Wednesday, March 31, 2010

Glaciers, Evolution, and Reefs ~ Darwin and the Agassizs

I’d been waiting for some personal event or insight of my own to hang this story on, then paleontologist Alton Dooley’s son had a class assignment to write about a famous scientist’s contributions to science, an assignment which prompted Dooley (père) to post “Darwin’s Other Theory” (his work, not his son’s) on his blog Updates from the Vertebrate Paleontology Lab. (Alton Dooley is Associate Curator of Paleontology at the Virginia Museum of Natural History.) Since it was a father-son thing and very relevant to the story, I thought it would be an appropriate moment to write this post.

The people and debate surrounding this “other” theory have been percolating in my mental filing cabinet for some time because the theory is the focal point for the third signal encounter in the 19th Century between, on the one side, naturalist Charles Darwin, and, on the other, either Swiss-born naturalist Louis Agassiz or his son Alexander, also a naturalist. (In the late 19th Century, Alexander Agassiz was one of the wealthiest men in the United States as a result of investing in cooper mines which he then managed and made productive.) These scientists were in some degree of conflict for much of their adult lives. If they’d been playing chess, there would be three matches to study:

• one ended almost before it started, though the loser, Darwin, took a couple of decades to admit it with any finality;
• another, the one that mattered most, saw Darwin devastating Louis Agassiz; and
• the last consumed Agassiz fils, who devoted much time and treasure to a battle settled only long after all were dead.

The irony was that Alexander Agassiz and Charles Darwin liked each other.

Since I’ve already covered two of the three battles between the Agassizs and Darwin (a little on each below), this post is principally about the third.

Match One – Glaciers

The first of these encounters centered on the explanation of the parallel “roads” or terraces along either side of the valley of Glen Roy in Scotland. (I’ve written about this previously.) In his first formal scientific paper (1839), Darwin argued that the land at Glen Roy had risen in stages from below or at sea level, at each stage, a shoreline was carved on either side of the valley as the water level in the valley fell. (He’d seen the action of geological uplift first hand during the earthquake he experienced in Chile in 1835.)

Louis Agassiz (pictured here) countered swiftly, arguing that glaciers plugged the egress from the valley, flooding it. At different times, the ice plugs shrank, letting out some of the water, and, so, parallel traces of shorelines were etched on the valley walls. Agassiz’s view prevailed in scientific circles, though Darwin fought on, until, at last, in 1861, he agreed he had to “give up the ghost.”

Match Two – Evolution

The second Agassiz vs. Darwin (pictured below) match (described in a prior posting) was precipitated by Darwin’s publication of On the Origin of Species By Means Of Natural Selection.

Agassiz, in 1859, held sway in the United States, probably the most well-known scientist in the country in both scientific and popular circles. He’d worked hard to create renowned scientific establishments in the U.S. to rival those in Europe. While teaching at Harvard, he’d trained many of the leading American biologists and paleontologists of his time. But, he could not embrace evolution because it so thoroughly undermined his belief in a God-driven plan for the natural world, in which species were independent, coming into being or going extinct through repeated catastrophes, all in keeping with that divine blueprint. Besides, by then, Agassiz had stopped doing science. He launched a campaign in the U.S. in opposition to the theory, an effort he carried out in popular journals and in lectures. He had some success with the general public but was thoroughly and embarrassingly routed in the scientific arena. He remained in opposition to the theory until his death in 1873.

Initially, Alexander took his father’s side (as a dutiful son should), though, by 1869, he was admitting privately to a belief in the theory (he confided it once to Darwin), a position he never made public. He visited Darwin a couple of times, visits both men enjoyed. After Darwin’s death, Agassiz would express disappointment when evidence emerged that Darwin had promoted, and even taken pleasure in, the attacks that the senior Agassiz had endured for his anti-evolution stance.

Match Three – Reefs

The third Darwin-Agassiz debate involved coral reef formation. As I understand it, any theory about coral formation has to resolve the problem that coral reefs are often found mid-ocean in areas of deep water, far exceeding the maximum depth (roughly 200 feet or so) at which living coral can survive. Darwin described the problem and his solution as follows:

The facts then stand as follows:—there are many large spaces of ocean, without any high land, interspersed with reefs and islets formed by the growth of those kinds of corals, which cannot live at great depths; and the existence of these reefs and low islets in such numbers and at such distant points, is inexplicable, excepting on the theory that their rocky bases slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards. No positive facts are opposed to this view, and some direct evidence, as well as general considerations, render it probable.
~ Darwin, The Structure and Distribution of Coral Reefs (3rd edition, 1897, of the volume originally published in 1842), p. 132..

