Phosphorus, Brachiopods, and Fate: A Story Marred in the Retelling

In her newspaper column, the popular science writer paints a bold and exciting picture of the role of phosphorus on evolution in deep time.  She draws on a new scientific opinion piece by two paleontologists.  This post describes how I think the columnist gets it wrong.

Phosphorus had been on my mind.  Number 15 on the periodic table, the element is volatile, toxic, and essential.  Prone to explosions, it’s frequently found under the “control” of calcium in the form of phosphate rock (Ca3(PO4)2).  “In what is perhaps the most disgusting method of discovering an element, phosphorus was first isolated in 1669 by Hennig Brand, a German physician and alchemist, by boiling, filtering and otherwise processing as many as 60 buckets of urine.”  (Jefferson Lab website, U.S. Department of Energy.)

One of my most recent encounters with this element was in Enola Holmes 2 on Netflix, the second movie in the mystery series and, though this outing is not quite up to the standard set by the first, it’s still fun.  (Millie Bobby Brown breaks the fourth wall with the best of them.)  The dire situation of “match girls” working in a London match factory in 1888 prompts the action of the movie.  The “strike anywhere” matches they made had tips coated in a mixture that contained white phosphorus, a highly unstable allotrope of the element.  Exposure to the fumes from the mixture wreaked havoc on the workers’ health.  (Lowell J. Satre, After the Match Girls' Strike: Bryant and May in the 1890s, Victorian Studies, Autumn, 1982, Vol. 26, No. 1.)

I’ve since learned that today’s safety (“strike on box”) matches use red phosphorus, a more stable allotrope, and that the element is not in the match tip, but embedded in the rough box sides.  The heat from the friction created by drawing the match across the box side converts the red to white phosphorus which combusts when exposed to air, igniting the match head.  (Match, Encyclopedia Britannica, accessed January 23, 2023.)  (Clearly, when I learn something new, I feel the need to share.)

So I was primed when I came across a column by science writer Natalie Angier about the hypothesis newly advanced by paleontologists Petr Kraft and Michal Mergl.  Angier’s piece, titled The Sad Fate of the Ancient, Well-Shelled Mariners (The New York Times, November 4, 2022, online version), describes in dramatic terms the scientists’ contention that the availability of phosphorus in the Paleozoic Era (roughly 541 to 252 million years ago, encompassing the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods) played a critical role in the evolution of invertebrate organisms utilizing phosphorus to build their shells.  She focuses on brachiopods.  Their fate, in Angier’s account, hinged on the availability of phosphorus.  In the Cambrian, a surfeit of phosphorus gave rise to hard phosphatic shells.  Come the Devonian, a diminished supply of phosphorus doomed these shell builders to extinction.

Early in her column, Angier writes: 

As researchers recently proposed in the journal Trends in Ecology and Evolution, the collapse of the brachiopod empire exemplifies a struggle that has defined life from the start: the quest for phosphorus.

And the brachiopods lost out in that quest with the rise of vertebrates making judicious use of phosphorus in fashioning their skeletons.

She concludes her piece with:

“It’s a big advantage to have these [phosphatic] shells,” Dr. Kraft said.  By comparison, the shell of a modern mollusk, made of calcium carbonate, cracks easily beneath a beachcomber’s feet.  But as the seas grew crowded and bony fishes appeared, phosphate supplies dwindled, and brachiopods could no longer freely scavenge what they needed to construct their expensive housing.  Bony fishes were judicious in their use of phosphate as a building material: their teeth, a few parts of the skeleton, and that was it.  And being mobile, fish could trap whatever phosphate and other nutrients filtered down from land to sea, before they reached the lumbering hard shells below.

This, at least, is the story she tells and attributes to Kraft and Mergl.

But is that the story the paleontologists actually told in their paper titled Struggle for Phosphorus and the Devonian Overturn (Trends in Ecology & Evolution, Volume 37, Issue 8, August, 2022, residing behind a paywall)?  Broadly speaking, that’s mostly correct but it’s certainly not when it comes to claims of collapse of the brachiopod “empire.”  The arguments advanced by the scientists in that regard are carefully nuanced, not so Angier’s retelling.

My focus is specifically on their and her treatment of brachiopods, a taxon about which I know a bit.  Brachiopods are invertebrates whose shells suggest mollusks, though their internal physiological structures are quite different.  (Also, brachiopod shells are singly symmetrical, that is, each half of a shell matches the other half.  In contrast, mollusk shells are not necessarily symmetrical individually, but a mollusk’s two shells are mirror images of each other.)  Brachiopods were particularly abundant during certain periods of the Paleozoic Era, but relatively few genera have managed to survive to the present.

