Monday, June 29, 2015

Calvert Marine Museum ~ Striking A Balance

I recently visited the Calvert Marine Museum (CMM) in Solomons, Maryland, and the word that I’ve been contemplating since that visit is balance, in various of its manifestations.

At a fundamental level, natural history museums (and, I guess, all museums) have to strike a balance between, and among, many valid, though often competing, elements.  Among these are the depth of its coverage and its breadth, the complexity of information conveyed and its accessibility to visitors, and the popularity of topics covered and their importance.  Challenging to say the least.

The CMM navigates nicely through the various shoals upon which a natural history museum might founder.  Typically with an appropriate level of detail, its exhibits capture the natural history and beauty, the complex maritime history, and some of the present troubles of the Chesapeake Bay, this continent’s largest estuary.  An attentive visitor will come away with a new, or, perhaps, renewed, appreciation of many aspects of ancient and contemporary life in, on, and along the waters of the Chesapeake.  Not much more that one might ask for from this museum.

To my delight, balance is itself one of the central themes that the CMM is conveying.  More precisely, balance in the natural world.  This is perhaps clearest in the contents and labels of the varied aquaria showing marine life in the Bay.  These explicitly present the issue of the balance between predator and prey.  Significantly, the museum makes it clear that we have to face the fact that many of the environmental balances that may have marked the Bay from ancient times have been disrupted by various invasive species.  None more destructive than . . .

Of course, I would argue that a museum dedicated to the Chesapeake Bay has to give a prominent place to fossils collected from the Calvert Cliffs whose Miocene sedimentary formations stretch from Chesapeake Beach in the north to Drum Point to the south, where the Patuxent River and Bay meet.  In this regard, the CMM does not disappoint.  The paleontology exhibits offer a richness of fossil shark teeth, the fossils most sought after from the cliffs.  A few appear below.

Yet, how refreshing is the central line of thinking about Calvert Cliffs fossils at the CMM (at least, as I reconstruct it).  I suspect it goes something like this – “Sure, we have to offer lots of shark teeth because that’s what the casual visitor and the typical fossil collector who might come through will want to see.  Nevertheless, that’s a skewed view of what lived here in the Miocene.  Above all, we’ve got to balance those teeth with lots and lots of mollusc shells.”  Amen, says this fossil shell collector.  Here are a few.

I was engrossed by the array of fossil shells.  The species diversity is staggering in its richness, reflecting, I assume, the fecundity of the Salisbury Embayment, the arm of the Atlantic that covered this area during the Miocene.

When I saw all of the various kinds of gastropods (some are shown in the first two pictures above), particularly my favorite oyster drills (subject of a previous post), my first reaction was to wonder whether these years in the Miocene might have been bad times for their bivalve prey (some are shown in the bottom three pictures).  But then again, when I consider the various bivalves, it’s not hard to notice how many of them sport thick shells, and, if not that, then a large size.  Paleontologists Gregory Dietl and Patricia H. Kelley have written, ". . . size and thickness of the shell are two general defenses in molluscs that are effective against shell-crushing crabs or fish, as well as shell-drilling snails, not to mention boring parasites."  ( The Fossil Record of Predator-Prey Arms Races:  Coevolution and Escalation Hypotheses, appearing in The Fossil Record of Predation, edited by Michal Kowaleswki and Patricia H. Kelley, The Paleontological Society Papers, Volume 8, October 2002, p. 364.)

And therein lies a story that I wish the CMM had sought to tell, at least a bit, with these fossils.  Yes, it's hinted at by the way the gastropods are grouped in an upper level of the display where they sit above the bivalve specimens.  I'll concede it's possible that perhaps the museum did explicitly address this somewhere for the fossil molluscs and I just missed it.  If that's the case, I apologize, but, if I missed it, I would assume so would most visitors.  Regardless, more might have been done with it.

