Sunday, August 30, 2015

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.
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