At the risk of committing interpretative errors on a par with those of Angier, it’s important to summarize what I think are the salient points that Kraft and Mergl make in their paper.  They begin by asserting that phosphorus is a critical “limiting” element for biological processes.  In living organisms, phosphorus is a component of DNA and RNA, of ATP (adenosine triphosphate), the energy source for living cells, and of cell membranes.  (It strikes me that our so-called “carbon-based” life forms might just as well be considered “phosphorus-based.”)  In vertebrates, phosphorus is used in forming skeletons.

They note that biomineralization of shells utilizes one of three types of minerals:  calcium carbonate, calcium phosphate, or silicon dioxide.  A core hypothesis they put forward is that, during the Paleozoic Era, fluctuations in the availability and biological uses of phosphorus directly influenced the evolutionary fate of certain shell-building taxa.  Because phosphorus was so abundant in the early Cambrian, they assert, many taxa arose that built their shells out of calcium phosphate.  It was “the golden age of phosphatic shells of the groups that produced them.”  Among the groups they identify in particular creating such cells are linguliform brachiopods, tommotids, and hydrozoans.  (As already noted, my interest is in the treatment of brachiopods, not the others about which I know little.)

During the Ordovician, organisms profligate in their use of phosphorus in shell building were at a disadvantage as availability of the element declined.  This was compounded, the scientists posit, by increased use of phosphorus in cells.  Linguliform brachiopods suffered and went into general decline.  In the latter part of the Ordovician and in the Silurian, phosphorus was more readily available, but the Devonian saw another marked reduction, particularly, they argue, because of a “key factor:”  the rise of vertebrates drawing significantly on phosphorus for skeleton building, sealing the fate of many phosphatic shell builders.  “They were victims of a combination of circumstances in the long-term trend of phosphorus availability.”

It's quite a provocative hypothesis, one that mostly removes the decline of invertebrates using calcium phosphate to build shells from the overall impact of the three mass extinctions that punctuated the Paleozoic (end-Ordovician, end-Devonian, and the largest of all mass extinctions, the end-Permian).  Kraft and Mergl argue that many builders of phosphatic shells went into decline in the interims between the mass extinctions.

It's also a very neat hypothesis whose arguments, if not read and interpreted carefully, I fear may reduce a very complex set of interacting factors to a relatively simple horse race with a set of winners and a set of losers, and a single cause, the relative abundance and uses of phosphorus.  The temptation to strip away any complexity and nuance from Kraft and Mergl’s opinion piece is one that Natalie Angier gives into in her New York Times column.  In it she fails to heed a key limitation that the scientists imposed on their claim for the impact on brachiopods of changes in the availability of phosphorus.

The Brachiopoda phylum (brachiopod “empire” as Angier characterizes it) did not collapse because of vertebrates’ increase utilization of phosphorus.  Kraft and Mergl did not make that claim.  Rather, they identified linguliform brachiopods as among the groups that, in particular, used phosphorus to craft their shells, and that this specific group of brachiopods suffered as the relative supply of phosphorus diminished, possibly due to greater use by vertebrates.

Bear in mind that the Brachiopod phylum was (and is) made up of taxa whose shells were composed of calcium phosphate and taxa with calcium carbonate shells.  So, at a minimum, reduction in the supply of phosphorus wouldn’t affect the latter.  Further, the origins of the brachiopods with calcium carbonate shells stretches far back into the early Cambrian when, apparently, phosphorus was abundant.  (Sandra J. Carlson, The Evolution of the Brachiopoda, The Annual Review of Earth and Planetary Sciences, Volume 44, 2016, p. 424.)  Thus, some brachiopods, very early on, were uninfluenced by the supply of phosphorus.  

The linguliform brachiopods that Kraft and Mergl cite specifically had inarticulated shells (no tooth and socket hinge) that were, yes, exclusively phosphatic in nature.  They did indeed decline during the Ordovician.  But that didn’t seal the fate of entire brachiopod empire because the articulated brachiopods (those with tooth and socket hinges), bearing calcium carbonate shells, flourished.  Of all known extinct and extant genera, 95 percent are articulated brachiopods.  So, the Ordovician and later seas were not, as Angier seemingly would have it, somehow bereft of brachiopods.  Instead, the balance in the brachiopod world seemingly had shifted to the articulated taxa which were less reliant on phosphorus.

For that matter, when is it thought that the brachiopod empire actually did collapse?  The end-Permian extinction event is the key.  The brachiopod kingdom went into serious decline toward the end of Permian.  (Decline does not equate to disappearance since brachiopods are still with us, including some members of the Linguliformea subphylum, builders of calcium phosphate shells.)  Paleontologist Douglas H. Erwin notes, “About 90% of brachiopod families and genera disappeared between the mid-Permian and the Early Triassic.”  (Extinction:  How Life on Earth Nearly Ended 250 Million Years Ago, 2006, p. 108.)