He posited that the sinking (or subsidence) of an island accounted for the main variations in the structures of coral reefs. He theorized that, first, coral would form a reef around the perimeter of an ocean island (a fringing reef). As the island sank, the coral reef would grow upwards in an effort to keep the living coral within the depth at which it can survive; a lagoon would now separate the sinking island from the reef (a barrier reef). When the island sank below the waves, the coral would seek to maintain its presence, continuously adding to a ring circling the spot where the island once stood (an atoll). The subsidence of the ocean floor that drew down the islands was, in Darwin’s view, the natural counterpart to the uplift he’d witnessed in his travels in Chile and elsewhere in South America. Dooley’s posting on Darwin’s reef formation theory is much better than the description I’ve just written (link here). His is nicely done, very succinct and accessible, and enhanced by some great pictures and graphics.

Frankly, I think what particularly galled Alexander Agassiz about Darwin’s reef theory was the way he had conceived of it. Darwin’s inspiration for the theory came from studying the charts of the Pacific route the Beagle would travel on its way home from South America. As he wrote in his Autobiography,

No other work of mine was begun in so deductive a spirit as this, for the whole theory was thought out on the west coast of S. America before I had seen a true coral reef. (p. 34)

The deliberate, time-consuming process of gathering the mountains of evidence needed to support a theory was one that he, Agassiz (pictured here), thought any self-respecting scientist had to undertake before advancing a theory. This was the kind of process Agassiz undertook in his decades-long effort to dethrone Darwin’s reef theory. (This was also the process Darwin followed with evolution.)

Agassiz’s rejection of Darwin’s method in this instance can be read in the sarcastic remarks he made in the opening to his study of The Islands and Coral Reefs of Fiji (Bulletin of the Museum of Comparative Zoölogy, May, 1899). Agassiz wrote (I’ve added emphasis to the particularly biting comments which I venture to guess were directed at Darwin):

On looking over the literature on coral reefs, one cannot fail to be struck with the amount of irrelevant matter which has been passed down from writer to writer. Statements made on hearsay have gradually become facts. The observations of inexperienced persons receive general recognition. Special cases are discussed without reference to their limited or exceptional application. The whole question is often threshed out de novo, so that it is difficult to separate the new from the old. And, finally, information gathered from charts is substituted for observation in dealing with questions which the latter alone can settle. (p. 5)

Agassiz was also frustrated by the unwillingness of Darwin’s supporters in this battle to recognize what Agassiz considered to be the incontrovertible evidence against the coral reef theory that had accumulated during the last quarter of the 19th Century.

To Agassiz, All Reefs Are Local
According to David Dobb’s superb account, Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral (2005), the strongest, clearest public statement by Alexander Agassiz rejecting Darwin’s reef theory and advancing his own came in the Fiji study. Agassiz had undertaken a voyage to the Fiji Islands in 1897, in part, because they had been touted by geologist James Dwight Dana as offering the clearest examples of subsidence at work in the creation of coral reef structures. (Despite never embracing evolution, Dana had played a role in dethroning Louis in the 1860s. Further, he had written substantive analyses of the reef question, coming down on the side of Darwin’s theory.) In contrast, Agassiz saw only evidence of uplift and then erosion leading to the formation of coral reefs, nothing of sinking islands.

The islands of the whole group have been elevated, and since their elevation have, . . . , remained nearly stationary, and exposed to a great and prolonged process of denudation and of aerial and submarine erosion, which has reduced them to their present height. . . . These atolls and islands, surrounded in part of wholly by encircling and barrier reefs, have not been built (as is claimed by Dana and Darwin) by the subsidence of the islands they enclose. They are not situated in an area of subsidence, but on the contrary in an area of elevation. The theory of Darwin and Dana is therefore not applicable to the Fiji Islands. (p. 135)

His ultimate conclusion was that Darwin’s theory had little applicability to the many coral reefs that scientists had studied worldwide, and that “all reefs are local” (to misquote former Speaker of the U.S. House of Representatives Tip O’Neill):

My observations in Fiji only emphasize what has been said so often, that there is no general theory of the formation of coral reefs, either of barrier reefs or atolls, applicable to all districts, and that each district must be examined by itself. (p. 144)

Agassiz died in 1910, never having published the summary account he said he was working on, the one that would tie all of his reef analyses together and bury Darwin. In fact, little trace of any serious work on it has been found.

The Evidence Comes In [Spoiler Alert]

It wasn’t until the middle of the 20th Century that definitive evidence was first gathered resolving the coral reef issue. This involved drilling through reefs. If the cores brought up by the drilling showed only extensive coral surmounting the island base, subsidence would have been at work. Cores that showed shallow ranges of coral on top of non-coral limestone sediments before reaching the island "basement" would support uplift and erosion.

