Tuesday, May 30, 2017

Brooding on Stragglers


I’ve always defined straggler as “someone who falls behind, who fails to keep up.”  So I was intrigued to find that in the literature on periodical cicadas those members of a brood who arrive before or after their expected year of arrival are called stragglers.  Clearly, my definition is much too narrow, missing the essence of what it means to straggle.  The Oxford English Dictionary defines straggler variously, but its first meaning is:  “One who wanders or roves without fixed direction; one who strays from his companions or from the regular route; a gadabout; a camp-follower, a tramp, a vagabond.”  The heart of this as it applies to periodical cicadas is “one who strays from his companions.”  It doesn’t matter whether the straggler arrives early or late, just that the straggler is separated from his or her companions.

In the past several weeks, neighborhoods near me in the Maryland suburbs of Washington, D.C. have seen the emergence of a massive (well, it seems like that) number of periodical cicadas, those of the genus Magicicada.  Lord, what a beautiful name for this genus.  Entomologist William T. Davis named this genus in 1925 but didn’t pause to explain his thinking.  (Cicada Tibicen, A South American Species, With Records and Descriptions of North American Cicadas, Journal of the New York Entomological Society, March, 1925.)  (Periodical cicadas are different from their brethren who emerge annually after relatively brief underground sojourns.)

Streets near me became killing fields with the smashed bodies of cicadas strewn everywhere, testament to the implacable force of the automobile.  Winds deposited in the gutters many, many shells from which the insects had crawled after their extended stay underground sipping on tree sap.  Vertical surfaces, such as trees or telephone poles, were the scenes of amazing transformations as the ghostly cicadas emerged from their shells and, shortly, took on color and substance, before launching into the air in brute-force flight, nothing graceful about it.





Delve into the life history and paleontological history of cicadas and you fall down Alice’s rabbit-hole.  No other insect has a longer total life cycle than the periodical cicadas who have two cycles, one of 13 years and the other of 17 years.  Complicating this story is the fact that there are three “species groups” of periodical cicadas: Decula, Decim, and Cassini.  Each of these groups independently evolved into forms that live under ground for either 13 or 17 years (well, for the most part that’s true, an issue I’ll return to momentarily).  These 13- and 17-year forms have organized themselves into “broods” with different years of emergence and different geographic ranges.  Underlying this developmental (and mathematical) complexity is one astounding attribute, what entomologist Stewart H. Berlocher has described as “the real kicker”:  nearly every brood includes members of the three “species groups.”  (Regularities and Irregularities in Periodical Cicada Evolution, PNAS, April 23, 2013.)

The idea of a “species” seems under some duress here.  Although the Decula, Decim, and Cassini groups interbreed probably rarely enough to be considered separate species, the species designation for the Magicicada, so current thinking goes, drops down one stage to the 13- and the 17-year cohorts of each species group.  This results in the following species of periodical cicadas:  Magicicada tredecim, M. neotredecim, M. tredecassini, M. tredecula, M. septendecim, M. cassini, M. septendecula.  The first four are 13-year species (M. neotredecim came into being recently, evolving from a cohort of M. septendecim, a 17-year species); the last three are 17-year species.  Berlocher notes, “All broods of each of these species have occasionally been proposed as species.”

In Maryland, our periodical cicadas are only members of 17-year broods:  Brood II (last appeared 2013, next expected in 2030), Brood V (in western Maryland – last appeared in 2016, next expected in 2033), Brood X (last appeared in 2004, next expected in 2021), and Brood XIV (lasted appeared in 2008, next expected in 2025).  (17 & 13 Year Cicadas, Cicada Mania website.)  In Maryland, each time one of these broods crawls out of the ground and sets the trees singing, each of the 17-year species, M. septendecim, M. cassini, and M. septendecula, is represented.