To belabor the point even further, I turn to a paper by Stephen Jay Gould and C. Bradford Calloway which challenged the once popular argument that brachiopods faded into relative obscurity while mollusk bivalves (specifically, clams) rose to prominence because the former failed to compete successfully.  (Phosphorus doesn’t play any role in this scenario.)  They tabulated the number of genera of brachiopods and clams found across a range of time intervals, beginning in the lower Cambrian.  (Clams and Brachiopods – Ships That Pass in the Night, Paleobiology, Volume 6, Number 4, 1980.)  They found:

The famous pattern of Paleozoic domination by brachiopods followed by a later hegemony of clams arises as a result of one incident:  the Permian extinction.  Brachiopods exceed clams throughout the Paleozoic.  The Permian event then affects brachiopods far more strongly than clams.  Both groups decline, but clams much less so, and the earliest Triassic stage finds clams ahead, a status they have never relinquished.  (p. 386)

Exploration of the causes of the end-Permian mass extinction is beyond the scope of this post, but nothing in my reading on the subject implicates phosphorus.

Here’s my take on the hypothesis advanced by Kraft and Mergl.  Frankly, I don’t see any room for the dramatic struggle that Angier depicts in her article which is, I think, misplaced, failing to acknowledge that brachiopods did not fade into obscurity because of a change in the availability of phosphorus.  In my opinion, the soundest takeaway from the Kraft and Mergl hypothesis is that fluctuation in the availability of phosphorus may well have influenced how evolution proceeded with regard to shell building.  An abundant supply in the Cambrian may have enabled many inarticulated brachiopods with their calcium phosphate shells to flourish, and they may have suffered as the supply of phosphorus decreased, but they never went away.  Not as exciting or newsworthy I guess.

A couple of final points.  The hypothesis being put forward by Kraft and Mergl isn’t really new.  As early as 1984, paleontologists Peter J. Cook and John H. Shergold posited that an abundance of phosphorus across the late Precambrian and early Cambrian may have given rise to a shelled fauna utilizing calcium phosphate as the building material.  (Phosphorus, Phosphorites, and Skeletal Evolution at the Precambrian-Cambrian Boundary, Nature, Volume 308, March 15, 1984.)  They are cited by Kraft and Mergl in a footnote.

Another aspect of Angier’s column I found rather confusing.  The print and online versions are illustrated with a photograph of an assemblage of fossils whose caption points specifically to one from the Leptaena genus of brachiopods.  I won’t violate copyright law and reproduce that photograph here.  Instead, here is a picture of a portion (about 20 mm across) of a Leptaena brachiopod from my fossil collection.  It's on a piece of limestone from the Brookville Formation (Indiana), Late Ordovician in age.

So, why, I wonder, would a brachiopod from this genus be used in Angier’s column, given that this is an articulated brachiopod whose shell is made of calcium carbonate?

The Last American Dinosaurs at the Smithsonian ~ Paleoecology of the Hell Creek Formation

In the last two million years of the Cretaceous Period, the area of Montana and the Dakotas presently marked by the Hell Creek Formation was decidedly green, a humid and semi-tropical landscape featuring rivers and forests.  The formation itself, according to paleontologists John R. Nudds and Paul A. Selden, “is a fluvial deposit, laid down by meandering rivers, which frequently flooded onto a broad alluvial coastal plain on the eastern side of the Rocky Mountains.  The rivers flowed east across this plain into a large epeiric [shallow, inland] sea, the Western Interior Seaway, which during Cretaceous times was retreating southwards and eastwards, exposing the coastal plain.”  (Fossil Ecosystems of North America, 2008, p. 182.)

The Last American Dinosaurs:  Discovering A Lost World, the new exhibit at the Smithsonian’s National Museum of Natural History, features the Hell Creek Formation, then and now.  Perhaps the exhibit's most immediately appreciated gifts are the dinosaurs which end the barren months that followed the closing of the museum’s fossil hall earlier in the year for renovation.  Two mounted skeleton casts of a Tyrannosaurus rex and a Triceratops horridus dominate the entrance to the exhibit (the casts affectionately known as Stan and Hatcher, respectively), and several other Triceratops skulls and the skull of an Edmontosaurus appear as well.

But, the heart of the new exhibit is captured by its subtitle.  If visitors make the effort to go beyond the dinosaur hook and consider the story being told by the exhibit, they will be rewarded, learning a bit about that 66-million-year-old “Lost World” – what it was like in its ecological complexity, and also how we have come to know it.  So much more than dinosaurs.

The best of the exhibit is its middle where the opposing walls offer complementary narratives.  On one wall is an array of recent pictures of the contemporary, arid landscape of the Hell Creek Formation showing museum scientists scouring the rocks for fossils, such as tiny teeth and impressions of plants.  Here is the fieldwork that underpins the exhibit.

  Among these images is a particularly lovely one focused on that critical piece of fieldwork equipment – toilet paper.