In 1950, on Eniwetok, an atoll in the Marshall Islands, deep cores were extracted through drilling in the reefs as part of an analysis of the local environment prior to upcoming atomic bomb testing (a sad fate). This drilling technology had been unavailable to the previous combatants in the fight. Dobbs dramatically recounts how this initial evidence came in (evidence that was supported repeatedly over the years by drilling elsewhere):

The first cores were clearly reef rock, as expected. As the drill passed the first few hundred feet and out of coral reef depth, the cores changed little. They still appeared to be reef rock. . . . So it went as the drills cut deeper – 500 feet, 1,000, 2,000, 3,000, 4,000. Finally, at 4,200 feet, the drills hit what was unequivocally basement, a greenish basalt, the volcanic mountain on which the reef had originated. . . . For more than thirty million years this reef had been growing – an inch a millennium – on a sinking volcano, thickening as the lava beneath it subsided. Darwin was right, Agassiz was wrong. (p. 254-5)

Closing Comment and Sources

I’m not sure why I find the Agassizs so interesting. I clearly don’t agree with Louis’ stance on evolution and don’t find much about his character very appealing. Perhaps, it’s easier to explain for Alexander, who seems quite quixotic, devoting a vast fortune to ocean voyages to study coral reefs around the world and then never being able to bring it all together. He is a tragic figure, unable to avenge his father’s scientific demise, and losing both his father and wife in a ten day period in 1873. Ultimately, though, I think it’s because the two men are fortunate to have been the subjects of fine books by skilled writers – Edward Lurie’s Louis Agassiz: A Life in Science (1988) and Dobbs’ Reef Madness (cited earlier).

I must give credit where credit’s due; though I read other material, my thoughts on the reef issue were shaped by Dobbs’ book. (All errors are my own.)

The photographs included in this posting are from the Smithsonian Institution. That of Louis Agassiz can be found here; that of Darwin can be found here; and that of Alexander Agassiz can be found here. They were located through the Collections Search Center.

Friday, March 26, 2010

Death Of A Fossil Collection

Home is so sad. It stays as it was left,
Shaped in the comfort of the last to go
As if to win them back. Instead, bereft
Of anyone to please, it withers so,
Having no heart to put aside the theft.

~ from the poem Home Is So Sad
by Philip Larkin

We walked through the living room where the pictures still hung and bookshelves were full. The mail was tossed onto a coffee table. Only a box or two were signs of the dismantling of two lives that had been taking place throughout much of the rest of the house for the past several weeks. This house was beyond that initial moment of bereavement, no longer a home “shaped in the comfort of the last to go.” On this gray day in March, the house was cold.

We climbed the stairs to a gray room. It had been a front bedroom or maybe an office, impossible to tell absent any furniture and with the jumble of boxes and the myriad lumps wrapped in tissue or newspaper randomly strewn across the floor. Now, it was yet another temporary resting place for the final remains of a husband and wife’s fossil and mineral collection. Maybe mineral had deserved top billing at one time, but not now. Rockhounds had already drilled through the collection, leaving mostly fossils. And even those had lost brethren. Reportedly, the Smithsonian had been offered, and taken, some of the prizes in this collection that had been built over decades as the couple traveled the world. Other amateur paleontologists had been through it as well. So, the death knell had sounded, and we were here with a mission, move these remains out of the house and do with them what seemed most appropriate. So sad.

I sat on the floor and began unwrapping. Triage. A cataloguing of final destinations. Curiously exciting because there wasn’t any direction to the exploration, no structure, no certainty of what would be found. It was so unlike a fossil hunting trip where the formation and possibilities are known, as are the improbabilities and the impossibilities. None of that here. Rather, a Jurassic stemless crinoid, spread across a slab of rock like an open armed spiral galaxy, was followed by a bag of worn Miocene fossil shark teeth with sand sprinkled throughout, which in turn gave way to a well-armored Devonian trilobite as fierce today as then. We seemed to be in a giant version of that drawer in the bureau that is a basin of attraction for all of miscellany left over in your pockets at day’s end.

Worse, labels had become detached or were missing. I could tell from those I found that this couple had taken their custody of fossils seriously. For many, they had carefully typed on slips of paper the genus and species of their specimens, and, often, the order and family (ironic in this house with this debris). They’d included the location where the fossils had been found and, usually, the geological formation, too. These they’d slipped into plastic bags with the specimens (too often these bags were not sealed) or affixed them to fossils (with glue that dried, cracked, gave way). Time and rummaging hands had created chaos. So sad.

Days later, for the several items that, after all of the divisions, had become mine, I prepared new labels, a few of which noted “unknown location,” and modified some of the old ones, so that all of them now specified from whose collection the fossils had come. Somehow that seemed only right. It’s what I would hope might happen when my collection is cast to the winds, that some of the orphans will come to carry a name tag mentioning whose hands had dug them out or caught them in the wash on some shore.

But, maybe, it’s not all so sad. There’s still some power in these last few remnants of a collection. For me, at this moment, that power is preserved in those original labels.

I considered them. I tried to translate the scribbled notation running perpendicular to the typing on one, wondering whether the handwriting was his or hers. The labels were a trace of past lives, much like the fossils they attempted to identify. I studied one, really looking at the typing, seeing that “o” and those “e”s in “Eocene,” those dirty letters, literally, the empty spaces in the letters a smear of gray.

And my image of the collectors changed. I saw my grandfather. No, he was not a collector, but he was a typer (you know what I mean). I still have notes he wrote on that manual typewriter of his that rose mysteriously from the bowels of a massive desk when a section of the desktop was lifted up. Whether true or not, my memory is that the keys on his typewriter invariably struck muddy imprints of those and other letters. It feels good to think of him. (I also guess I am a member of the last generation that will remember the dirty letters of printed text.)

