Interpreting Fossil Tableaus Frozen in Time

No, this post isn’t about the polar vortex.

I’ve come to the conclusion that one of paleontology’s core missions is the reconstruction of ancient communities of plants and animals.  At the heart of this effort is the challenge of deciphering and understanding what the fossil record says about the biological communities of organisms that lived and interacted in specific places in deep time.

One key to meeting this challenge is the field of taphonomy, first defined by paleontologist J.A. Efremov as:  “. . . the study of the transition (in all its details) of animal remains from the biosphere into the lithosphere . . . .”  In essence, analysis of how organic remains become rock.  (Taphonomy:  New Branch of Paleontology, Pan-American Geologist, September 1940.)  The utility of this field of study is how it enables us to determine, as paleontologist Anna K. Behrensmeyer and her colleagues have written, “how faithfully biological history has been recorded” in the fossil record.  (Taphonomy and Paleobiology, Paleobiology, January 2000.)

Not surprisingly, it turns out we must approach the fossil record with great care because what we see in the fossils from any specific site may not provide a reliable picture of the biological reality at that time in that place.

The particular interest that prompts this post is quite narrow and, as it often true of this blog, idiosyncratic.  I am wrestling with a few of the basic questions that might be asked of fossils that offer the remains of organisms affixed to each other, in other words, specimens and moments frozen in a fossil tableau.  Two such are pictured below.

The first picture shows the exterior and interior of a shell from the scallop Chesapecten nefrens.  This shell measures roughly 4.5 inches vertically and was found at the Scientists’ Cliffs portion (Choptank Formation) of the Calvert Cliffs on the western shore of the Chesapeake Bay.  Roughly 12 to 16 million years old, the scallop sports the remains of a number of barnacles, including remnants of the basal edges of several barnacle shells.  I won’t hazard an opinion on the species (one or more) of barnacle that encrusts this scallop.

The second photo shows two views of a 3.25 inch tall shell of the gastropod Ecphora gardnerae germonae.  An amazing gift from a friend, this specimen comes from the St. Mary’s Formation, further south along the Calvert Cliffs than the scallop, and is approximately 8 to 10 million years old.  The basal edges of barnacle shells abound on this Ecphora specimen.  Again, I’ve made no effort to identify the species of encrusting barnacle.

And here’s my rather simplistic set of questions.  For fossil specimens like those shown above, what can we say about the interaction of the different species that, through fossilization, are fixed in this kind of direct relationship with each other?  Did these individual specimens live at the exact same time in the same place?  Were the hosts alive when their “guests” took up residence?

My assumption has been that, in most cases, the best we can do is draw from modern analogs of the interaction of similar kinds of organisms and only offer educated suppositions about what these tableaus from deep time are telling us.  But I should have realized that brilliant people find brilliant ways to tease out greater meaning than the rest of us are resigned to.

An article that only recently came my way offers a kind of direct response to my assumption.  The renowned evolutionary biologist Geerat Vermeij has coauthored a fascinating piece on a specific species of Miocene barnacle, Chesaconcavus chesapeakensis, and its interactions with the gastropod Conradconfusus parilis and the scallop Chesapecten santamaria.  (Vermeij and Sara K. Ruch, Barnacles, Their Molluscan Hosts, and Comparative Ecology in the St. Mary’s Formation (Late Miocene) of Maryland, USA, Journal of Paleontology, Volume 92, Number 2, 2018.)  These species lived during the Tortonian age of the Miocene epoch, between 7 and 12 million years ago.

Working with a collection of these fossil species, Vermeij and Ruch concluded that representatives of this barnacle species came to be anchored on a high percentage of the fossil specimens of C. parilis and C. santamaria they had for analysis, and did so only when those gastropods and scallops were alive.  No question in their minds that these barnacles settled on the snail and mollusc shells when the hosts were definitely going about their daily business.

How can these scientists know that these hosts were alive, carrying living barnacles on their shells?

Their logic is simple and, to me, persuasive.  Consider, first, the barnacles and gastropods.  Vermeij and Ruch found that, of the gastropod specimens in their study collection that sported barnacles, none had barnacles encroaching on their hosts' apertures.  Had the barnacles done so, they would have interfered markedly with their hosts’ ability to survive.  No barnacles were found to have taken up residence on the interior of the gastropod shells.  Further, and, to my mind, quite telling, the shape of many of the barnacles was curved toward the growing edge of the gastropod shells signaling that they were growing even as their hosts, themselves, grew.

As for the barnacles on the scallops, they, too, refrained from overlapping the outer edges of the shells, so did not impede the normal opening and closing of the articulated valves.  Also, in no case, did barnacles inhabit the interiors.

Accepting the logic of this analysis, one obvious takeaway is that clearly the species under study lived at precisely the same time and in the same place.  That is, they were members of the same community, interacting as members of biological communities do.  Quite beautiful I think.

But I hesitate to take too much inspiration from the work of Vermeij and Ruch because that might mean I’d have to consider approaching the specimens depicted in the tableaus presented earlier with their analysis in mind.  My fossils are, after all, single samples, not part of a broader representative collection of Chesapecten or Ecphora that might be subjected to the methodology these scientists brought to bear.  Further, there are many basic pieces of information I’m missing (e.g., what species of barnacles are these?).  I’m smart enough to know my limits and am content just to enjoy the Vermeij and Ruch tour de force.

A Little Bit of Big Data

Data crunching.  Over the many years that I’ve written this blog, I’ve touched on different aspects of paleontology and the activities of paleontologists but, for the most part, I’ve ignored one kind of endeavor that has increasingly marked this field in recent decades and that is data crunching.