Stragglers.  Herein lies another great source of confusion in this already complex life cycle.  The bulk of a brood emerge as expected at the end of their defined 13- and 17-year cycles, but varying numbers of so-called stragglers can emerge before or after their expected arrival year.  Generally, stragglers emerge four years early, one year early, or one year late.  In Maryland, stragglers from Brood X were awaited this year (four years before their 2021 due date) and, apparently, they’ve arrived.  Or, have they?  The verdict is not in.

A bit to my south, in North and South Carolina, and Georgia, Brood VI, a 17-year brood, has emerged on schedule.  Are the periodical cicadas here in Maryland a northern extension of Brood VI?  Entomologist Jonathan Neal raises this question on his blog Living with Insects in the post titled The 2017 Cicada Mystery (May 21, 2017), writing,
Brood VI emerged in NC, TN and GA earlier this year. Brood VI in the past has had heavy emergence in IL, MI and WI. The question cicada sleuths are asking: “Is the current emergence in MD Brood part of Brood X that is emerging 4 years early? Or could the emergence be part of Brood VI. If it is Brood VI, are there factors that cause it to increase its range or cause populations to fluctuate over time? If it is Brood X, what is causing large numbers to emerge early[?”] Many studies and tests will be needed to arrive at a conclusion.
Are they simply stragglers of Brood X arriving four years early?

Or are they the products of the Brood X group that emerged in 2000 (four years before their 2004 due date)?  Biologist Gene Kritsky, on the Mount St. Joseph University’s MSJ Cicada Website, observes that, at least in southwest Ohio, these Brood X stragglers came out in 2000 in numbers that were sufficient to overwhelm predators and allow some of them to mate and reproduce.  It’s those stragglers that he expects to be emerging now.  Are they becoming a separate brood, no longer stragglers from another brood?

So we’re left with a big unknown.  Absent data on the numbers of periodical cicadas that emerged in 2000 in Maryland, it’s hard to know which of these possibilities – stragglers from Brood X, range extenders from Brood VI, or products of the stragglers that emerged in 2000 – is the case here.

Other aspects of these life cycles have attracted lots of scientific interest.  The hypotheses about why these periodical cicadas have so synchronized their life cycles that all three species groups nearly always emerge in enormous numbers at the same time tend to center on safety in numbers.  Predators are overwhelmed, so many individual cicadas survive to reproduce.  That evolution has moved to 13 and 17, prime numbers, is often explained by reference to the challenge those periods pose to predators trying to harmonize their own shorter life cycles to those of their prey.  It has also been suggested that these periods ensure that cicadas breeding on different cycles will very seldom emerge at the same time, avoiding the prospect of crossbreeding that would doom the offspring.  (See Susan Milius, Mystery in Synchrony:  Cicadas’ Odd Life Cycle Poses Evolutionary Conundrums, Science News, July 13, 2013.)

Further, straggling is a longstanding phenomenon.  Some periodical cicadas are always early or late to the party.  Opinion about the current straggling raises the prospect that climate change has had an impact with warmer conditions prompting faster growth and earlier emergence.  As writer Scott Dance describes it, “[S]cientists say there are always some subsets of the 17-year cicada broods that don’t wait the full cycle before emerging.  These experts think cicadas ‘count’ in fours, and if they are big enough after 13 years, some crawl out sooner.”  (This Isn’t a Cicada Year, So Why Are They Now Showing Up Across the Mid-Atlantic?, Washington Post, May 20, 2017.)  But research by entomologist Richard Karban shows that the members of any specific brood do not develop in lockstep, some complete certain stages before their compatriots, but, in general, the early finishers, as science writer Susan Milius puts it, “end up waiting for the signal to emerge, giving the laggards time to catch up.”  (Mystery in Synchrony citing research by Richard Karban.)

