The Enigmatic Scaphopoda Reflect the Downside of Distinctiveness

I find the members of the Scaphopoda Class of mollusks to be beautiful and quite distinctive.  This post is partly about how that distinctiveness may work against them in paleontological terms.

My interest in these animals was sparked by several fossil shells (shown below) from Dentalium attenuatum Say 1824, an extinct member of this class.  These were a gift from a friend who collected them at Windmill Point on the St. Marys River.  The formation shedding fossil shells at this location is considered Upper Miocene, deposited about 11.6-5.6 million years ago.

Marine biologist Helen Scales, when describing the various classes of mollusks, had this to say of the members of the Scaphopoda Class:

Scaphopods or tusk shells are fairly self-explanatory:  they look like little tusks.  Often they live buried in the seabed, head down, with the tips of their shells poking out.  (Spirals in Time:  The Secret Life and Curious Afterlife of Seashells, 2015, p. 27.)

In a wonderful aside in her discussion of the changing thinking about the relationships among different taxa of the mollusk family, she wrote of Scaphopoda:

The scaphopods continue to be a pain in the neck to identify; we simply don't know enough about them to be sure where exactly they fit in.  (Scales, p. 35.)

Though there is uncertainty about the kinship of scaphopods with all of the other mollusk taxa, there is no doubt that they belong in the mollusca phylum.  (Scales, p. 28.)  Further, we also know that they are the newest of mollusk classes to show up in the fossil record, appearing perhaps as early as the Mississippian in the Carboniferous Period, about 362.5 million years ago.  (Patrick D. Reynolds, The Scaphopoda, Advances in Marine Biology, Volume 42, 2002, p. 139.)

To elaborate on Helen Scales' brief description, each of the extinct and extant scaphopods generally positions itself in sea bottom sediment with its larger opening (the aperture) buried and its narrower opening (the apex) exposed.  The aperture houses the animal's "creeping" foot and its mouth.  Water is drawn in through the exposed apex and waste is excreted from the same opening.  (See descriptions of tusk shells in:   National Audubon Society Field Guide to North American Seashells by Harald A. Rehder, 1981, p. 655; and Invertebrate Fossils by Raymond C. Moore, et al., 1952, p. 273-275.)

I think it undeniable that the Scaphopoda Class, as a whole, is distinctive:   shells of all members share the same morphology, one which readily distinguishes them from other mollusks.  At the same time, it may be challenging to differentiate among species within the class.  That morphological sameness has long been noted as a potential issue.  For example, when the D. attenuatum shells were first described in 1824 by conchologist and entomologist Thomas Say (1787-1834), he noted of the Dentalium genera as a whole:

The species of this genus are very closely allied to each other, and at the same time they exhibit so few characters, that it is with difficulty that some of them are determined.  (An Account of Some of the Fossil Shells of Maryland, Journal of the Academy of Natural Sciences of Philadelphia, Volume IV, Part 1, 1824, p. 155.)

They exhibit so few characters:  from a taxonomic perspective, that's a fairly damning statement.  Later, paleontologist Raymond Moore et al., in Invertebrate Fossils (cited above), characterized the Scaphopoda as being "simple tapered tubular shells [exhibiting] few sorts of difference other than size and surface markings."  (Moore, p. 275)  The authors,  prompted by the general sameness of the various species in this class, concluded that these shells "are not very useful as index fossils."  (Moore, p. 275)

Hmmm.  Though their dismissal of the shells of scaphopoda as potential index fossils seems to have been sparked solely by the relative uniformity of the morphology of the individual species in the class, Moore et al. may not have been wrong about the possible utility of these shells as index fossils.  I turned to Hervey Woodburn Shimer and Robert Rakes Shrock, who, in their Index Fossils of North America (1944), defined an index fossil as:

one which identifies and dates the strata or succession of strata in which it lies.  The ideal index species has narrow or restricted stratigraphic range and broad, even worldwide geographic distribution. . . .  A genus which has narrow stratigraphic range and rather broad geographic distribution is now considered an index fossil.  (Shimer and Shrock, p. 1.)