More importantly, fieldwork is placed in its proper context.  As visitors step back from this montage of pictures, they encounter a display, not only of field equipment, but also of what happens after the fieldwork, from how specimens are safeguarded in the field to the careful and painstaking prep work in the museum’s labs.  In fact, someone standing before this montage need only turn to the right to look through the windows of the FossiLab and see ongoing work on fossils.

On the opposite wall from the pictures of collecting at Hell Creek is a wonderful mural by Smithsonian scientific illustrator Mary Parrish.  It’s primary element is her rendering of a stream scene in the Hell Creek area some 66 million years ago.  She acknowledges that she’s filled it with more species than were likely to be in such a location at one time, but the scene captures the essence of what that environment was like – wet, lush, and green.  The key to this scene is that dinosaurs do not dominate it; they are a part of an environment graced by plants and other animals.  It’s an ecosystem, not a blockbuster movie scene.

I hope visitors take the couple of minutes needed to watch the video associated with the mural.  In it, Parrish explains the process she follows in creating her artwork.  It’s excellent; indeed, all of the videos in this exhibit are first rate.  (Several of the videos, including Parrish's, can be found here.)  Still, I do have a complaint about how her mural is treated.  The mural itself includes more than this pre-extinction scene and some of the material that went into making it.  It encompasses the destruction of the extinction event and the recovery of the landscape in the Paleocene Epoch, yet visitors, I suspect, will not quite get that.  Perhaps because of the unfortunate position of a pillar, it was decided to interrupt the mural’s flow with a tall, metal frame holding displays about the asteroid hit.  It connects to the pillar and stands at right angles to the mural, dividing it in two.

The fossil material around the Parrish’s stream scene reinforces its message.  Visitors will see a beautiful cast skeleton of Didelphodon vorax, the largest mammal living 66 million years ago.  I am quite taken by this cat-sized marsupial, both because it’s bigger than I, in my ignorance, believed mammals to have become when they co-existed with dinosaurs, and because it disappeared in the end-Cretaceous extinction event, along with some other mammal taxa.  This extinction may have opened a door to mammal diversification and size increase, but it came with a mammalian cost.

Parrish carefully places one in her mural.  Seen here, clinging to a limb, beneath a philodendron leaf.

Plants, plants, and more plants.  The mural is awash in vegetation.  Displayed immediately before the mural are fossil impressions of leaves of plants that lived then.

Further, a slab of leaf impressions suggests how abundant the vegetation was.

Of the many other aspects of the story of this lost ecosystem that merit attention, there is one I will mention in closing – what message it might hold for us now.  Given the contemporary demise of many species, the exhibit asks whether we are in the midst of the sixth mass extinction.

There’s a great deal to like about the exhibit, and, happily, I think it’s a harbinger of what we will experience when the renovated fossil hall opens in 2019.

Witticisms in the Face of Mass Extinction

I fear that this posting will be taken as a retelling of an old joke or several jokes from paleontology.  That’s one advantage of being a paleontological amateur of recent standing, these are new to me.  Actually, the term “jokes” misses the mark, these are “witticisms” fashioned by clever paleontologists to describe real phenomena.  I found these all at once, rather than spread out over the decade in which they came into being.  That timing may explain some of my unease about this cluster of witticisms, the focus of this posting.

Earlier this week I spent a dreary morning in the Library of Congress’ Science and Business Reading Room.  The weather outside offered lowering clouds that threatened rain, and a building wind.  This complemented the atmosphere inside with its dim lights, dark wood desks, and a sepulchral echo whenever someone dared to walk the aisles.  All totally appropriate for the texts that lay on my desk, texts recently released from the bowels of the library.  The books were not old themselves – none of the volumes on my desk dated from earlier than the mid-1980s, but the tales they told were ancient – tales of mass extinctions.

(Given these atmospherics, this moment in the drafting of this posting constitutes a fork in the road.  In one direction lies the story I intend to tell, a relatively light-hearted one with a bit of my curmudgeonly attitude attached to it.  In the other direction floats a “ghost” story, perhaps something involving a dusty volume with engravings from which spring long extinct creatures.  Say, for instance, a rash of trilobites breaking the quiet with their skittering across the desks as they flee from the plate volume of Reverend William Buckland’s Geology and Mineralogy Considered with Reference to Natural Theology (1837).

Ah, that would be a ghost story inspired by those magical texts conjured up by Montague (“Monty”) Rhodes James a century ago (such as those he published in Ghost Stories of an Antiquary, 1904).  James, of King's College, Cambridge, wrote ghost stories in the British tradition, stories laced with horror and the macabre, not ghosts, per se.  Part of the horror of my tale would be the prospect of other extinct creatures sliding or thundering from the engravings.  Perhaps some other time, there are paleontological witticisms to explore first.)