Apparently, these 13- and 17-year cycles are relatively recent phenomenon in geological and paleontological terms.  Based on genetic studies, biologist Teiji Sota and his colleagues, conclude that the three species groups (Decula, Decim, and Cassini) separated some 3.9 million years ago, during the Pliocene Epoch.  They posit that this speciation process occurred when populations of their common ancestor became geographically isolated and evolved into separate species groups (allopatric speciation) and then, later, came back together to evolve their synchronized life cycles.  (Independent Divergence of 13- and 17-Y Life Cycles Among Three Periodical Cicada Lineages, PNAS, Volume 110, No. 17, April 23, 2013.)

Does the work by Sota et al. mean that no cicadas before, say, 3.9 million years ago had such long life cycles?  I don’t know.  They suggest that because these species have shown repeated shifting between 13- and 17-year cycles during their existence, there is probably a genetic basis for these life cycles that predates the separation into the three species groups.  Was it manifest before that point?  Again, I don’t know.  Cicadas, themselves, (not necessarily periodical ones) do go back, far earlier than the Pliocene.  The wonderful fossil cicada (unidentified as to genus and species) shown below is from the Eocene Florissant site in Colorado.  It's some 34 million years old.


(This picture is reproduced from the National Park Service’s Florissant website.)

Apparently the earliest cicada fossil that has garnered general agreement that it is, in fact, that of a cicada dates from the Early Cretaceous (more than roughly 100 million years ago).  (George Poinar, Jr., and Gene Kritsky, Morphological conservatism in the foreleg structure of cicada hatchlings, Novicicada burmanica n. gen., n. sp. in Burmese amber, N. youngi n. gen., n. sp. in Dominican amber and the extant Magicicada septendecim (Fisher) (Hemiptera: Cicadidae), 2012, accepted manuscript subsequently published in Historical Biology, Volume 24, Issue 5, 2012.)  But Poinar and Kritsky believe that the cicadid group goes back much farther, perhaps to the Permian Period.

I’ll close with some wonderful speculation (I assume that's what it is, speculation) by entomologist Scott Richard Shaw in his book Planet of the Bugs (reviewed previously in this blog).  He posits that insects played an essential role in the development of dinosaur diversity by being the source of protein for smaller, herbivorous dinosaurs which, in turn, were a source of protein, along with other insect-eating animals, for carnivorous dinosaurs.  Efforts in the insect world to deal with dinosaurs' incessant munching on its denizens prompted, Shaw observes, various kinds of avoidance strategies among insects.  Some turned to “behavioral escape mechanisms” in which case, writes Shaw, 
[I]t’s certainly possible that mayflies’ and cicadas’ mass synchronized emergences adapted and were fine-tuned in response to intense dinosaur predation.  (p. 121)
A neat thought.

Sunday, April 30, 2017

Sexual Dimorphism ~ Another Challenging Variable


This post was prompted by the recent publication of a fascinating article by paleontologist Jordan C. Mallon about our ability or, more specifically, our inability to distinguish between males and females in the available fossils for several prominent dinosaur taxa.  (Recognizing Sexual Dimorphism in the Fossil Record:  Lessons from Nonavian Dinosaurs, Paleobiology, 2017.)

Sexual dimorphism is present when characteristics of the males and the females of a given species differ.  These differences may be reflected in male and female morphology including such features as body shape (e.g., display features such as crests), body size, or coloring, and may also be found in variations in behavior.  Sexual dimorphism is an important attribute that is central to evolutionary biology, particularly as it relates to sexual selection.

With extant animals, some aspects of sexual dimorphism may be obvious as with the African lion species (Panthera leo) whose males sport a distinctive mane and may be somewhat larger than the females.  Pictured below are mounted specimens on display in the Hall of Mammals at the Smithsonian’s National Museum of Natural History.