It’s puzzling that I’ve had this blind spot because for most of those years I’ve been a volunteer at the Smithsonian’s National Museum of Natural History assisting projects that are quintessentially data-driven efforts.  In a post several years ago, I noted that the popular perception of how paleontologists spend their time conflicted with the reality of what such scientists actually did.  Every hour spent in the field collecting fossils, generated many hours of lab work, prepping and studying each fossil found.  My take on this issue was certainly incomplete because at that juncture I made no mention of the very different way in which, at present, many paleontologists endeavor to extract meaning from fossils, that is:  staring into a computer screen while manipulating large, often complex sets of data derived from collected fossils.  Yes, “big data” is now very much a part of the work that paleontologists writ large do.

Science historian David Sepkoski, in a delightful commentary on big data in paleontology, writes from a very personal perspective.  His father, paleontologist Jack Sepkoski (who died much too young at age 50 in 1999), made seminal contributions to our understanding of the diversity of life on earth and on patterns of extinction by carefully compiling and analyzing extensive data on marine families and genera.  As a child, David saw his father as an Indiana Jones figure (though the latter was an archaeologist):

The illusion was shattered some years later when I figured out what he actually did:  far from spending his time climbing dangerous cliffs and digging up dinosaurs, Jack Sepkoski spent most of his career in front of a computer, building what would become the first comprehensive database on the fossil record of life.  The analysis that he and his colleagues performed revealed new understandings of phenomena such as diversification and extinction, and changed the way that paleontologists work.  But he was about as different from Indiana Jones as you can get.  (What a Fossil Revolution Reveals About the History of “Big Data”, Aeon, February 12, 2018.)

By building this database and using computer power to manipulate it, Jack Sepkoski and his colleague paleontologist David M. Raup were able to discern the major extinction events that have punctuated life on Earth from deep time to the present.  The ability to assemble a vast quantity of data and analyze it has transformed the research avenues explored by paleontologists.   In an interview, Raup’s wife said of her husband, “He used to say he went into paleontology because it was a field with a lot of data that no one was analyzing.” (Bruce Weber, David M. Raup, Who Transformed Field of Paleontology, Dies at 82, New York Times, July 15, 2015.)  That’s certainly not true now.

A look backward is in order at this juncture.  The manipulation of data to discern patterns in the fossil record isn’t what’s new about this phenomenon, rather, it’s how prevalent it’s become.  I’m quite taken by the work that British geologist John Phillips did in the middle of the 19th century analyzing the data he had available to him at that point to depict the relative abundance of species over time.  Perhaps, most importantly, in his 1860 volume titled Life on Earth:  Its Origin and Succession, Phillips published what paleontologist Douglas H. Erwin has called “the first illustration of the diversity of life through time.”  (Erwin, Extinction:  How Life on Earth Nearly Ended 250 Million Years Ago, 2006, p. 20.)

Phillips’ graphic shows a quite modern take on changes in the diversity of life over long timescales, changes that Erwin describes as “major turnovers in the dominant fossils.”

I cannot leave Phillips without mentioning that as a child, after the death of his parents, he was raised by his uncle William Smith, the now famous geologist.  It was Smith who recognized there was a recurrent order of rock strata and that certain fossils were found only in specific strata, giving rise to the science of stratigraphy.  He created Britain’s first geologic map.  In addition, Phillips merits applause for having bucked the trend followed by so many of his contemporaries (Darwin included); he gave his 1860 volume a wonderfully brief and to the point title.

In his article on big data in paleontology, David Sepkoski makes much the same argument as I’ve made here regarding the very early use of data aggregation, though he turns to the German paleontologist Heinrich Georg Bronn.  Bronn, also in the middle of the 19th century, created a “paper ‘database’ of fossil groups” which, by means of what we now refer to as spindle diagrams, he illustrated the origin, duration, and extinction of taxa.

I think one sign that big data has come to occupy an important place in paleontology is the growing significance of the Paleobiology Database (PBDB) for analysis in this field.  The PBDB, the product of an international group of paleontologists, seeks to become a comprehensive database recording the taxonomy and collection-based occurrences of fossils around the world and in all time periods.  It is primarily funded by the National Science Foundation and the Department of Geoscience at the University of Wisconsin – Madison.  The PBDB is accessible to the general public, offering various tools for analyzing its data on (as of April 30, 2018):  1,366,816 fossil occurrences of 369,683 taxa housed in 192,792 collections.  There have been 310 official publications based on PBDB data.

A recent example of such a publication is Diversity Change During the Rise of Tetrapods and the Impact of the ‘Carboniferous Rainforest Collapse’, by Emma M. Dunne et al. (Proceedings of the Royal Society B, 2018) which used the PBDB to explore how the Carboniferous rainforest collapse (CRC) might have affected patterns of diversity change among tetrapods from the late Carboniferous into the early Permian.  This analysis, contradicting an earlier hypothesis, found that the CRC was associated with increased interconnectedness among communities, rather than increased endemism.  Dunne and her colleagues turned to the PBDB but only after spearheading an effort to add to it all of the currently published data on the terrestrial tetrapod fossil record across the Paleozoic Era.

The PBDB is a fascinating and challenging tool for looking at ancient life, and I’ve only just scratched the surface of how its riches can be used (and probably won’t ever be able to dig much deeper).  At the most prosaic level, it offers the collectors among us myriad references about different locations at which fossils have been found; some of those sites may still be available for hunting fossils.  More intellectually challenging is utilizing the PBDB to place in context fossils found in the field.  For this, the PBDB is invaluable, offering an avenue to the taxonomic history of a wide array of fossil taxa.  Among the data available are the formations in which these taxa have been found.