Those who study mass extinction events distinguish them from background extinction, that continuous process of winnowing taxa (taxonomic groups), a sort of white noise of extinction.  In contrast, mass extinctions, according to paleontologist Peter D. Ward, “are geologically short intervals of intense species death.”  He notes that, “During the last 530 million years of earth history, the time since the advent of commonly skeletonized creatures on earth, there have been about 15 mass extinctions.  Five of these may have involved as many as 50 percent of the earth’s species.”  He considers three of these to be “major” because they “completely reorganized the ecosystems in the sea and, more relevant to humanity, on land.”  These would be those of the End-Permian (truly, the mother of all extinctions), Late Triassic, and End-Cretaceous.  (Rivers in Time:  The Search for Clues to Earth’s Mass Extinctions, 2000, p. 6.)

It was a morning of contrasts in the LC reading room.  For all of the death that marked mass extinctions and the profound impact these events had on earth’s living organisms, mine, though, was mostly a light-hearted charge.  Critical to analysis of extinction in general, and mass extinction in particular, is determining whether a taxon or taxa have in fact gone extinct.  It’s no simple exercise.  I had been struck by how thoughtful paleontologists have been in identifying the subtle processes that might affect that determination in the fossil record.  Equally striking, though, is how analyses of these situations prompted several paleontologists to engage in clever wordplay to capture their essence, wordplay that, in my mind, may challenge the usual gravitas of the science.  That was my pursuit – the origins of several of the best of those scientific witticisms.  It was really nothing new, very well-trod ground.

Here then are the three bits of wordplay that I explored.

Lazarus Effect

In the early 1980s, paleontologist David Jablonski demonstrated his familiarity with the New Testament when he gave a name to the phenomenon of taxa that disappear from the fossil record in mass extinction intervals only to resurface some time later, apparently not victims, but survivors.  “This disappearance and apparent extinction of taxa that later reappear unscathed can be termed the Lazarus effect.”  (Causes and Consequences of Mass Extinctions:  A Comparative Approach, in Dynamics of Extinction, edited by David K. Elliott, 1986, p. 197.)

It’s a clever and memorable label for such an occurrence.  Perhaps, he suggests, these Lazarus taxa sought refuge elsewhere, riding out the storm, so to speak.

I don’t intend to demean this first term, which I think is a stroke of genius, or the phenomenon it describes.  The Lazarus effect is not just a curiosity.  Jablonski uses it to great advantage, seeing it as “a rough indication of the completeness of the fossil record for the interval in question. . . .  The magnitude of the Lazarus effect is an indication of the distortion suffered by the fossil record in that time interval.”  (p. 197)  Further, “the Lazarus effect, can be used to assess patterns of extinction near mass extinction boundaries:  apparent gradual declines in taxonomic diversity leading to the extinction event can only be accepted as genuine if they exceed the magnitude of the Lazarus effect.”  (p. 211)

Elvis Taxa

In a 1993 piece, paleontologists Douglas H. Erwin and Mary L. Droser consider a related issue – the accurate identification of Lazarus taxa.  (Elvis Taxa, Palaios, Volume 8, Number 6, December, 1993, p. 623-624.)

They acknowledge the importance of the Lazarus effect for its utility in characterizing the quality of the fossil record, and, based on the length of time between the disappearance and reappearance of the Lazarus taxa, its contribution to an understanding of some aspect of the recovery of the environment after the extinction event.  But they stress that its usefulness depends upon correctly identifying the Lazarus taxa.  They note, “Extensive homoplasy and morphologic simplicity may confound recognition of Lazarus taxa.”

As I understand it, homoplasy describes the situation where unrelated taxa share very similar or identical morphological traits.  (See, for example, Homoplasy, A Good Thread to Pull to Understand the Evolutionary Ball of Yarn, ScienceDaily, February 24, 2011.)  As a result, finding such a taxon may lead to the conclusion that its look-alike had risen from the grave when it hadn’t.  “These apparent Lazarus taxa are a taxonomic artifact.”  Erwin and Droser “suggest that such [ersatz Lazarus] taxa should be known as Elvis taxa, in recognition of the many Elvis impersonators who have appeared since the death of The King.”

They then argue for the positive contribution of such taxa for our understanding of possible limits to the amount of “play” in evolution.

Elvis taxa, properly recognized, illustrate the pervasiveness of homoplasy but also the constraints on morphological evolution and community construction.  If Elvis taxa are as common as Lazarus taxa they may indicate that morphology may be more highly constrained than commonly believed, or that particular roles require particular morphologies.

I have to admit that this particular label strikes me as a bit too playful.  Even Erwin and Droser appear to be sensitive on this score.  They acknowledge,

New terms should be proposed with caution, when they describe a particularly important phenomenon, and never in jest.  In addition, terms should be short and memorable if they are to achieve any currency.

Hmmm, . . . Elvis taxa . . . memorable, sure, but I have to think these two paleontologists had a laugh, or several, when they coined the name.  They explicitly reject following in Jablonski’s biblical footsteps, aspiring to what they characterize as “a more topical approach.”