A word of caution.  Having living animals available for study offers no certainty that we will, in fact, accurately identify sexually dimorphic traits.  The lions in the Hall of Mammal, as posed, promote the widely accepted canard that the female lions do all of the hunting for the pride and the males just lounge around.  It was only in this decade that careful tracking of male and female lions in Kruger National Park (South Africa) using GPS and other technology showed that females hunt in open areas while males do their hunting in denser vegetation shielded from observation.  (Scott R. Loarie, et al., Lion Hunting Behaviour and Vegetation Structure in an African Savanna, Animal Behaviour, Volume 85, 2013.)

Certain patterns of sexual dimorphism have been detected in extant animals.  For example, with respect to size and morphology, biologists Ehab Abouheif and Daphne J. Fairbairn observe, “In most species of animals, females attain larger body sizes than do males (e.g., most spiders, insects, fish, amphibians, reptiles), whereas in most birds and mammals, males are the larger sex.”  (A Comparative Analysis of Allometry for Sexual Size Dimorphism:  Assessing Rensch’s Rule, The American Naturalist, Volume 149, Number. 3, March, 1997, p. 540, references omitted.)

But all bets are off with extinct animals known from the fossil record.  Mallon is not alone in positing that the barriers to identifying the sex of extinct species are particularly high.  Armed only with fossils, we find ourselves without much of what might help to distinguish males from females including gender-specific behavior as well as some morphological attributes such as sex organs and coloring.  Without the information that observation of living animals provides, paleontologists face a daunting challenge.  As evolutionary ecologist Robert J. Knell and his colleagues have written,
Identifying sexual dimorphism in animals known only as fossils is often difficult; specimens of a particular species are sometimes rare, unique, or unavailable, and reliably identifying sex in fossils is often difficult or impossible.”  (Robert J. Knell, et al., Sexual Selection in Prehistoric Animals:  Detection and Implications, Trends in Ecology and Evolution, 2012.)
On the one hand, if the sex-related differences preserved in the fossil record are slight, then, as Knell et al. note, one must work with large samples (IF such sample sizes are actually available).  Apparently, it hasn’t been unprecedented for claims to be made about the sexes of dinosaur fossils based on bones from just two specimens (which is clearly an inadequate sample size).  At times, it may be differences in the sizes of features that distinguish males from females, not the presence or absence of such features in either of the genders.

On the other hand, “when dimorphism is strong, there is a risk that different sexes will be described as different species.”  (Knell, et al.)  That is, relying on morphology to identify species, as is a common and necessary practice in paleontology, poses the risk that significant differences in fossils, possibly related to sexual dimorphism, might be treated as indications of the presence of different species, not the two sexes of the same species.

From his analysis, Mallon concludes that, with the dinosaur fossils at hand, paleontologists, for the most part, cannot tell whether an individual dinosaur was male or female.  In this vein, I would add that one must ignore the nicknames given some of the iconic dinosaur fossil skeletons such as the Field Museum’s Tyrannosaurus rex which is known as “Sue,” the Triceratops horridus cast on display in the Last American Dinosaurs exhibit at the Smithsonian’s National Museum of Natural History which is fondly called “Hatcher” and its companion T. rex cast in the same exhibit carrying the nickname “Stan.”  (The National Museum has Hatcher's fossil bones, while its Stan is just one of many casts that can be found worldwide.  Stan's original fossil bones are in the Black Hills Institute.)  These nicknames honor the collectors who found the original fossils – Sue Hendrickson, John Bell Hatcher and Stan Sacrison.  In all fairness, the Field Museum’s T. rex was nicknamed Sue by Peter Larson because he believes this specimen was, in fact, female.  (See, Jacqueline Ronson, Sue the Tyrannosaurus Has a Sexual Identity Crisis, Inverse Science, December 1, 2016.)

Yes, there are some dinosaur fossils that can be identified as female because they carry eggs or have a medullary bone which, in contemporary birds, supplies the calcium used to fashion egg shells.  Mallon concludes that, for the taxa he studied (nine prominent species including Tyrannosaurus rex, Allosaurus fragilis, a couple of Stegosaurus species, and Protoceratops andrewsi), the available evidence offers “no support for sexual dimorphism.”  Mallon takes pains to make it clear he is not saying that such dinosaurs were not sexually dimorphic, simply that, in his considered view, the fossil record is inadequate to make such a claim.

