology that Hendricks would later follow, the conversion to a negative places a “dark color on light background (the true arrangement).” (The Use of Ultraviolet Light in the Study of Fossil Shells,
, Volume 17, Number 1, 1974. It resides behind a paywall which I briefly lowered at a minimal cost in order to read it.)
The picture below is from Hendricks’ study. It shows three cone shells. From the top, they are seen: under regular light, under UV light, and in negative images. The three species are from left to right:
Conus (Lautoconus?) carlottae
Conus (Dauciconus) garrisoni
Conus (Stephanoconus) bellacoensis
(Hendricks’ article, Glowing Seashells: Diversity of Fossilized Coloration Patterns . . . , has been distributed under the
Creative Commons Attribution License
which I understand permits my use of this image. See article.)
In her article, Krueger described how, in the 1960s, Axel Olsson experimented with a wide variety of reagents in an effort to enhance fossils’ reaction to UV light. He settled on chlorine bleach which strengthened fluorescence in shells that already exhibited some and drew it out of shells that had not previously fluoresced. Krueger expanded on Olsson’s efforts, reporting that a stronger bleach solution and a longer soaking time greatly enhanced the results. Further, she noted that bleach did not harm the shells, a critical reassurance for my subsequent effort to duplicate this methodology.
Krueger stressed that what’s revealed through this methodology is the
that once graced the shells.
In the intervening half century and more from the work by Olsson and Krueger to Hendricks’ piece in 2015, it would appear that, although the core equipment employed, specifically the cameras, has improved significantly, the basic methodology is unchanged. Oh, to be sure, the complicated process Krueger described for generating a negative image from a UV image is a thing of the past. Photo editing software, such as
, generates a negative image at the click of a mouse. Nevertheless, Krueger would recognize the steps Hendricks followed.
As the picture above shows, the results Hendricks achieved were stunning. Ironically, it’s the very attractiveness of the color images of the inverted UV pictures that troubles me. Make no mistake, Hendricks carefully and clearly stated that the results attained by this methodology reveal color patterns, not the actual colors. Nevertheless, he decided to publish the reversed images in color because, he argued, variation in colors on individual shell may mark some differences in the underlying pigment elements. That seems reasonable, but it’s not a convincing justification in this instance because I don’t think the analysis in his article rested on such color variability. Better, I think, to have published the negatives in grayscale and not run the risk of his findings being misinterpreted.
And, yes, they were misinterpreted. For example, there’s the article by Marissa Fessenden that appeared in the
Smithsonian Magazine’s Smart News
on April 2, 2015. The title says it all: UV Light Reveals the Colors of Fossil Shells. Sure the title may have been composed by an editor and the subtitle mentions color
, but, in the text of the article, Fessenden wrote: “Now researchers are using UV light to coax color from porcelain white seashell fossils.” Though it would have cost him eyeballs, Hendricks might have more seriously considered going with grayscale.
My Efforts to Apply This Methodology
I was encouraged enough by this literature to think that I could replicate the results others had obtained. Hey, I had fossil gastropods; a handheld long wave UV light; bleach in the laundry room, an iPhone with a pretty capable camera, and photo editing software, including
, with an invert or negative function. How could I fail to succeed?
My beta testing of the methodology and my equipment started with the fossil shell of a small snail,
, found in the Little Cove Point bed of the St. Marys Formation on the western shore of the Chesapeake. This auger shell is of late Miocene age (roughly 11.6 to 5.3 million years ago). (I would note that the
has been “recombined” as
The photograph below shows three images of the shell. From left to right, we see the shell: under normal light, under UV light, and as a negative of the UV picture in grayscale.
I was particularly struck by how the large bottom whorl is marked by two patches of coloring, a division of which the specimen under normal light gives no hint. Further, I was impressed that the pattern elicited by the UV light in this specimen is somewhat similar to patterns seen in several modern
shells, a group of which is shown below.
From left to right, we have
. This image is from the richly illustrated and informative website
The Seashells of New South Wales.
Its author, Des Beechey, Senior Fellow at the Australian Museum, generously gave me permission for its use.