Zombie Effect

In Dinosaur Extinction and the End of an Era:  What the Fossils Say (1996), paleontologist J. David Archibald waxes enthusiastic about the fossil record despite its limitations.

The spotty nature of the geological record is not unique to natural history.  All histories bear this burden.  There is, however, no cause for despair.  The information that has been preserved in the rock and retrieved by human effort is truly a wondrous précis of past life. (p. 64)

But he strikes a serious cautionary note, “The difficulty comes when we must determine whether our record is accurately portraying the biological past.”  (p. 64)

He focuses his concern for the accuracy of the record on the implications of several different phenomena, including the Lazarus effect.  He introduces a new term, the Zombie effect, for a potentially misleading artifact of the fossil record resulting from the reworking of fossils from older rocks to younger ones, leading to the erroneous conclusion that some taxon was alive after it had actually gone extinct.  These reworked fossils, Archibald writes, “lurk in later sediments like the living dead.”  (p. 68)

He describes organic (e.g., burrowing) and mechanical (e.g., changes in streams) processes leading to the Zombie effect.  And then reinforces his initial cautionary warning,

I argue that the Zombie effect is far more common that we paleontologists would like to believe.  Basically, we should be suspect of any fossil bone that is not articulated with a good bit of the rest of the skeleton.  (p. 70)

Surely Archibald has fun with this term, making sure he works in phrases like “the living dead” and “exhumed remains of organisms that lived earlier” to describe the Zombie effect.  I suppose once the literature sports terms like Lazarus effect and Elvis taxa, Zombie effect doesn’t seem so far out of line.

Still, even though I particularly like the Lazarus effect (I suspect it was not offered in jest), I would suggest that these very real, very important phenomena are somewhat devalued and robbed of some their deserved gravitas by the cumulative effect of these witticisms.  Restraint is in order.  But, after reading a column by Edward Willett (Guess How Some Fossils Are a Lot Like Elvis, Regina Leader-Post, February 1, 2007), I have to wonder whether that's possible.  Some two decades after Archibald brought forth the Zombie Effect, Willett mentions that some would describe “so-far-undiscovered bones that must be hidden somewhere” as a Jimmy Hoffa taxon.  May it stay buried.

Perhaps this turned out to be a ghost story after all.

Of Mice and Megafauna

The travails associated with opening my summer cottage each spring help explain why I enjoy some of the recent research on the extinction of the North and South American megafauna. This fauna of very large mammals which went extinct at the end of the Pleistocene Epoch (about 11,500 years ago), included such creatures as 15 foot tall woolly mammoths (image on left below), “smaller” mastodons at roughly 8 feet high (image on right), giant ground sloths the size of elephants, bear-sized giant beavers, and saber-toothed cats on the order of today’s Siberian tiger.













The link between the megafauna extinction research and my summer cottage is the lowly house mouse (Mus musculus) which checks in with a body of between 3 to 4 inches in length, complemented by a tail of perhaps equal length.

In the slender volume entitled The Ecology of a Summer House (1984), the late biologist Vincent Dethier painted a loving portrait of nature within the confines of a summer bungalow in Maine over the course of summer and into winter, ending with the deep snows of December. A renowned expert on flies and on insect behavior, he brought a scientist’s precise perspective to the subject, and coupled it with an artist’s sensitivity to life and death. As was only natural, mice and summer homes were joined in the book.

For as long as I can remember there had been wood mice in the bungalow. They were year-round residents, true natives. Each summer when we opened the house there would be numerous signs of their winter occupancy despite all efforts to discourage it. (p. 22)

Wood mice (Apodemus sylvaticus), despite their destructiveness and, perhaps because of their “air of delicate charm,” were treated by Dethier with a gentle hand. Still, he was enough of scientist to experiment with one mouse mother and her pups to see if she really knew how many she had (she didn’t).

Every spring, when I first approach the front door of my summer cottage, it is with a sense of anticipation tinged with definite dread, the latter a feeling I suspect Dethier never experienced with his bungalow. Though the cottage has been without its human occupants for nearly all of the fall and winter seasons, it has not been unoccupied. The wintering-over residents are most likely to have included, among others, house mice, not Dethier’s cuter wood mice.

A first order of business in opening the cottage for the season is searching for evidence of mice amid the dust and cobwebs. Sure, this involves some scanning for destruction, but that’s usually well hidden, waiting to be discovered late one night when, in desperate need of sleep, I unfold the sofa bed or reach into the bottom of the chest with the blankets.

The best evidence, the telltale sign, that few, some, or hordes of mice partied here in my absence is mouse scat. Though it’s hard to be precise in using this evidence to measure the extent to which the mice wintered over within these walls, with experience I have developed an instinctive internal gauge about these things.

Upon reflection, I have had to conclude that my annual spring “analysis” of mouse scat and its implications for the state of the cottage predisposes me to appreciate recent research on the extinction of the American megafauna.