This is certainly a thought-provoking, revisionist conclusion and it’s not just relevant for dinosaurs.  Mallon lays out the conditions under which sexual dimorphism might be successfully detected in different fossil taxa.

For me personally, his article brought me up short.  I have to admit that for the years that I’ve collected vertebrate fossils (principally shark teeth), I have never pondered the gender of the animals and how that might be reflected in the fossils I have at hand.  So much for my intellectual curiosity and my efforts at taxonomy.

Considering just fossil shark teeth, several of the most important variables that might account for differences in the teeth under study are:

  • natural variation among individuals within the same species (hey, this variability is the material that natural selection work with),
  • chronological age of the specimens (not only might teeth from younger sharks be smaller, they might also differ in terms of shape),
  • position of the teeth in the jaws (most shark families are heterodonts, that is, teeth vary by location in the jaws; depending upon the shark, teeth from the upper jaws can differ from those in the lower jaw, and/or they can vary by location within a specific jaw),
  • differential influence of the environment on the health and development of individuals from the same species,
  • taphonomic processes (i.e., what happens to an organism in death) which may introduce variability in the fossils that emerge millions of years later, and
  • sexual dimorphism.

This list is based primarily on paleontologist Bretton W. Kent’s Fossil Sharks of the Chesapeake Bay Region (1994, p. 2).  Though he does include sexual dimorphism as a factor behind some variation in fossil shark teeth, as far as I can tell, it’s only in a very few species that sexual dimorphism has been identified.  He cites it in just two fossil shark species found in the Chesapeake Bay region – Hexanchus (cow sharks), and Rhizopriondon (sharpnose sharks).

Ultimately, it’s somewhat ironic that, despite his warnings and the unlikelihood of detecting sexual dimorphism in most of my collected fossils, Mallon’s piece has prompted me to be sensitive to that possibility.

Thursday, March 23, 2017

A Point With Multiple Dimensions


I manage to confine my collecting to a few, discrete interests, though the temptation is always there to add another.  I admit I felt a tug as I read novelist Gary Shteyngart’s account of his obsession with expensive, mechanical wristwatches, sparked by Donald Trump’s rise to power over the course of 2016.  Yes, it seemed to him that the time is out of joint.

How almost irresistible when he writes lovingly about his four-thousand-dollar Nomos Minimatik Champagner watch with its “exhibition-case back” revealing its inner mechanics:
[It] is a riot of sunburst decoration, tempered blue screws, and a small constellation of rubies.  A tiny golden balance wheel spins back and forth, regulating the time . . . , and this action gives the watch the appearance of being alive.  It is not uncommon for some watch enthusiasts to call this part of the watch its “heart,” or even its “soul.”  (Shteyngart, 2017, p. 38.  Full citations to all reference are provided in the Sources section at the end of this post.)
But, no, collecting mechanical watches would be far too expensive and possibly smack of affectation.  Even the few interests I’m feeding are too many.

Nevertheless, there is a virtue to multiple interests because, at some rare moments, my interests intersect, suggesting there may be some sort of logic to my collecting mania, not simply a psychological need.  So it is with a recent purchase, a rather small Native American projectile point with a broken tip from New York.  Though certainly not of high quality nor expensive, this point is profoundly appealing because in it are met three of my collecting passions – points, minerals, and fossils.
This is a triangular Madison Point, an identification based on William A. Ritchie’s A Typology and Nomenclature for New York Projectile Points (1961, p. 33-34), which squares with what the seller asserted.  According to Noel D. Justice, Madison Points were part of many “cultural phases across eastern North America” during the Late Woodland period, probably dating from 800 to perhaps as late as 1350 CE (Justice, 1987, p. 224, 227).  Ritchie describes Madison Points as “very finely chipped by pressure flaking” (p. 34), though I assume that the artisan who fashioned this point used some direct percussion flaking initially to shape it.  Ritchie also notes, “Among the northern Iroquois the principal material employed was Onondaga flint from the exposures of central New York and the Ontario Peninsula.”  (p. 34)  Absent any provenience (beyond the geographic state in which it was recovered), I won’t make any such claims for this particular point.