And, there’s more to these labels than just these personal memories.

The orphans from that collection have tried to cheer me up with humor and some learning about new places. Like Chunky, Mississippi. Savor that name.

A web journey to Chunky intending to discover its connection to shark teeth skipped me to Meridian and then further east, across the state line into Alabama, to the Shark Tooth Creek where fossil teeth from Cretaceous sharks leach out of the Mooreville Chalk. On one website with a video of families walking the creek in search of shark teeth, someone has posted a comment. In this day and age, I have to assume he or she was serious, something that should have made me sadder, but, instead, made me laugh all the harder. The comment read:

A shark living in a creek? No wonder they became extinct.

Saturday, March 20, 2010

I Do It Wrong ~ The Smithsonian’s Hall of Human Origins

The newly opened David H. Koch Hall of Human Origins, which celebrates the 100th anniversary of the Smithsonian’s National Museum of Natural History, is impressive, well laid out, and ambitious. So, it’s ironic that curator Rick Potts’ overarching intentions fell flat and not because there is anything wrong with them. I’ll share my impressions of the new hall, first, and then try to explain my way out of the conundrum of really liking it while, at the same time, rebuffing Potts.

Though I may be something of a contrarian by nature, that’s not why I came into the new exhibition the wrong way, through the “backdoor.” It was an honest mistake, but, hey, as the exhibition makes clear, I’m a member of an adaptable species. Visitors are supposed to enter the new hall through a “time tunnel” that opens off the Sant Ocean Hall (that hall is well worth a visit). I misread the article in the Washington Post and very carefully walked in from the Mammal Hall (stuffed mammals move me, but not as the folks behind that exhibition intended, so, skip that hall).

I was greeted by a group of models and casts of fossil skulls, each on its own pedestal. A diverse crew, to be sure; among them, the earliest hominid in the exhibition, the 6-7 million year old Sahelanthropus tchadensis with a narrowing and protruding snout, as well as, Paranthropus boisei with its alien and prominent sagittal crest, and Homo heidelbergensis seeming, in comparison to either of those others, so modern. But, for each, the black shadows in their vacant eye sockets seemed to signal that their lives were, indeed, nasty, brutish, and short – a recurrent theme in the exhibit.

In retrospect, beginning with the fossils seems right. Not just because, as I describe later, the intended entrance leaves something to be desired. The fossils are where the science begins. So, as I went through the exhibition in reverse order, the fossil skulls acquired eyes, donned flesh, grew hair, acquired the attributes of humans, a process representing our growing understanding of who and what these hominids were. This is how it has worked in our study of early humans and prehumans, and, I think, it makes for a deeper appreciation of what we have learned.

Overall, the new hall is welcoming (particularly if you come in the back way), a wide open L-shape that never feels confined or cluttered despite containing a wealth of material – over 75 skulls (nearly all models or casts) dominate this space, with a complement of several life-sized bronze statues of hominids engaged in the work of survival, 8 startlingly life-like busts of several hominids (see H. heidelbergensis below), many, many tools ranging from the bulky and crude to the streamlined and refined, as well as beautiful examples of prehistoric art work. Along one side of the hall are three shallow cave-like openings with displays that make a feeble gesture toward interactivity. These displays highlight some of the research that has reconstructed the lives of ancient hominids, actually involving caves.

The exhibition is organized around a couple of questions. One entire wall is dominated by a series of displays that offers answers to a framing question: What does it mean to be human? Described by curator Potts as the “spine” of the hall, this area explores such aspects of humans and human experience as walking upright, new tools/new food, changing body shape and size, bigger brains, and social life.

Another question I saw cutting through all of this was: What role did climate change play in the evolution of humans? The exhibition’s answer is certain: climate change has made a huge difference in the high stakes hominid species-survival lottery. Lives were, indeed, Hobbesian. Climate change takes on the role of an evolution engine. In the human family tree, Homo sapiens is the sole survivor, in part because of our great capacity to adapt to variable climate. Yet, there was nothing certain about that survival. Roughly 74,000 years ago, according to exhibition materials, modern humans were down to the last 10,000 adults of reproductive age, a consequence of severe climate change.

I would recommend that, before visiting the new hall, a prospective visitor watch a brief video in which Potts describes what guided the development of the exhibition. Entitled “Designing the Exhibition,” the video provides a helpful framework within which to place what is on display. Frankly, some of what Potts intended will emerge naturally, while some of it may too subtle to come through without help. (The video is at the official Smithsonian website for the new hall.)

Finally, at the end of my visit, I reached the time tunnel which was supposed to have been my portal into, rather, than my exit from, the new hall. It offered a relatively short, darkened passageway with walls onto which was projected a panoramic view of paintings of some of the actors in the human family drama, including the fully fleshed busts of Homo floresiensis (aka the “Hobbit”), Homo neanderthalensis, and Australopithecus afarensis. I really wonder at the impact of this tunnel on the casual visitor to the exhibition, particularly if this is his or her introduction. The dates below the faces clearly showed that, if you came in the proper way, you would be traveling from the near present to the distant past in our history, but the pictures also may plant the misleading idea that the hominids featured belong in some sort of direct lineage of ours – one evolving from the other. And that’s just wrong. Take but one example, H. floresiensis, who is, I think, really just a relatively recent evolutionary sideshow.