What follows is a fairly simple-minded example of a bit of research I undertook with the PBDB into the fossil record of the Ecphora, that beautiful gastropod so familiar to those of us who have collected along the western shore of the Chesapeake Bay.  Pictured below is an Ecphora from the St. Marys Formation (this fossil is presumably younger than 13.8 million years old).

For my adventure in the PBDB, I provide a series of screen shots of the database in action (very low key action, mind you).  Here’s the welcoming splash page for the PBDB (in most cases in the images below only a portion of the screen is shown):

In this instance, I started with the Navigator.  Here a map of the world is covered with a multitude of colored dots, many of them lying atop others, each representing a collection of fossils, color coordinated with the time scale shown at the bottom.

Limiting the taxa (using the beetle icon) to the genus Ecphora generates the following view of the world map highlighting (barely visible) the very few collections in which this genus appears.

With a single exception, the collections with Ecphora are located in the US.  In the view below, the dots are clearer though very much clustered in a mass along the mid-Atlantic coast of the country.  Deep yellow dots predominate, marking sites yielding Miocene Epoch fossils.  The few greenish and pale greenish yellow dots identify sites in different parts of the late Cretaceous Period (older than 66 million years) which are markedly older than the Ecphora sites elsewhere on the map.  The latter all date from the Oligocene through the Pliocene, leaving a gap of some 32 million years or more from those earlier finds.



Zooming in just on the Chesapeake Bay region, brings into focus the wealth of places where Ecphora specimens, now in collections, have been found.

Clicking on each dot will open up a window giving information about the location, the references in the literature to this site, and the occurrence of an Ecphora species at that location.  The picture below shows this information for the site in Texas that is one of the Cretaceous outliers for Ecphora.

Clicking on the link in the middle of the window which reads “Sohl Wolf City,” reveals the following information about the site.

Opening the “Occurrences” tab at the top of the window tells me which Ecphora species is found here.

Ecphora proquadricostata.   All of the Cretaceous sites in PBDB featuring Ecphora identify either E. proquadricostata or E. sp. as the species in question.  These Cretaceous sightings are a problem I’ll turn to in a bit.

I decided to explore the time ranges for each of the Ecphora species in the PBDB, asking when in the fossil record each species lived.  I wanted to see how much they overlapped with each other and when the whole genus first showed up and when it blinked out.  To do that, I downloaded PBDB’s information on the first and last appearances of each of these Ecphora species using a PBDB-based web app called Fossilworks.  In Fossilworks I requested PBDB occurrence data on all species of the Ecphora genus.  I opened the “Download” tab (see below) and clicked on “Collection, Occurrence, or Specimen Data.”  In the form that opened, I identified the taxon I was interested in as Ecphora.  I made sure to indicate that the taxonomic level I was interested in was set to “Species.”

I then clicked on “Collection Fields” tab and specified in the form that opened up what data I wanted, including minimum and maximum ages.

The screen shot above doesn’t show the whole “Collection Fields” form.  At the bottom of is a button to “Create Data Set,” which I clicked.  A window opened up showing the data sets that had been created.

I used the taxonomic ranges data set and, when clicked, it opened in Excel.  From that point on, the time range analysis I performed basically followed a very handy PBDB tutorial video.

The product of my efforts manipulating the PBDB data appears below – an Excel chart that shows the time span for each of the Ecphora species in the PBDB.  The present day is on the right of the chart and the bars for each species tracks its life span as a species, dating back to those late Cretaceous sightings of E. proquadricostata.  As the screen shot above indicates, the underlying data are from 128 separate occurrences of the Ecphora genus.

I haven’t cleaned this chart up very much since, at this point, I don’t have any plans to do more with it.  An initial cleaning would involve dropping the Ecphora sp. entry whose bar basically shows that a species identification wasn't possible in several collections that fell across the purported lifespan of the genus (at least, at the beginning and at the end).

More problematic is the inclusion of E. proquadricostata.  As already noted, this species appears in the Cretaceous and then disappears after the end of that period, long before any of the other species show up.  What’s going on here?  Well, exploring this has introduced me to one of the limits of the data aggregation behind the PBDB.  E. proquadricostata is a species over which experts have contended.  Is it truly an Ecphora or does it belong to some other genus entirely?  Paleontologist Norman F. Sohl (author of the reference to the Texas site discussed above) wrote that, given the close similarity he detected between specimens of this species and all of the other Ecphora (known from the Oligocene onward), “it would be unwise to separate this species from the genus Ecphora purely on the basis of time lapse.”  (Neogastropoda Opisthobranchia and Basommatophora From the Ripley, Owl Creek, and Prairie Bluff Formations, Geological Survey Professional Paper 331-B, 1964.)  In contrast, paleobiologist Geerat J. Vermeij, citing the arguments of other authors, agreed strongly that this Late Cretaceous species is definitely not a true Ecphora.  (Morphology and Possible Relationships of Ecphora (Cenozoic Gastropod:  Muricidae, The Nautilus, Volume 109, Number 4, 1995.)

If I were to drop both of those questionable entries, then this graphic squares quite nicely with at least one school of thought about the early evolution of the Ecphora species.  As I noted in a previous post on the chemical composition of the Ecphora shells, some have posited that E. wheeleri was the first true species in this genus and that E. tampaensis evolved from it.