This research grapples with what appear to me to be among the core questions of paleobiology: When did some set of events occur? What were the causes? What were the consequences? In this case, the key event is the extinction of the megafauna. Scientists know these large animals were still around at roughly 15,000 years ago and by the end of the Pleistocene were gone. Timing is everything. It is particularly critical for weighing the various alternative explanations offered up for this extinction. These extinction theories include (1) climate change dooming the megafauna, (2) newly arrived Paleo-Indians hunting the animals to extinction, (3) those same Paleo-Indians bringing some virulent disease that decimated the megafauna, or (4) the impact of a comet setting off a catastrophic chain of events that led to the extinction. (For an overview of these theories, see End of the Big Beasts by Peter Tyson, on NOVA Beta Evolution page, March 1, 2009.)

I think that one avenue of research on the megafaunal extinction is particularly brilliant and therein lies the summer cottage link. This thinking begins with the understanding that the herbivores among the megafauna consumed a huge amount of plant biomass and, as a result, must have generated copious amounts of dung, as in, say, mastodon scat. It also stands to reason that this waste would have become home to dung-living fungi, particularly Sporormiella, which produce spores on dung. Further, as a result of ingesting so much cellulose, it makes sense that the herbivore megafauna emitted vast amounts of methane.

A couple of recent analyses are very clever in using this scenario to fashion answers to questions surrounding the megafaunal extinction. Jacquelyn Gill of the University of Wisconsin and her colleagues analyze cores of sediment taken from the bottom of a lake in Indiana (supplemented with data from New York lakes), and measure changes in the presence in the cores of Sporormiella spores. (Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America, Science, November 20, 2009). The spores are washed from the dung into the lakes by “slopewash” and their relative abundance at different levels in the cores are taken to reflect the waxing and waning of the megafauna. I love this example of that “indirect” scientific approach – if something cannot be measured or witnessed directly, find some associated effect that can be.

What do Gill et al. find? They conclude that the inception of a significant decline in the Sporormiella spores pegs the beginning of the megafaunal decline at 14,800 years ago, with full collapse at about 13,700 years, and apparently extinction at about 11,500 years. So, the ultimate extirpation of this fauna took awhile. With these dates in hand, they offer various conclusions, among them, that the “rapid-extinction hypotheses” are wrong, so no comet impact (even the most likely candidate occurred at 12,900 years ago which is well after the onset of the decline) and no “Paleo-Indian blitzkrieg” bringing the megafauna down. They acknowledge that humans may well have contributed to the decline. Of interest, among other findings, they posit that significant changes in vegetation followed the megafaunal decline, and that the megafauna decline began during a warm period and well before the sudden cooling associated with the Younger Dryas period.

The second piece of research is one I just came across. It is a tightly reasoned, mathematical analysis that explores the potential impact of the megafauna extinction on the amount of methane in the atmosphere and the consequences for the climate. The authors, led by biologist Felisa A. Smith of the University of New Mexico, marshal data on 114 megafauna herbivores that died out at the end of the Pleistocene. (Felisa A. Smith, et al. Methane emissions from extinct megafauna, Nature Geoscience, published online May 23, 2010.) Using estimated data on the body masses of these herbivores, their per square kilometer density, and their ranges, the authors calculate that these animals’ combined annual methane production was 9.6 teragrams (9,600,000 metric tons). When those animals went extinct, that methane contribution to the atmosphere ended. Since methane is a greenhouse gas and its loss could have had substantial consequences for the climate, Smith et al. relate their findings to data on atmospheric methane concentrations derived from ice-core records. Their ultimate conclusion?

We find that the loss of megafauna could explain 12.5 to 100% of the atmospheric decrease in methane observed at the onset of the Younger Dryas. . . . [O]ur calculations suggest that decreased methane emissions caused by the extinction of the New World megafauna could have played a role in the Younger Dryas cooling event.

Though each of these studies addresses a different, though related, set of questions, there is one critical area in which they don’t agree – the time period over which the extinction occurred. As noted, the megafaunal march to extinction revealed in Gill’s data begins about 14,800 years ago, reaches collapse about 13,700 years, and extinction by 11,500 years. Smith, in contrast, places the timing of the extinction across a time period from 13,400 years ago to perhaps 12,500 years, supported in this by the decline in methane concentrations shown in the ice-core data. As a consequence of these differences, Gill would have the decline and collapse of the megafauna take place well before the beginning of the Younger Dryas cooling, a climatic change Smith suggests could itself be attributed in part to the extinction. Significantly (I think), the ice-core data in Smith’s piece seem to show rising methane concentrations for several hundred years after Gill would have the megafauna decline begin in earnest. More research and thinking may be in order.

Regardless, both of these analyses show intellectual virtuosity in their efforts to extract meaning from their data. I enjoyed them both. Smith’s analysis was especially hard to resist because in her acknowledgements she thanks the NPR news quiz show Wait Wait . . . Don’t Tell Me for “providing critical stimulus and motivation to pursue the project.”