There's nothing in that definition about whether the difficulty of distinguishing one species from another might affect what could be considered an index fossil, though that could be the case.  The actual focus of Shimer and Shrock's definition is on the temporal and geographic distribution of a species:

Is it found in a short and well defined interval in the stratigraphic column and, so, could it be used to identify and date the interval?

Is it distributed widely enough that it could be useful for correlating formations across a broad geographic range?

I wondered what we do know of the distribution of the scaphopods in the Maryland Miocene formations.   Of this class of animal here, paleontologist Harold E. Vokes et al., in their useful Miocene Fossils of Maryland (Maryland Geological Survey, second edition 1999), wrote that the scaphopods "occur throughout the [Maryland] Miocene deposits."  (Vokes, p. 18.)  I became hung up over that "throughout."  Undoubtedly, this does not mean that each species in this class is found everywhere in the stratigraphic column.  So what does it mean?

The seminal work on the Maryland Miocene published by the Maryland Geological Survey in 1904 is somewhat helpful in teasing this out.  (Miocene, Text, Maryland Geological Survey, 1904, p. eviii-evix.)  Five scaphopod species are identified in that publication.  (I think it interesting that there is no statement in this volume regarding any particular difficulty in distinguishing among these species.)  Data presented on the distribution of these five species among the Maryland Miocene formations show they may be widely, though unevenly, distributed in these formations.  The distribution of these species across the three formations covered in the report is this:  three of the species had been collected in localities in all three formations, two of the species were collected in only one formation (a different one for each).  Does that pattern offer any support to using scaphopod species as index fossils or militate against such an application?  Frankly, I don't think the data from this particular publication are sufficient to inform such a decision.

A better sense of the adequacy, or, rather, inadequacy, of the scaphopods as potential index fossils may be gained from their complete absence in the stratigraphic work on the mid-Atlantic coastal Miocene formations by paleontologist Lauck W. Ward.  In Molluscan Biostratigraphy of the Miocene, Middle Atlantic Coastal Plain of North America (Virginia Museum of Natural History, Memoir Number 2, 1992), he proposed stratigraphic zones for these formations based on the distribution of various mollusk taxa, defining the boundaries of zones in his schema by the first appearance of a mollusk taxon in the stratigraphic column.  For instance, the zone from which the D. attenuatum shells in my collection came is the Chesapecten/Glossus fraterna Interval-zone, which begins where fossil shells of the species Chesapecten santamaria first appear, and ends where fossil shells of Glossus fraterna enter the fossil record here.  None of Ward's zones in this work involve scaphopods.  (There is also no mention of scaphopods in Ward's later work with George W. Andrews titled Stratigraphy of the Calvert, Choptank, and St. Marys Formations (Miocene) in the Chesapeake Bay Area, Maryland and Virginia (Virginia Museum of Natural History, Memoir Number 9, 2008, p. 48).)

Ward characterized the mollusk species that he used in setting the boundaries for the zones as "well represented."  Further, confidence in appropriateness of these taxa to mark zone boundaries was heightened

because of the relatively complete phyletic record of the aforementioned taxa and because this record is found over a wide geographic area.  (Ward, Molluscan Biostratigraphy, p. 9.)

The scaphopoda species are missing because they, in general, did not meet these index standards.  (Ward indicated as much in a private communication.)  I would note that, for each molluscan zone, he also listed mollusk species restricted to that zone, as well as, various mollusk species common in, but not restricted to, that zone.  Scaphopods are excluded from those lists as well.

In sum, it would appear that our knowledge of scaphophod fossils is limited, here in Maryland and elsewhere.  Further, biologist Patrick D. Reynolds has argued that the morphological sameness of the scaphopods, coupled with the taxon's uncertain phylogeny in the mollusk family tree, "renders the scaphopods an enigmatic group."  (Reynolds, p. 139, emphasis added.)  Indeed.

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, not 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.

The Methodology

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 Photoshop 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, Curator, 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, and 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 pattern not the colors 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 Photoshop, 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 patterns, 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 Photoshop Essentials, 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, Terebra simplex, 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 Paleobiology Database states that Terebra simplex has been “recombined” as Laevihastula simplex.)

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 Terebra shells, a group of which is shown below.

From left to right, we have T. amanda, T. lima, T. nebulosa, T. nitida, T. triseriata, T. punctatostriata, and T. pymgmaea.  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 Questions

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.