For me, the story of how the balance of power between predator and prey is struck and how it changes holds an endless fascination, particularly when it comes to gastropods and their bivalve prey.  My collection of fossil shells offers clear evidence of this struggle in the partial and complete holes drilled into many bivalve shells (and some gastropod shells).  Indeed, examined closely, some of the specimens on display at the CMM show evidence of drilling.

As I prepared this post, I explored some of the literature on the evolutionary importance and consequences of predatory-prey interactions.  I initially thought of devoting the post to that topic, but, frankly, while I have found the debate on this topic to be deeply interesting, my own understanding of it has become less and less clear.  I've concluded that I don’t know enough to do any kind of justice to the thought-provoking debate between the two primary hypotheses – coevolution and escalation – whose distinctions I find subtle and elusive.  For an excellent discussion of these hypotheses, I recommend the piece cited above by Dietl and Kelley.  To give a flavor of the differences between these two hypotheses (and reveal my ignorance), I would note that, in the predator-prey configuration, coevolution involves an evolutionary response by each of the interacting predator and prey species – one responding to the other, and vice versa.  In contrast (I think), the escalation hypothesis posits that the driving force for evolutionary change is a species’ enemies.  As a consequence, under this hypothesis, predator-prey interactions are more likely to promote an evolutionary response by the prey, while evolutionary change by the predator will often be a response to pressures from the predator’s own enemies, not to any change in its prey.

As an aside, I would note that some of Kelley’s work has involved analyses of the evidence of interactions between certain Miocene gastropod and bivalve taxa using fossils from formations in these very Calvert Cliffs.  She found evidence that increased shell thickness accompanied anti-predation success by bivalve species, but no concomitant prey-related evolution by the gastropod species she studied.  Rather, what changes she noted in the predators appeared to be more likely responses to their enemies.  (See, for example, Coevolutionary Patterns of Naticid Gastropods of the Chesapeake Group:  An Example of Coevolution?, Journal of Paleontology, Volume 66, 1992.)

No, I’m not asking that this predator-prey story in all of its subtlety be told in the CMM, but I wanted more than the hint I got.

So, because the museum doesn’t deserve a post that ends on a somewhat negative note, I have to say that I was greatly impressed by a display that greets visitors to the paleontology exhibits.  Titled There’s More Than One Way To Make A Fossil, it succinctly identifies the various ways that organic remains become fossils.  Certainly not technical in its explanations, its virtue is its conciseness and in the message it conveys – its title says it all.  It displays fossils reflecting unaltered preservation, permineralization, recrystallization, replacement, carbonization, compression, as well as preservation by being captured in tar or amber, and through the creation of molds, casts, and traces.  Forgive the poor photograph which appears below, it doesn’t do the display justice.

Tuesday, June 9, 2015

Brazil Nut Effect ~ Exploring the Glen Rose Limestone, Part 2

My first impression wasn’t wrong, it was just incomplete because I’d fallen victim to the Brazil nut effect.

In my previous post, I described the shells of Orbitolina texana, an agglutinated foraminifera, which appear in staggering numbers in a sample of material collected in Texas from the Glen Rose Limestone.  As far as I could see, this material, laid down in the Lower Cretaceous about 113 to 110 million years ago, consists almost completely of these foram shells.  For this present post, I took a closer look.

I washed the sample and then worked it through two sieves, creating two portions that differed by the size of their component pieces:  the first with anything larger than 1 mm, and the second with anything equal to, or smaller than, 1 mm and larger than 0.177 mm (177 microns).  I should add that any very large object, such as the example of a Salenia texana (a type of sea urchin) shown below, was removed in advance from the first portion.  (This fossil is about 2 cm across and stands about 1 cm tall.)

From each sieve, I shook material into tall envelopes secured with clips.  (In a prior post, I considered some aspect of the effects on microfossil searching of the mesh size of sieves.)