I’ve also heard the time tunnel described as a way to suggest the confluence of changing hominids with changing climate (ah, a subtext for the entire exhibition). If so, it fails on that score, too. The changing environment depicted below the hominid busts simply does not register on the casual viewer whose attention will, I suspect, remain focused on the faces. In the final analysis, the supposed message of the time tunnel doesn’t come through and something misguided may, instead.

There’s a powerful subtext to the entire exhibition. All of those other species displayed here have come and gone. The exhibition states in no uncertain terms – forget all of the features people have used in the past to separate and divide members of our species, forget race. We are all one species. That’s what counts.

Now to tackle directly the conundrum posited at the outset. I think Potts’ intentions for the exhibition weren’t realized with me because he succeeded in another way. To be honest, the framing questions he posed – what makes us human and what’s climate got to do with it – just didn’t seem very interesting, particularly the first. Rather, I could not move beyond my awe in the face of the more than six dozen fossil skulls on display here. And I didn’t want to. Maybe on future visits, but, not this time. I found it difficult to turn away from the original Cro-Magnon skull displayed beside an actual Neanderthal skull from La Ferrassie (France). (Both of these skulls are on loan and will be returning to France in 3 months’ time.) I recognized this particular Neanderthal skull as one of us. I don’t know how else to describe it. Recognition.

So, it’s fossils, fossils, fossils – by far, the most compelling feature of the new Hall of Human Origins.

Saturday, March 13, 2010

A Scientific Patina to an Unscientific Endeavor

I hunt for fossils along the western shoreline of the Chesapeake Bay, with the Calvert Cliffs towering over me as I walk the narrow beaches at their base.

So, I was pretty eager to read a recent article in the Journal of Vertebrate Paleontology by Christy C. Visaggi and Stephen J. Godfrey (marine biologist and paleontologist, respectively) in which they presented a quantitative analysis of the distribution of Miocene fossil shark teeth by genus and species along the Calvert Cliffs. [Variation in Composition and Abundance of Miocene Shark Teeth From Calvert Cliffs, Maryland, Journal of Vertebrate Paleontology, January 2010]

To be honest, after reading it, my excitement was generated less by the interesting results of their study, and more by a key aspect of the methodology they used for sampling teeth from the cliffs. That provided me with one of those moments when my avocational interest in fossils might actually intersect with scientific analysis, and when I have a chance to make a fool of myself.

Let me set the scene. The Calvert Cliffs, stretching for more than 30 miles down the coast, are composed of sediments deposited periodically during a period of roughly 10 million years, from about 18 million years ago to about 8 mya during the Miocene Epoch. Erosion from the cliffs exposes a multitude of marine fossils. Some of these fossils are found in situ in the cliff face (digging is illegal in the cliffs on public land and legal on private land only with permission), but most are caught in the waves and then captured by collectors as “float” along the shore. The fossils from the Calvert Cliffs reflect the breathtakingly rich marine fauna that characterized this arm of the Atlantic Ocean that, during the Miocene (and earlier), ebbed and flowed here in the so-called Salisbury Embayment.

Three different formations make up the cliffs – the Calvert Formation deposited during the Early and Middle Miocene, the Choptank Formation deposited during the Middle Miocene, and the St. Mary’s Formation deposited in the Late Miocene. Significantly, these formations are tilted, sinking at the rate of about 11 ft/mile as one moves from north to south. As a result, the earliest (oldest) formation, the Calvert, is exposed only at the northern end of the western shoreline because it drops out of sight (and out of the reach of the forces of erosion) as one treks south along the cliffs, until only the St. Mary’s Formation is exposed at the southern end of the cliffs. For reasons that will be clear, my collecting is focused on the northern end, that is, on fossils eroding out of the Calvert Formation. The pictures above are of the Calvert Formation, though it is still beyond me to identify the specific layers shown.

Scientists Clench Their Teeth

Visaggi and Godfrey examined teeth secured in two independent ways – 1,866 teeth currently residing in museum collections gathered originally in situ from the various layers of the cliffs by collectors (presumably, professional scientists and others), and 24,409 teeth gathered along the shore as float material.

It’s this latter source of teeth where the story gets particularly interesting for me. These teeth were collected by Calvert Cliffs residents, volunteers with the Calvert Marine Museum (Solomons, Maryland), and members of the CMM fossil club, and were originally donated to the CMM’s Discovery Room where children search through sand to discover fossil shark teeth. During a three-year period, 40 donations with good provenance data were diverted to the analysis of shark teeth distribution along the cliffs.