Although not dramatic, my results were sufficiently encouraging to go forward with a broader test.
I assembled a clutch of Miocene fossil gastropod shells. I soaked shells over several days and then examined them under UV light. The result were, to say the least, very disappointing. The shells refused to do more than offer up minute snatches of fluorescence which I couldn’t capture with my camera. Sadly, there is nothing to show from the full implementation.
In retrospect this probably should have been expected. Besides the serious limitations of my equipment, the methodology actually has a fairly high risk of failure. Hendricks noted that of the shells he analyzed, only some 60 percent “showed well-preserved evidence of their ancient coloration patterns when viewed under UV light.”
I would consider another round but, despite taking precautions, I worry about an aspect of this methodology. Playing around with ultraviolet light, even of the long wave variety in “black lights,” carries a significant risk to one’s eyes and skin. Most of the authors whose works I read in preparation for this project warned of these dangers in quite stark terms.
The more I read about the use of UV light to generate ancient color patterns the more curious I found the entire situation. Here we have a methodology which many believe accomplishes an important and quite useful objective. At the same time, the foundation upon which this methodology is based is riddled with several fundamental questions. Yes, it may do what its advocates have said it does, but looming over it all is the question: Why?
Actually, it's a series of multifaceted "whys" which the advocates clearly acknowledge. For instance, in her 1974 piece, Krueger posited that UV light excited electrons in pigment material remaining in the fossil shells which, when they fell back to their previous state, emitted energy in the form of light. But she admitted that there were myriad unknowns about this phenomenon including the following: Why does the pigment material fluoresce in fossil shells but not in modern shells? Why does bleach enhance or generate fluorescence in this material?
Further, Krueger noted that, while modern shells exposed for lengths of time to sunlight lose their color patterns through oxidation, the bleach soaking also acting, she thought, as an oxidizing agent surprisingly regenerated the lost color patterns.
With Krueger's questions in mind, consider what Hendricks wrote many decades later in his paper:
While the use of UV light to reveal ancient shell coloration patterns has proved to be a useful technique for understanding the systematics of some fossil mollusks,
we still do not have a clear understanding of exactly what compounds are responsible for pigmentation in modern shells, much less what matter is actually fluorescing in the fossil shells
. (emphasis added)He continued:
. . ., it seems that oxidation – caused either naturally by exposure of shells to sunlight over prolonged periods of time, or artificially by soaking in bleach for several days – plays a role in causing formerly pigmented regions of fossil shells to fluoresce under UV light, though
the reason is not currently understood
. (emphasis added)Sobering that such questions remain unanswered.
Actually, there’s an even more fundamental issue that has to be addressed:
Why do shells in living mollusks exhibit such a bewildering array of intricate color patterns
in the first place
?This is a particularly pertinent question given that many mollusks with colorful shells spend their lives under cover, either hidden away in the bottom sediment of their watery environment or under a drab and fuzzy protein layer (the periostracum).
Marine biologist Helen Scales, in her recent book titled
Spirals in Time: The Secret Life and Curious Afterlife of Seashells
(2015), reviewed the progression of research on the why of seashell color patterns. For a long time, such patterns were considered inconsequential and, because they were believed to be unaffected by natural selection, were thought to be unconstrained as to their diversity. More recent work challenges that perspective as scientists explore more deeply the process through which the patterns are actually created. An emerging explanation is that specific pigment-generating cells on the outer edge of mollusks’ mantle, where new shell material is laid down, are neurologically triggered to produce pigment. The suggestion here is that these triggers may come from the mantle’s “reading” of the past record of color patterns which, in essence, identifies the correct place in which to lay down new shell and, in the process, extend the previous pattern. Scales wrote that we may come to understand
. . . that shell patterns aren’t frivolous playthings but important registration markers for shell-making that have been subject to the forces of natural selection, and have evolved over time. It may not matter exactly what kind of patterns are made, as long as there is some way for a mollusc to figure out where to put its mantle before continuing to make shell. (p. 75)A thought-provoking hypothesis.