For this post, I’m not interested in that first portion (greater than 1 mm) which, under the microscope or with the naked eye, for that matter, basically confirms what one could learn just by looking at the ground where this material was collected  (the previous post includes a photograph of material in situ):  aside from some fairly poorly preserved larger invertebrate fossils, the matrix appears to be composed almost exclusively of Orbitolina texana shells

It’s when I began to search through the second portion (material between 0.177 mm and 1 mm in size) that things got interesting (well, at least, they did for me).

For awhile, what I shook from the second-portion envelope into the sorting tray seemed little different from what I’d seen in the first portion with the larger specimens.  There were many beautiful, albeit small, O. texana shells.  These, I believe, are from juveniles.  Scattered amid the O. texana shells were some fragments of sea urchin spines and pieces of urchin plates with tubercles (where spines were attached).

So, early in the process of working down through this second envelope, the picture of the Glen Rose Limestone at this location that I’d already formed was unchallenged.  I remained convinced that this fauna was thoroughly dominated by a single species of foram.

But, then, as the material from lower in the envelope tumbled into the sorting tray, my perspective changed, the picture became more complex.

There had been a more diverse population at this site than I’d imagined.  Fossil shells from ostracodes began to appear frequently, now dominating the sorting tray.  Equally impressive, these ostracode shells came from a rich array of species.  Pictured below are several examples of Eocytheropteran trinitiensis (first picture), Cythereis ornata (middle picture - note, the specimen at the top center is a carapace (i.e., having both valves) and is resting on its dorsal side), and Paracypris weatherfordensis (bottom picture).

Identification of these specimens is based on H.C. Vanderpool, Fossils From the Trinity Group (Lower Comanchean), Journal of Paleontology, Volume 2, Number 2, June, 1928, and Frederick M. Swain and Philip M. Brown, Cretaceous Ostracoda From Wells in the Southeastern United States, North Carolina Department of Conservation and Development, Bulletin Number 78, 1964 (this latter piece revises many of the ostracode name assigned by Vanderpool).  For these references and others in this post, I provide links only to publications that do not reside behind paywalls.

Vanderpool identified nine different species of ostracode that he found in the two Glen Rose locations he sampled.  He characterized five of these as "abundant."  Swain and Brown revised several of Vanderpool's identifications, including one in which they concluded he had considered adult and juvenile forms of one species as two separate ones.

This is an aside, but I have to note that, in light of what I've found in my Glen Rose sample, it's quite stunning to read Vanderpool's assessment of the microfauna in the environment in which the Trinity Group formations (which include the Glen Rose Limestone) were laid down.  It suggests the difficulty in basing such an assessment on merely a handful of sites (six in his case, two of which featured Glen Rose Limestone, or one in my case).  From what he found, he concluded that the near-shore environment of these locations posed such challenges that only "the more hardy forms" of microfauna could handle it, primary among them were, in his estimation, the ostracodes.  In contrast, foraminifera, he concluded, appeared only "sparingly."  Yes, I'll grant him that ostracodes do turn up in the Glen Rose in large numbers, but "sparingly" is not the adverb for the O. texana at the site of my sample.

Though I don’t know if most of the ostracodes I was finding co-existed with the hordes of O. texana forams, I suspect they did.  Somehow these ostracodes in the substrate earned their living while navigating around the O. texana.

As my sense of the Glen Rose microfauna changed (coming closer, perhaps, to the reality of this formation at this time and in this place), I realized that this modification in perspective tracked with how far down I’d gone into the container with that second portion of material.  Indeed, the material toward the bottom of this envelope was markedly smaller than that at the top.

And, so, the Brazil nut effect had come into play.

A good place to start is with breakfast.  Most mornings, my breakfast includes some Chex Granola Mix which I most enjoy when a new package is first opened.  It’s then that the big “honey nut clusters” and whole Chex squares tumble into my bowl.  Several mornings later, as I near the end of the bag, I’m down to bits of nuts, a bunch of oat and rice flakes, and fragments of Chex squares – not so enticing.  The view below, looking down into a newly opened package, shows the big stuff gathered at the top.