The authors divided the aggregate float sample of over 24,000 teeth into ten unequal groups based on where along the cliff shoreline they were collected. For example, the group from the beach at the northernmost end was designated CC10 and contained 1,716 teeth, while the southernmost group was CC1 with 614 teeth. Each one of the ten groups was analyzed separately, as was the total float sample.

Each of these sources of teeth – the in situ collection and the float collection – comes with decided biases. Among the various biases discussed by the authors were these two:

Payoff bias
– Collectors of float material have a natural tendency to head north to hunt for teeth because that’s where, by reputation, the payoff is the biggest. In their analysis, the authors emphasized the proportional distribution of teeth for different sections of the cliffs in an effort to mitigate against that potential bias. They suggested that this fossil wealth in the north reflected a deep, open marine paleoenvironment highly supportive of sharks that prevailed when the Calvert Formation was being laid down.

Size and Prize bias – Most collectors are likely to be biased against small teeth through a lack of interest in them or an inability to spot them. Teeth that are large, rare, or unusual are likely to be the focus of collectors’ energy. Interestingly, the authors conclude these sought-after teeth were likely to be overrepresented among the in situ teeth (adding these to a museum’s collection would be a priority), but underrepresented among the float teeth (the typical collector of these teeth would keep the best finds). The authors suggested that for the large, rare, or unusual teeth, the actual proportional representation would fall between those of the in situ and the float samples.

Scientific Guise

I appreciate the cleverness in amassing over 24,000 teeth from donations to the museum to create an impressive sample of teeth from the cliffs. Despite the clear biases and limitations of relying on float material, the sheer number of teeth suggests some confidence in what can be gleaned from this sample. Still, there might some interesting idiosyncrasies of float collectors that influence what is gathered.

[Later edit: I dropped a paragraph in which I speculated about other biases of float collectors and the number of individuals who might have been involved. It was misdirected because I failed to realize that 40 donations might have involved lots of people contributing to each donation, not a maximum of 40 contributing the whole sample (which raised the question of just a few collectors influencing the whole).]

So, what’s a rank amateur to do? Explore the idiosyncrasies, of course. I decided to devote my next trip to the Calvert Cliffs to answering this question:

How does the distribution of genus and species among the teeth I might collect on a single day compare to the distribution of the float sample reported by Visaggi and Godfrey?

A foolish venture, I recognize. One that was patently unscientific – a single sample taken by one collector on one day for, it turned out, three and a half hours (my back could only take so much stooping and bending on the beach) certainly doesn’t carry any weight. Any divergence from the paper’s results could be easily explained away. But, my motives were pure, I think, and it gave the trip some structure and I could delude myself that it added a very thin scientific patina to the effort. Plus, it was a damned sight more fun than going back through the teeth I had already collected from the Calvert Formation to see what their distribution looked like.

Let’s start with a few of my credentials to engage in this folly. I am a collector of float material and I reflect the payoff bias in spades since my collecting from the Calvert Cliffs is concentrated in an area at the northern end where the float samples CC10 (1,716 teeth) and, possibly, CC9 (7,523 teeth) came from.

It’s interesting to consider the possible influence of the size and prize bias on my collecting this day. I clearly have this bias – it’s hard not to. But, since I wasn’t donating my finds, it wouldn’t matter for the results. I wouldn’t be skimming off the “good stuff.” As for the tiny stuff, I decided to gather up every tooth that I spotted, regardless of size. This resolution presumably didn’t reflect one held by the collectors of the float material in the Visaggi and Godfrey study. Also, to be honest, I couldn’t follow through on this for the entire three and a half hours I was collecting. For roughly the last hour, I just couldn’t bear to stoop for yet another speck washing up around my boots.

What the Scientists Found

First, I think it’s appropriate to review the general results that Visaggi and Godfrey present in their paper because they are broadly relevant to my little exercise.

1) Overall, along the Calvert Cliffs, teeth come predominantly from gray or requiem sharks (Carcharhinus spp.), sand tiger sharks (Carcharias spp.), tiger sharks (Galeocerdo spp.), snaggletooth sharks (Hemipristis serra), and mako sharks (Isurus spp.). (“spp.” is the plural abbreviation for species – in other words, there are multiple species within the specific genus.)

2) The vast majority, about 90%, of all teeth, in situ or float, are found in the generally northern end of this expanse of shoreline, the section associated with the Calvert Formation.

3) The further south one goes, the relative distribution of teeth from these five genera of sharks changes – grays and sand tigers diminish in their representation among the teeth being found, while teeth from snaggletooth sharks and makos increase proportionately.

Relevant Study Samples Meet Mine

More relevant than their overall findings for my project were the results from the float samples Visaggi and Godfrey analyzed that came from the stretch of beach where I collect. This is principally the sample they identified as CC10, though it’s unclear precisely where the boundary between CC9 and CC10 falls. I suspect my wanderings took me from CC10 across that boundary, hence I not only compared my sample to the CC10 results, but also to a second aggregate sample I created from data in the article for CC9 added to that for CC10.

