Saturday, January 28, 2023

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?

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