But, until someone takes the step to rename E. proquadricostata moving it to another genus, analysis of the Ecphora genus using the PBDB will give the genus an exceedingly long time range that may, in fact, not be accurate.  Caveat emptor.  (To explore the risks and limits of computer analyses of big data sets, one might start with writer Kalev Leetaru’s provocative piece titled How Bad Data Practice is Leading to Bad Research which appeared in Forbes on February 19, 2018.  It's not focused on the sciences per se but does identify some of the issues the may affect big data use in all fields.)

Nevertheless, the PBDB offers users a taste of this other aspect of the life of a paleontologist – basking in the glow of the computer screen, being much too sedentary for much too long a time.  Field work anyone?

Intriguing Chemistry of the Ecphora ~ Peeling Back the Layers

Shell fusiform, ventricose, with revolving costae; . . . .
~ T.A. Conrad, 1843

Research on the chemistry of the layers comprising the shell of the Ecphora, an extinct genus of gastropod, has led to some fascinating results.  My already strong sense of the beauty of this shell has deepened.  At the same time, the Ecphora seems to be in some sort of identity crisis; to me, its taxonomy is a frustrating and confusing mess.  Happily, there’s hope that the chemistry of its layers may also help address that.  Prompted by the acquisition of a particular Ecphora specimen (more on that later), I’ve begun to explore the literature on the Ecphora chemistry.  This post offers up some of what I’ve learned.

I am not alone in finding a singularly appealing grace in the flowing, intricate shape and red-brown to tan color of Ecphora shells.  Perhaps I’m stretching it a bit, but I think even naturalist Timothy A. Conrad’s turgid description of the genus’ shells (given above) manages to convey a sense of their aesthetic specialness.  (Proceedings of the Academy of Natural Sciences, p. 310, 1843.)  They are tapered at either end (think of a plump sewing spindle – fusiform), rounded out (ventricose), and covered with revolving (I’d prefer swirling) ribs (costae).

Though paleontologist Edward Petuch, whose fingerprints show up all though the Ecphora taxonomy, describes “ecphorine” shells as “bizarrely shaped,” he notes, in the same text, that they are prized for their “unusual shell sculpture, large size, and general intrinsic beauty.”  (Edward J. Petuch and Mardie Drolshagen, Molluscan Paleontology of the Chesapeake Miocene, 2009, p. 35.)

The specimen pictured below strongly exhibits the prototypical features of the ecphorine group of fossil shells – from size to shape to color (although the color of an Ecphora shell fades with exposure to sunlight, this specimen is grayer than many).

Ecphorine.  This adjective is being used by folks writing about the taxonomic group that includes the Ecphora genus partly because of the taxonomic muddle.  Unclear about whether several similar taxa are all in the genus Ecphora or some other genera?  "Ecphorine" covers a multitude of sins.  I don’t find a consensus out there about which are valid genera and what their relationships are.  Indeed, this group seems to be a battleground between lumpers and splitters.  My conservative (i.e., lumper) nature inclines me to follow Joseph G. Carter and his colleagues in the (possibly dated) paper titled Morphological and Microstructural Evidence for Origin and Early Evolution of Ecphora (Mollusca:  Gastropoda) (Journal of Paleontology, Volume 68, number 4, 1994, hiding behind a paywall).  In it, Carter et al. consider the Ecphora genus to be relatively broad in the species it encompasses and relatively old, dating from the early Oligocene.  As a result, in this post, I will use the generic name Ecphora expansively (and try to avoid the adjective ecphorine) to describe specimens sporting quintessential features of an Ecphora, including relatively large size, prominent (swirling) ribs, spindle-shaped body with a rounded and inflated midsection, and some red-brown to tan coloring.

I’m confident that the shell pictured above is from an Ecphora, but, given the taxonomic mess of this taxa, I will only suggest that it’s from E. quadricostata (Say, 1824).  I acquired this specimen from a dealer at a show.  Compounding the taxonomic confusion is the fact that I don't know exactly where this specimen was found.  The dealer would only say he "thought it was collected in Virginia."

Initially attracted by the specimen's size (it’s bigger than those I’ve collected on my own), I found another feature truly irresistible:  the chalky white layer that lines the interior of the shell.  It can be seen poking out of the spire at the top of the shell where the exterior grayish layer has broken away.  (Those initial swirls at the top of the gastropod’s shell are known as the protoconch, which, in shells of adult Ecphora, are typically broken off.)

Many of the Ecphora specimens in my collection exhibit some of that internal layer, but not to the extent of this one.  Indeed, the interior white layer is one of the defining elements of nearly all genera and species in the taxonomic group to which the Ecphora belongs.  Both layers (highlighted below) are composed of calcium carbonate (CaCO₃) but the interior layer is in the form of aragonite, while the exterior is calcite.

Bearing the same chemical formula, calcite and aragonite differ in their crystalline structure.  Calcite is stable and, in contrast, aragonite is metastable meaning that it can, over a long period of time or under heat, be transformed to calcite.  In fact, the instability of aragonite greatly increases the chances that an aragonitic shell will dissolve in the fossilization process, leaving a mold or an internal cast.  It may also explains why many Ecphora are found with relatively little of the aragonite layer still present.

The current scientific thinking is that whether calcium carbonate precipitates in seawater as calcite or aragonite depends upon the ratio of magnesium ions to calcium ions in the water.  The lower that ratio, the more likely calcite is to form; the higher the ratio, aragonite or, perhaps, high-magnesium calcite (which results from some substitution of Ca ions with Mg ions) is the likely precipitate.  (David L. Chandler, Mystery Solved:  Why Seashells' Mineral Forms Differently in Seawater, MIT News Office, MIT News, March 2, 2015.)  And here, as a result, the tale takes a decided twist.