Nevertheless, my point is manufactured from Onondaga chert.  This gray chert is often referred to as flint, though, as I have learned, geologists apply the term chert to all of those rocks that artifact and rock collectors tag with such terms as flint or jasper based on differences in color.  As Prothero and Lavin write, “The color differences are not only superficial, but frequently misleading, especially since heat treatment may change the color range of a particular chert.”  (Prothero and Lavin, 1990, p. 561.)

Chert has a geological life story understood in general, but apparently not in its smallest details.  It is a hard sedimentary rock composed of silica (silicon dioxide) in the form of very tiny crystals, either only discernable under a microscope (microcrystalline) or not even then (cryptocrystalline).  This material comes into being through what has been characterized as “an obscure chemical process”  (Roberts, 1996, p. 24) involving the precipitation of silica from solution.  Chert can be found as nodules, lenses, or beds depending upon the environment within which it forms.  Given its fine crystalline composition, chert fractures conchoidally, that is, “fracture faces have smooth, curved surfaces” with sharp edges (Ostrom and Peters, 2012, p. 12) which makes it ideal for fashioning projectile points, scrappers, and knives.

Assuming the chert in this point was found as a nodule, which is likely given its origin in the Onondaga Formation dating from the Devonian Period, it is also highly likely that the source of its silica was the hard silicon elements (spicules) that sponges secrete to give themselves structure and a means of protection.  (Maliva et al., 1989, p. 523)

From their analysis of thin slices of the chert from different sites in the northeastern U.S., Prothero and Lavin find that the chert in Native American implements can often be easily identified as to its place of origin.  I found their description of chert from western New York State particularly applicable.  They write, in part, of this chert:  “Small (0.1 mm in diameter) dolomite rhombs are abundant, but there are few large rhombs.  Fossils and pyrite are common . . . .”  (Prothero and Lavin, p. 570)

Two elements of that description ring very true for the point I have in hand.  First, there are the dolomite rhombs which are the rhomboidal crystals or impressions of those crystals left by some dolomite caught up in the geological process creating the chert.  The mineral dolomite is a calcium magnesium carbonate and is the primary component of dolomite, a biogenetic sedimentary rock.  Pictured below is a close-up (30x) of a small portion of one face of my point.  This chert, under magnification, appears less strongly colored than a more removed view would have it since the color is, I surmise, often the cumulative effect of impurities in the mineral.  I have marked several of the rhomboidal shapes that appear on the surface of this section of the point; these are, I assume, dolomite rhombs.


Then there are the fossils in Onondaga chert which is where another of my collecting interests finds play in this point.  Pictured below is the full point (minus, of course, its tip) with an inset showing a closer look at a small hole in its surface – a fossil mold.
Given the common occurrence of fossils in Onondaga chert, it isn’t at all that surprising that Native American stone artifacts fashioned from this material would, sometimes, contain a fossil or two.  Source of the mold in my point?  The seller suggested coral and he may well be right.  But, the impression in the point strongly reminds me of columnals from some types of crinoids.  These animals, also known as sea lilies, create long structures of “stems” topped with long branch-like appendages for food gathering.  Secreted calcium carbonate pieces (ossicles) provide crinoids with structure and protection; in the stem, the stacked individual ossicles are known as columnals.  Crinoids are still with us, but their fossil record goes back well before the Devonian Period, which means that, yes, they were around when and (based on my reading) where the Onondaga Formation was laid down.