My sample consisted of the 157 teeth I collected on one cold, windy day in March, from about 2 hours before low tide to about 1 ½ hours after. Lots of wave action, and steady erosion from the cliffs. Little feeling in my fingers. Altogether, a typical winter day at the bay.


The table and graphic below present the relative distribution among the genera accounting for the largest proportion of teeth found. Any genus accounting for at least 1% of the total of any specific sample – mine, CC10, or CC9+10 – is included. These eight genera include the five listed above and the following: cow sharks (Notorynchus primigenius), lemon sharks (Negaprion eurybathrodon), and hammerhead sharks (Sphyrna laevissima). These eight account for between 87% (my teeth) and 92% (CC10) of the teeth in these samples.

Genus/SpeciesMy TeethCC10 TeethCC9+10 Teeth

Notorynchus primigenius0%4.6%1.2%
Carcharias spp.7.6%10.1%8.8%
Isurus spp.3.2%4.5%2.5%
Hemipristis serra7.0%8.0%9.4%
Carcharhinus spp.45.2%56.9%50.7%
Galeocerdo spp.17.2%7.0%17.6%
Negaprion eurybathrodon3.8%0.1%0.1%
Sphyrna laevissima2.6%0.6%0.9%

Some nice examples of teeth in these predominant categories are shown below. This first one contains some teeth from grays.

This next picture shows some tiger shark teeth. These are all from the contortus species. (In this little exercise, I’ve followed Visaggi and Godfrey in grouping these specific teeth under the Galeocerdo genus.)

A few sand tiger shark teeth.

Finally, a trio of snaggletooth teeth, all three of these are uppers and would be pointing down in the shark’s mouth – two have serious root damage and the third is quite pretty.


So, what was learned? Well, though I doubt this will offer much comfort to Visaggi and Godfrey, my sample actually closely tracked the distribution of these genera in both CC10 and CC9+10. Gray shark teeth (Carcharhinus spp.) dominated in all three samples. The better match overall with my sample was with the combined float CC9+10.

Anomalies? Sure. I found no cow shark teeth (Notorynchus primigenius) on this day, and my handful of lemon shark teeth (Negaprion eurybathrodon) and hammerhead shark teeth (Sphyrna laevissima) represented a much larger percentage of my sample than in either of the others.

Knowing that I was collecting for distributional analysis made a difference. As noted, I set out to act as a vacuum for much of the day, as a result I chased down all stray teeth (at least for awhile), painfully bending frozen fingers to pick yet another tooth from the waves, from under a downed tree, or from a chink in a clayey block of fallen cliff face. This may account for my luck with lemon and hammerhead shark teeth which are smallish teeth and, on another day, ones I might otherwise have skipped or missed. That doesn’t explain why my sample was missing other little teeth that did show up in the CC10 and CC9 samples, though still at marginal rates.

While I was on the beach, I was continuously comparing this day to the myriad others I’d spent gathering teeth in this same area. For much of the time, I bemoaned how bereft of the really good stuff – read the “large, rare, and unusual” – the beach was. They had taken an inopportune leave of absence. Then, I had one of those moments when I felt I’d stepped into some sort of parallel fantasy universe, actually, the fantasy universe of fossil hunters. In this universe, the fossil gods deigned to reward me. Almost everywhere I looked, there were wonderful teeth, one stuck in the sand, black glistening crown pointing to the cliff top, another two tumbling together in the surf, another one nestling next to the prize I had just grabbed with a sharp intake of my breath. It didn’t last long, but it was grand while it did, and all the while, I was sure it made my sample on this day totally irrelevant. But, not so. The good stuff either fit into expected categories or was outweighed by the other teeth gathered earlier. Besides, who was I kidding, my sample, despite any scientific gloss I tried to give it, was probably irrelevant from the outset


Moral of the story – one day’s collection does not a valid sample make, though it’s not bad.

Thursday, March 4, 2010

Classical Gas

In which the blogger ponders breaking wind, benefits of a toxic gas, mass extinctions (ah, the paleontological link), and taking blood pressure in mice – yes, the usual mélange.

A Passing Gas

Pediatrician Howard Bennett contributes a column to the kids’ page of the Washington Post and his little identifying blurb at the bottom of each column says that “he writes about gross things” – very true. His columns are direct, informative, fun, and never condescending to his (supposed) young audience. A quintessential example is the column that ran on February 22, 2010, with the daring title (daring for the Post and, yes, it generated complaints) of :

“Ever Wondered Why People Fart?”

Thank you for asking. Yes, I have.

According to Bennett, it’s a combination of the air we swallow (some of which reemerges as belches – topic of his previous month’s column, of course) and the job done on undigested food by the bacteria that inhabit our gut. These bacteria add gases, including oxygen, nitrogen, hydrogen, carbon dioxide, methane, and hydrogen sulfide, to the swallowed air and that gaseous mixture has to get out some how.

As for that smell, credit goes to the hydrogen sulfide some of the bacteria produce as their own waste product from chowing down. Hydrogen sulfide (H2S) – that’s the paleontological link here. More on that in a moment.