To my surprise, it turns out that over the course of the Phanerozoic Eon (beginning with Cambrian to the present), the planet’s seas have cycled between what are identified as “calcite seas” and “aragonite seas.”  At different times, the seas are more conducive to the precipitation of calcite or of aragonite.  Early and briefly in the Cambrian (which began 540 million years ago), we had aragonite seas that were followed by long-lived calcite seas.  In the Carboniferous Period (perhaps roughly around 340 mya – take this and all ensuing dates with a huge grain of salt), things shifted to a second phase of aragonite seas, which lasted until the mid-Jurassic (to roughly 170 mya).  These calcite seas endured until late in the Paleogene Period (to maybe 30 mya, during the early Oligocene).  From then until now, we’ve been in a third aragonite sea phase.  (There's a body of research on the driving engine for these changes, but it's not relevant to this post.)

The very crude dates provided above should not be taken to mean there were abrupt global shifts from one seawater chemistry to another.  Geologist Lawrence Hardie has suggested that these transformations may have occurred over 10-million-year periods.  (I derived these very soft dates by eyeballing graphics that appear in paleontologist Steven M. Stanley’s Earth System History, 2nd edition, 2005, p. 242, and in an article by Steven M. Stanley and Lawrence A. Hardie, titled Secular Oscillations in the Carbonate Mineralogy of Reef-Building and Sediment-Producing Organisms Driven by Tectonically Forced Shifts in Seawater Chemistry, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 144, 1998, p. 6.  Paywall, I believe, for the latter.)

These shifts in seawater chemistry have significant implications for marine life, particularly influencing the kind of calcium carbonate secreted by less complicated organisms in the creation of their shells.  The proliferation of these simpler organisms is somewhat at the mercy of seawater chemistry; they thrive when the chemistry favors the kind of CaCO₃ they secrete and decline when it does not.  As a result, for example, the changing patterns of aragonitic and calcitic reef-building follow the aragonite/calcite sea cycles.  In contrast, taxa that have greater biological control over their calcification processes can buck the shifting patterns in seawater chemistry to their long term benefit.  Stanley and Hardie note, “Taxa that engage in more sophisticated biomineralization cannot take full advantage of beneficial seawater chemistry in the same way [as the simpler organisms], but all else being equal, the carbonate productivity of these groups, such as the Mollusca, has been more stable throughout the Phanerozoic.”  (p.16.)

The most direct influence of seawater chemistry on those more complex organisms may have come when those taxa first appeared on the scene.  Biologist Susannah M. Porter has argued that the propensity for any major taxa to secrete either calcite or aragonite skeletons (i.e., shells for mollusks) is a function of seawater chemistry at the time when each taxa made its initial appearance.  She suggests that “when skeletons first evolved, natural selection favored the mineral easiest to precipitate” and the taxa remained locked into that mineral despite subsequent changes in the magnesium/calcium ion ratios.  (Susannah M. Porter, Seawater Chemistry and Early Carbonate Biomineralization, Science, Volume 316, Number 5829, June 1, 2007.)  Mollusks appeared during the early Cambrian when the seas were aragonite, so, according to Porter, it is not surprising that many mollusk taxa have shells composed of solely or mostly of aragonite, despite the several changes in water chemistry they have experienced since.

But what influence would seawater chemistry have on subsequent evolutionary change in a more complex organism, say, change like the addition of a different kind of calcium carbonate layer?  Case in point, the Ecphora.  According to Carter et al., in the paper cited above, Ecphora wheeleri, from the early Oligocene Epoch (which began about 34 mya), was the first species of the Ecphora genus, and its shells were composed entirely of aragonite.  (Reflecting the chaos of Ecphora scientific nomenclature, the name E. wheeleri seems to have fallen out of use, though any currently accepted name for the Oligocene species discussed by Carter et al. has successfully eluded all of my research efforts.)  Regardless, E. wheeleri (or whatever it’s now called) evolved into E. tampaensis (still a valid name, though Petuch renamed it Ecphorosycon tampaensis) which was the first species in the genus to secrete any calcite.  In that particular species, the calcite was limited to the ribs.  Subsequent Ecphora species sported a complete, external calcite layer.  The genus expired in the late Pliocene Epoch.  (This point of extinction is provided by the Paleobiology Database.  Petuch agrees with that being the end of the line for the ecphorine taxa (p. 37).)

Why evolve an outer calcite layer anyway?  Carter and his colleagues speculate that an external layer of calcite offered an evolutionary advantage to the Ecphora because it better resisted the acids used by other gastropod predators in attempting to drilling through the shell.

With Porter’s analysis in mind, I am intrigued that there was a shift from calcite seas to aragonite seas going on relatively close (if I can trust my squishy dates) to when the Ecphora was evolving a calcite outer layer.  That doesn’t seem a propitious time to do so.  Certainly the seawater change did not preclude the Ecphora from following the evolutionary path that led to an external calcite layer, though it might have made that more difficult.  I wonder, though, is the calcite layer possibly made of high-magnesium calcite which precipitates more readily in aragonite seas?  I’ve found nothing in the literature to answer that question.

Of course, the fact that the calcite layer did evolve is one of the key reasons that these beautiful shells are able to make it through the fossilizing process in the first place.  The layer plays another critical role:  it is responsible for maintaining the Ecphora shell’s distinctive color across millions of years.  Though aragonite over long periods of time can be transformed into calcite, most of the aragonite shells of the mollusks contemporaneous to the Ecphora fossilize to a dull, chalky white aragonite, that is, when they don’t dissolve completely.  In contrast, the color of Ecphora shells tens of millions years old can still stand out vibrantly.  Why?