This particle segregation is popularly known as the Brazil nut effect because it is so evident in containers of mixed nuts where the Brazil nuts (or other large nuts) gather at the top.  (On occasion, it's been called the muesli effect.)  Rather than being some trivial, though interesting, topic, the Brazil nut effect is critical for a range of industries, such as food processing or pharmaceuticals, in which controlling how dry mixtures behave is important.  It’s also a key context within which microfossils are hunted.

This much-studied phenomenon continues to confound.  The original hypothesis was that, in containers with a mixture of objects (some larger than others), shaking causes the larger ones to percolate to the top.  (These larger objects are called intruders in the literature.)  Under this reasoning, it was assumed that, in a shaken container, spaces created beneath the intruders fill with smaller members of the mixture.  With more shaking, this filling process ultimately boosts the intruders to the top.   Once there, their sheer size precludes them shifting down.

However, further research concluded that, instead of percolation, convection is predominantly at work with an upwelling in the middle of the mixture (pulling up everything, not just the intruders) and a downwelling at the outer margins.  The problem for the intruders is that, once they’ve ridden the upwelling to the top, they get stuck; they’re just too big to descend with the margins.

Unfortunately, for my sense of clarity, the story doesn’t end there.  Apparently, same-sized light and heavy intruders can behave differently, challenging both the percolation and convection explanations.  In this reverse Brazil nut effect, the lighter intruders, unlike their same-sized heavier counterparts, can work their way down to the bottom of the container.  But, wait, there’s more.  Density and size of elements in the mixtures can interact in complex ways.  For instance, holding the density constant, a size threshold comes into play with those objects below it falling and those above rising.  If, instead, the size of intruders is held constant and density is allowed to vary, it appears that intruders at either extreme – high density or low density – will rise faster than those intruders whose density is between the extremes.  If that weren’t enough, it seems that the frequency of vibrations applied to the mixture makes a difference as does the ambient air pressure.

After penning a concise overview of the state of understanding of this phenomenon (and its counterpart), biomedical engineer Troy Shinbrot, who co-authored the paper that first identified the reverse Brazil nut effect, couldn’t resist this wonderful quotation, which he attributed to Mark Twain (The Brazil Nut Effect – In Reverse, Nature, Volume 429, May 27, 2004, p. 352-353):
 The researches of many commentators have already thrown much darkness on this subject, and it is probable that, if they continue, we shall soon know nothing at all about it.

Note [added later]:

I did test whether the Brazil nut effect was actually in effect with my portions of Glen Rose material, but these were a bit indirect.  I transferred some of the first portion (greater than 1 mm) material into a glass jar and gently shook it. Most of the largest pieces in the jar slowly gathered at the top.  I attempted to replicate what shaking might do to the second portion (between 0.177 mm and 1 mm) by adding a few slightly larger O. texana shells to material from that portion that I had already gone through in my hunt for microfossils.  (I simply could not bring myself to add back the ostracodes I had found and see if they ended up lower in the container.)  These larger O. texana shells, as expected, popped to the surface after only a few shakes.  Conclusive evidence?  No, but certainly supportive.

Addendum:  Dating the Glen Rose Limestone

As far as I can tell, the Glen Rose Limestone, part of the Trinity Group, dates from close to the end of the Aptian through much of the lower Albian Ages (from roughly 113 to about 106 million years).  The sample I’m working with probably is from the Lower Glen Rose (about 113 to 110 million years ago).  I crudely estimated these dates from Figure 1 in the paper by Ernest A. Mancini and Robert W. Scott, titled Sequence Stratigraphy of Comanchean Cretaceous Outcrop Strata of Northeast and South-Central Texas:  Implications for Enhanced Petroleum Exploration (Gulf Coast Association of Geological Societies Transactions, Volume 56, 2006, p. 541).  As for the idea that this material had to be from the Lower Glen Rose, I have relied on Raymond Douglass' assertion that O. texana is only found in that part of the Glen Rose (The Foraminiferal Genus Orbitolina in North America, U.S. Geological Survey, 1960, p. 6).

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