As for the role of humans in the demise of the megafauna, we don’t have an answer, but many of us certainly are inclined to believe it wasn’t inconsequential. I turn back to Dethier and the wood mice in his summer bungalow.

The mice, like all other creatures in the house, lived in the territory of nature’s greatest predator, ignorant of the power of life and death that he held over them. Nothing is too large to be slain or too small to be obliterated. In the world at large we can kill them all, from the sperm whale to the virus. What was I to do with the mice? (p. 24)

Source of Images

Both images are from the Smithsonian Institution and were taken by Dane A. Penland in 1977. The mammoth picture can be found here and that of the mastodon here.

Extinction and Adaptability

Some Permian (299-251 million years ago) fossil shark teeth came my way recently. These teeth from Xenacanthus texensis, a fresh water shark, are tiny (usually under 2 mm in height) and very weird, with a two-bladed crown and a little cusplet between the blades. Curiously, these sharks managed to trickle through the End-Permian Extinction – the Big Kahuna of extinctions (so far) – only to disappear during the succeeding period, the Triassic. [Later edit: I need to clarify that I'm not sure this particular species X. texensis outlasted the Permian. The order Xenacanthiformes as a whole did, but then the order went extinct in the Triassic.]

Surviving a mass extinction . . . . Interesting thought. I blame evolutionary ecologist Clive Finlayson for the extinction pall I currently find myself under. His recent Wall Street Journal piece (October 10-11, 2009, link here) identifying the five best books on extinction(s) was what really started me thinking about this. (I describe the article in more detail in the column at the right – at least, it is true as of the date of this posting.)

Finlayson has been studying the fate of Homo neanderthalensis. There isn’t a consensus theory on the cause or causes of their extinction after they enjoyed a 200,000 year period of dominance in Europe and western Asia. The competing theories range widely, from H. sapiens having a more energy efficient body structure to H. sapiens acting like we always do when we invade. I haven’t read Clive Finlayson’s new book, The Humans Who Went Extinct: Why Neanderthals Died Out and We Survived, but a recent Scientific American article (Twilight of the Neandertals, Kate Wong, August 2009) describes his thinking which centers, in part, on adaptability. The Neanderthals, according to this theory, had survived many climates changes during their period of dominance but, unlike modern humans, were unable to adapt to a string of rapid swings in climate that punctuated the end of their time on Earth.

Adaptability . . . . I was fuming earlier this week about unwelcome changes. Minor but upsetting nonetheless. A new format for my local newspaper (including little, ersatz pen and ink sketches of the columnists at the top of their columns – do I need to know that a favorite columnist has a double chin? should it make a difference? does it?); my first glimpse of an issue of Natural History magazine in many years (sadly, a pale reflection of its former robust self); and the introduction of Windows 7 (no further comment on that last). If these things are enough to upset my equilibrium in an epoch of war, pestilence, climate change, . . . .

The history of life on Earth is as much a history of extinctions as it is of survival. Faced with the reality that 99.9 percent of all species that ever were are no longer, writer Christopher Cokinos concludes, “Civilization is not a given. Extinction is.” (The Consolations of Extinction, Orion, May/June, 2007 – link here) Come on, give me those consolations. That some do survive is one he offers (but seems to take it away later). He counsels equanimity in the face of the inevitable. In the midst of the woeful Holocene (our current epoch) extinction, accelerated by human action, his advice is do what you can to ameliorate H. sapiens’ impact. Stay calm even when you realize that ultimately even the planet is toast. He writes, “I’m saying too much grief for the world means less energy to help it along.” In essence, there’s no point to the grief.

Perhaps that’s a perspective that comes from age, from long experience with natural cycles. To the 19th century (but deeply contemporary) poet Gerard Manley Hopkins (perhaps my favorite poet), grief in the face of the inevitable end of life is perhaps felt most sharply by the young. In his poem Spring and Fall: to a young child, he offers no consolation to Margaret for her sorrow over the “unleaving” (falling leaves) of Goldengrove. Not surprisingly, the Fall season represents the end of many things, including life. Of course, there’s also the Christian concept of the “Fall of Man” and what presumably flowed from that. (What I took away from all those undergraduate English courses – any great poem in the Western canon can be analyzed with at least one of the Big Trinity – sex, death, or Christianity, and, in Hopkins’ case, usually all three.)

At the poem’s conclusion, Hopkins asserts that Margaret’s grief is really over her own shared fate, telling her:

It ís the blight man was born for,
It is Margaret you mourn for.

Speaking to me, earlier in the poem, he makes it clear that he thinks it is different (though not better) for those with some life experience:

Áh! ás the heart grows older
It will come to such sights colder
By and by, nor spare a sigh
Though worlds of wanwood leafmeal lie;

Sadly, no consolation that. Perhaps, in a strange way, wishful thinking, because I still don’t “come to such sights colder.”