In mollusk shells, aragonite or calcite crystallizes on matrices formed of proteins and polysaccharides which are often complemented with shell pigmentation coming from the mollusks’ diet.  For the fossil Ecphora shells, it’s the surviving calcite outer layer that preserves the color, and that color was a clue suggesting to paleontologists that, perhaps, that layer preserved other organic material.  Paleontologist J.R. Nance and his colleagues collected and broke down the outer layers of various Ecphora specimens found along the Calvert Cliffs in Maryland and then analyzed the material.  Earlier this year, they reported that they had found “protein-rich polymetric shell-binding material and associated pigments in [Ecphora] specimens as old as 18 Ma [million years old].”  (Preserved Macroscopic Polymeric Sheets of Shell-Binding Protein in the Middle Miocene (8 to 18 Ma) Gastropod Ecphora, Geochemical Perspectives Letters, January 20, 2015, p. 2.)  “In this context, intact proteinaceous shell-binding material in 8 to 18 Ma Ecphora represents some of the oldest and best-preserved examples of original protein observed in a fossil shell.”  (p. 7)

Finally, even more exciting, Nance et al. suggest their work raises the “possibility of amino acid sequencing and phylogenetic analysis through 10 million years of gastropod evolution.”  (p. 8)  I wonder if, thanks to that calcite layer, we might eventually bring some order to the Ecphora taxonomic confusion, order befitting the beauty of these fossil shells.

Strange Small Eruptions

  

Horatio         In what particular thought to work I know not;

     But in the gross and scope of my opinion,

     This bodes some strange eruption to our state.

~ Hamlet, Act I, Scene I

I haven’t thought through the precise implications of my fascination with the world of microfossils, but in general I sense that it “bodes some strange eruption to our state.”  Perhaps not tragedy, but certainly change in behavior. 

Case in point, the fossil shell pictured below.  It prompted new behavior and led to some small eruptions (though nothing really earthshaking).

In a recent post, I initially identified this shell as coming from an Ecphora tricostata.  I have reason to think something’s rotten in my analysis, given the location where it was found (Scientists’ Cliffs), the formation I think it came from (Calvert), and the likely age of the material (Middle Miocene – some 16 to 12 million years old).  The definitive study of the stratigraphy of this area posits that E. meganae, not E. tricostata, is found in these portions of the Calvert Formation, but my fossil favors the latter more than the former.  (Lauck W. Ward and George W. Andrews, Stratigraphy of the Calvert, Choptank, and St. Marys Formations (Miocene) in The Chesapeake Bay Area, Maryland and Virginia, Virginia Museum of Natural History, 2008.)

It’s symptomatic of my current obsession with the micro that, rather than continue to puzzle over the identification of this Ecphora, I am now consumed by the gray clay matrix that surrounded and filled the shell.  Some three months ago, when I prepared the fossil, I ignored some expert advice and removed the matrix from what, unfortunately, turned out to be a rather fragile shell.  In the process, the internal, central pillar (columella) broke free.  A shame.

But, in a move that I recognized at the time as somewhat quixotic and strange, I bagged the matrix I removed from the shell, slapped a label on the bag, and set it aside.

A couple of weeks ago, I came across the bag and thought, “What the hell, let’s see what’s in here.”

I wont go through the tedious details of my amateurish handling of these few grams of Miocene material, except to say that it involved one percent solutions of Calgon (in water), lots of soaking, lots of sieving, but no baking.  And still it’s plagued with lumps.  (The comedy of errors that is my preparation of material for microfossil searching is described in painful detail in a previous post.  I’ve added some scientific sieves to my equipment, providing a patina of the professional to my inherently sophomoric efforts.)

There was something of the miraculous when, under the microscope, from the gray miasma of this matrix, intricate microfossils made their first appearance, ghosts of minute Miocene fauna.  Amazing what now gets my heart racing.

From within that fractured Ecphora came a flood of foraminifera shells, accompanied by a much  sparser scattering of ostracode shells.  Remarkably, despite several hours of picking, I haven’t exhausted this sample yet.  At this juncture, the critical limit to my time before the microscope is what my hunched over shoulders can bear.

Though I am retrieving foraminifera shells in many different and complex shapes, it’s a couple of shells from the ostracodes, those miniscule crustaceans, that have captured my imagination for the moment.  These two fossils are ornate, exploding with blunt and pointed spines.  Among the many spines are some that, upon closer examination, form three curved rows marking portions of the length of each shell.

These two specimens are both about 0.8 mm long.  Ostracodes have right and left valves hinged at the dorsal edge (top edge of each specimen in the photograph).  The top specimen shown is a right valve, the bottom one is a left valve (the valves are certainly from two individuals).

I’ve identified this Miocene ostracode as Actinocythereis exanthemata (Ulrich and Bassler, 1904).  My primary source for this identification is Richard M. Forester’s paper titled A Systematic Revision of the Ostracode Species Described by Ulrich and Bassler and by Malkin from the Chesapeake Group in Maryland and Virginia (U.S. Geological Survey, Geological Survey Professional Paper 1128, 1980, p. 11 and plate 3, figures 7 and 8).