I’m still lingering on the flatulence and the smell. Biologist Betsey Dexter Dyer knows bacteria and she wants us to know them, too. In her book, A Field Guide to Bacteria (2003), Dyer identifies the macroscopic field marks of bacteria, those signs around us of the presence of different species of bacteria that we can see (without a microscope), smell, taste (think I’ll pass), and touch. As for the bacteria in our intestinal tract, it’s not surprising they’re there, because, as Dyer writes, “Guts are safe, nutrient-rich places, extremely popular as habitats . . . .” (p. 16-17) The gut-dwelling bacteria work at fermenting and processing food our own enzymes cannot. Dyer ascribes the smell of flatulence to sulfate-reducing bacteria, and some fermenter bacteria, particularly when they process foods with abundant sulfur compounds (e.g., broccoli). These sulfate-reducing bacteria in our gut produce hydrogen sulfide as a waste product.

But, wait, isn’t hydrogen sulfide, even in relatively slight concentrations, highly toxic? Yup, it kills. Farting, clearly, is no joking matter.

“Yet, paradoxically, we need H2S to survive,” asserts biologist Rui Wang in a recent Scientific American article on the positive and potentially lifesaving attributes of hydrogen sulfide. His primary focus is not on the gas being generated in our intestines, but rather on that produced by the human body in blood vessels. (Toxic Gas, Lifesaver, by Rui Wang, Scientific American, March 2010) It turns out that an enzyme found in blood vessels combines with a specific amino acid to produce hydrogen sulfide, among other compounds. Working with rats, Wang found that H2S helps to regulate blood pressure by causing smooth muscle cells to relax, thereby dilating blood vessels. (The effects of another gas, nitric oxide, on reducing blood pressure are well established.) Wang notes that the potentially positive reach of hydrogen sulfide in the human body may extend beyond the cardiovascular system; it may have positive effects in the nervous system. Regulation of metabolism may also be one of its effects, leading Wang to some dramatic speculation about hydrogen sulfide hibernation to stabilize trauma victims at disaster sites. Pretty heady stuff.

Smelly Extinctions – An Upside?

So, a little hydrogen sulfide, good; more, bad. Each instance of hydrogen sulfide production in the human body appears to have an upside, despite the toxicity of the compound. Wang suggests that this capacity of humans to utilize hydrogen sulfide has its roots in our long, long, long ago ancestors’ survival of the massive Permian extinction. The theory of the causes of this extinction, described by Wang, features enormous amounts of hydrogen sulfide generated by bacteria thriving in oxygen-depleted oceans. When that gas, so the theory goes, bubbled to the surface of the ocean, it made the atmosphere toxic. Wang writes:

The importance of H2S in human physiological processes is probably a holdover from that long-ago time. The creatures that survived this catastrophe were the ones able to tolerate and, in certain cases, even consume hydrogen sulfide, and we humans have retained some of that affinity for the gas.

If true, I wonder if our tolerance for hydrogen sulfide produced by sulfate-reducing bacteria in our intestines is another legacy of that survival.

With regard to the Permian extinction and hydrogen sulfide, Wang cites an article by paleontologist Peter D. Ward (Impact from the Deep, Scientific American, October, 2006). To say that Ward is prolific is a gross understatement. He’s written many informative and entertaining books on this and related topics, and there are various videos out there of him lecturing on these issues, including a succinct and amusing one posted on dotSub from about a year ago.

Ward, as I understand him, posits that possibly only the extinction at the end of the Cretaceous was the direct outcome of an impact with Earth of a large extraterrestrial object. He labels others, such as the Permian and the one marking the divide between the Triassic and Jurassic, as “greenhouse extinctions” and a central villain in each is hydrogen sulfide. Ward asserts that the greenhouse extinctions follow a generally similar sequence of steps (a few are cited in this post – for more detail, see Under A Green Sky (2007), particularly p. 137-138): temperatures on the planet rise relatively suddenly due to the release of carbon dioxide and methane from vast areas of volcanic action called blood basalts; the warming of the world changes ocean circulation patterns leading to a growing presence of warm low-oxygen water at the ocean bottom; over time, this anoxic water pushes up toward the top, reaching sunlight which sparks massive growth of bacteria in this water that produce hydrogen sulfide in enormous quantities; this gas bubbles from the ocean into the atmosphere killing some life as it goes and subsequently destroys the ozone layer, delivering the coup de grâce.

Now, that’s what my family, invariably in a conversation around the dinner table, would certainly call “TF” or terminal flatulence.

Postscript – Working with Mice

To refine his research further, Wang and others developed a line of mice lacking the enzyme that enables production of hydrogen sulfide in blood vessels. They analyzed the impact on blood pressure of the absence of this compound in these mice and then its reintroduction.

So, how is blood pressure measured in mice?

With little blood pressure cuffs, attached to their tails.

Not sure I’d trust those readings. Surely, a lab mouse’s blood pressure must soar as a lab worker approaches with the little blood pressure cuff (“oh, no, not the tail again”) – yes, a classic case of “white coat hypertension.”
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