In 1904, E.O. Ulrich and R.S. Bassler, wrote the initial description of this species for the Maryland Geological Survey’s Miocene: Text (Volume I, 1904, p. 117-118).  They contributed the Ostracoda discussion to the Systematic Paleontology portion of the book.  Amid their lengthy characterization of this fossil, which they named Cythere exanthemata, is a pithy phrase that nicely captures the essence of their description – this ostracode shell has an “extremely nodose and spiny carapace.”  (p. 117)

Nodose challenged my vocabulary, but it’s a great word.  According to the New Oxford American Dictionary, it's an adjective meaning “having or characterized by hard or tight lumps; knotty.”  The neat noun form is nodosity.  Savor that word.  Makes sense that nodule has the same Latin root.

Here are the drawings of C. exanthemata that Ulrich and Bassler included among the plates in the second volume of the work on the Maryland Miocene.

 They acknowledged some concern about precisely where C. exanthemata should come to rest taxonomically, but noted they were at work on a monograph which, “it is hoped, may result in a more natural and serviceable classification of the fossil species than the one now in use.”  I don’t think they ever followed through on this.  The taxonomic history of A. exanthemata prepared by Forester in 1980 includes no subsequent publication of theirs.

Now, of the 1904 duo who tackled the Maryland Miocene ostracodes, Ulrich had the reputation of being a splitter – seeing different species where others didn’t.  I’m not certain about Bassler in this regard.  (The career paths of the two men intrigue me and will be the subject of a future post.)

As a post-doctoral fellow at the U.S. Geological Survey, Forester reanalyzed the taxonomic decisions regarding these ostracodes made by Ulrich and Bassler in 1904 and several D.S. Malkin had made in a 1953 article (Biostratigraphic Study of Miocene Ostracoda of New Jersey, Maryland, and Virginia, Journal of Paleontology).  Forester concluded that Ulrich and Bassler were guilty of both splitting and lumping.  In some instances, they identified males, females, and juveniles of the same species as distinct species.  In others, the array of specimens they offered as examples of a single species should have been considered different species under current practice.  So, he redid the species.

Unfortunately, even after Forester’s work, the identification of A. exanthemata is, in my eyes, still rather obscure.  In his treatment of A. exanthemata, Forester drew a fine distinction among specimens that previous researchers had considered of the same species.  He concluded that figure 4 in the Ulrich and Bassler’s drawings of C. exanthemata (see above) was actually A. marylandica.  (Ulrich and Bassler noted that figure 4 was a drawing of the largest specimen of the purported species that they'd found.)  Forester acknowledged that “Actinocythereis exanthemata is most frequently confused with the lower Miocene to Holocene species A. marylandica (Howe and Hough, 1935),” and identified similar instances of this confusion by other researchers in the decades since 1904.  But, he asserted, A. marylandica is “larger and more robust” and has a “slightly different” spine arrangement.

That’s not much help for me, actually.  The size distinction isn’t useful without more specimens to work with and, it’s not really certain that any differences in the configuration of the spines are evidence of much.  In a later study of South Carolina ostracodes, Thomas M. Cronin commented that “[t]he degree of variability in the spines precludes separation [of these two species] on the basis of this characteristic.”  (Evolution of Neogene and Quaternary marine Ostracode, United States Atlantic Coastal Plain:  Evolution and speciation in Ostracoda, IV, appearing in Studies Related to the Charleston, South Carolina Earthquake of 1886 – Neogene and Quaternary Lithostratigraphy and Biostratigraphy, U.S. Geological Survey Professional Paper 1367, 1990, p. c-35.)

[Later edit:  I dropped a paragraph from the original post which suggested that the two species may actually be males and females of the same species.  I believe I misread a study by Frederick M. Swain (Some Upper Miocene and Pliocene(?) Ostracoda of Atlantic Coaster Region for Use in Hydrogeologic Studies, U.S. Geological Survey Professional Paper 821, 1974, p. 30.).]

Nevertheless, as far as I can unpack Forester’s description of A. exanthemata, I think my two specimens fit.  And, in this instance, I’m going to follow the National Football League approach (instant replay overrides the ruling on the field only in the face of indisputable evidence) and stay with A. exanthemata.

Of course, I also have a nonscientific motivation, I love that species name – exanthemata.  Its Greek root, exanthema, means “an eruption.”

[Much later and decidedly sheepish edit:  I'm really beginning to think that these two ostracode specimens may be from Henryhowella evax, not Actinocythereis at all.  H. evax is apparently more ovate than A. exanthemata.  Further, the spines on A. exanthemata appear more pronounced than those on the specimens above.  If this rethinking of mine is correct, it leaves me looking more than a bit foolish for having rambled on at length in this post about a couple of misidentified fossil ostracode shells.  My apologies.]

Fragility of the Ecphora

It seems a minor miracle to find, amid the shell debris marking the shoreline, an even partial fossil shell from an Ecphora, an extinct sea snail or gastropod.  Paleontologists have used the Ecphora as an index fossil; the presence of specific Ecphora species can be used to date rock and other fossils.  For collectors, of whatever stripe, the Ecphora shells cast a spell by their sheer beauty which is heightened, for me, by the seemingly near impossibility of their survival as intact fossils during that final passage from burial in rock and sand to exposure.  The beaches where I hunt fossils along the Calvert Cliffs formation on the western shore of the Chesapeake are littered with little reddish brown shards of Ecphora shells, a stark reminder of the hazardous life of a fossil.  The fatality rate of these fossils after exposure to the elements must be staggering.

Ecphora are distinguished by the ribs or costae that run like exposed, elevated rails around the exterior of the shell.  The number and shape of those ribs are largely determinative of the specific species of Ecphora that one may have in hand.  The complexity and perceived delicacy of these ribs explains part of their attraction.  Often these shells have an appealing russet color, unusual in a fossilized mollusc, an attribute that also seduces the collector.

Last week, on a hunt in record breaking heat along a Chesapeake Bay beach, I came upon the specimen pictured below.  It sat exposed, wet, upright, and vulnerable amid chunks of clayey material that had fallen from the cliffs.  For seasoned collectors of Ecphora, this isn’t much, a seriously damaged specimen.  For me, it is special, as close as I’ve come to finding a complete specimen of whatever species of Ecphora.

The specimen shown above (1 3/4 inches long, 1 1/8 inches high, and 1 3/8 inches wide) I’ve identified as Ecphora tricostata Martin, an early to mid-Miocene fossil (some 15 to 17 million years ago).  The three prominent costae narrowed the field appreciably as did the location of the find – Calvert Formation.  For confirmation, I relied primarily on Ward and Gilinksy’s article entitled Ecphora (Gastropoda:  Muricidae) from the Chesapeake Group of Maryland and Virginia.  It appeared in the March 15, 1988, edition of The Academy of Natural Sciences of Philadelphia’s journal Notulae Naturae.  It fits in terms of location and its original description:  “body whorl large, with three very prominent elevated revolving ribs with a fourth rudimentary one below it on the largest specimens” (p. 3)  I detected the rudimentary rib on the specimen I found (I think it can be made out in the picture of bottom of the shell).  The other possible Ecphora with three ribs that may be found in the Calvert Formation is E. pamlico Wilson – the location fits but, among other distinguishing attributes, E. pamlico lacks the rudimentary fourth rib and has ribs that are not as prominent as those on my specimen.  (At least, that’s how I'm reading and applying the literature - the two species are clearly closely related.)

The fragility of, and fatality rate among, the Ecphora are not limited to their physical survival but also to their experience of the vicissitudes of the taxonomic process (clearly, not something unique to the Ecphora).  The naming and renaming of Ecphora species is a delightful and ongoing story

The 1980s witnessed a modest taxonomic blossoming of Ecphora species.  In particular, Druid Wilson of the Smithsonian nurtured the process in an article entitled Species of Ecphora, Including the Subgenus Stenomphalus, in the Pungo River Formation (appearing in Geology and Paleontology of the Lee Creek Mine, North Carolina, II, 1987 – full text appears on the Smithsonian website).  In this article, Wilson offered up three new species of Ecphora.

[An aside:  Because it’s germane to a tiny part of my story, the taxonomic name of each individual species given in this posting includes the name of the individual who was the original author of each species’ name.  Following proper nomenclature, if that genus and species combination were modified after initial publication, then the original author’s name is enclosed in parentheses.]

Most important for those collecting Ecphora in Maryland, Wilson stripped the state of the Ecphora quadricostata (Say).  He asserted that “[i]t is now well known that most if not all of the fossils described by [Thomas] Say in 1824 came from Virginia rather than Maryland . . . .” (p. 22)  As a result, he wrote, the “common Ecphora of the St. Marys Miocene of Maryland masquerad[ed] under the name ‘Ecphora quadricostata,’ which properly belongs to the Yorktown species of Virginia.”  (p. 23)  The specific species of Ecphora with four ribs coming from the St. Marys Formation along the Chesapeake that had been misidentified all these years, he renamed as Ecphora gardnerae.  The species name gardnerae is in honor of Julia A. Gardner (1882-1960), a geologist with the U.S. Geological Survey and a specialist in molluscs.

The following year, Ward and Gilinsky went Wilson one better, concluding that there were actually several subspecies of E. gardnerae, so the one Wilson identified was given a subspecies name and is now known as E. gardnerae gardnerae Wilson.  (p. 7)

Adding a fillip to this tale is the fact that, in 1984, the Governor of Maryland had signed legislation naming Ecphora quadricostata (Say) as the official fossil shell for the state.  Now, science had pulled out the rug from under the state.  When legislation correcting the error was enacted in 1994, the state had as its official fossil shell the “Ecphora gardnerae gardnerae (Wilson)” – well, they got it almost right.  Technically Wilson’s name shouldn’t be in parentheses.

The 1999 edition of Miocene Fossils of Maryland, by Harold E. Vokes, et al., published by the Maryland Geological Survey, identifies three Ecphora species as being found in Maryland (missing from this publication is E. pamlico Wilson).  In the illustration below, taken from Miocene Fossils of Maryland, they are #2 – E. gardnerae gardnerae Wilson, #3 – E. meganae Ward and Gilinsky, and #4 –  E. tricostata Martin.  The first shell image (#1) is a reproduction of what is purportedly among the first scientific illustrations of a U.S. fossil, published in the 1770 edition of Martin Lister's Historiae Conchyliorum.  It’s clearly an Ecphora, but which one?  The Maryland Geological Survey suggests that it's E. gardnerae gardnerae.  To others, that's certainly unclear.  Remnant of the masquerade?  (Numbers 5 and 6 are of a species of different genus of gastropod – Siphonalia devexa (Conrad).)  (The Ecphora portion of Miocene Fossils of Maryland is available on the MGS website.)

Maryland in this instance was caught by the crosscurrents coming from a vigorous scientific exploration and reexamination of the Ecphora genus.  Not sure what the excuse is for Maryland choosing, in 2004, something called the "Patuxent River Stone" as its state gem.  This has stirred controversy from the beginning.  Indeed, in 2002, legislation to do this was reported unfavorably from committee.  So, what is this stone really?  A recognized gem or not?  Agate, as its proponents label it, or, perhaps, something much more mundane like iron-stained quartz?  Ah, give me the scientific currents swirling around the beautiful Ecphora any day.