Pictured below is a bit of a Devonian crinoid fossil (first image) I found at a site in Maryland exposing Needmore Shale (which has been stratigraphically correlated to the Onondaga Formation in the Devonian’s Eifelian Age, about 393 – 388 million years ago).  The arrow points to where some columnals remain (the grooves to the left of the head of the arrow are where columnals have been lost).  The columnal in the second image, the first one remaining in the stem, appears to match nicely the mold left in my point.



Sure, not conclusive evidence, but wonderfully suggestive.

Shteyngart’s New Yorker essay is, ultimately, quite dark.  As the presidential campaign unfolds in 2016, his need to buy watches spirals out of control; he is “in deep.”  I may be contorting his message, but when he writes of how people cope and coped in Putin-era and Soviet-era Russia (e.g., collecting deluxe shaving equipment or compulsively doing mental math problems), I read him as saying that, in a society out of joint, we turn to find refuge and security in “the particular and microscopic” because they are “the only things that could still prove reliable.”

He may have a point.


Sources

Noel D. Justice, Stone Age Spear and Arrow Points of the Midcontinental and Eastern United States, 1987.

Robert G. Maliva et al., Secular Changes in Chert Distribution:  A Reflection of Evolving Biological Participation in the Silica Cycle, PALAIOS, Volume 4, Number 6, December 1989.

Meredith E. Ostrom and Roger M. Peters, Wisconsin Rocks and Minerals, Wisconsin Geological and Natural History Survey, ES046, 2012.

Donald R. Prothero and Lucianne Lavin, Chert Petrography and Its Potential as an Analytical Tool in Archaeology, in N.P. Lasca and J. Donahue, editors, Archaeological Geology of North America, Geological Society of America, Centennial Special Volume 4, 1990.

William A. Ritchie, A Typology and Nomenclature for New York Projectile Points, New York State Museum and Science Service, Bulletin Number 384, April 1961.

David C. Roberts, A Field Guide to Geology:  Eastern North America, 1996.

Gary Shteyngart, Time Out:  Confessions of a Watch Geek, The New Yorker, March 20, 2017.

Tuesday, February 28, 2017

Lost in Another Taxonomic Adventure

You are in a maze of twisty little passages, all alike.
~ Will Crowther’s Colossal Cave Adventure

The classic, late 1970s text-based computer game, the Colossal Cave Adventure (also carrying various other names, including Adventure or ADVENT), begins:
You are standing at the end of a road before a small brick building.
With patience, you can explore the building, follow the nearby stream to a locked grate, and enter a colossal multi-chambered cave.  With even greater patience, you can explore the myriad passages and chambers of the cavern, accumulating treasures, avoiding, in the process, such threats as axe-throwing dwarves.

The game was a source of endless frustration for me because, when I was lost in the cave’s passages, I didn’t have the large reservoir of patience needed to map their layout (though I knew an option was dropping items as I went).  That’s why it’s an appropriate analogy (at least, for me) to the process of identifying particular fossil specimens.  All too often, in this effort at retracing steps taken by taxonomists, I end up lost in a maze.
In a stream in woods near the town of Caledonia, Alabama, a Paleocene strata outcrops.  With commitment and patience, you may discover that the stream yields a fossil-rich matrix, abounding in two kinds of microfossils – ostracodes and foraminifera.
So might begin my present adventure in navigating through a taxonomic "cavern."

Both fauna from this geologic strata have been described in some detail in separate treatises; in both cases, well over half a century ago.  For his master’s thesis, Gordon C. Munsey, Jr., studied the ostracodes collected here.  (An ostracode is a microscopic crustacean living inside the two calcium carbonate shells it secretes.)  The paper he wrote based on the thesis is titled A Paleocene Ostracode Fauna From the Coal Bluff Marl Member of the Naheola Formation of Alabama (Munsey, 1953; a highlighted author's name links to the publication in question, and full citations to all publications cited are provided in the Sources section at the end of this post).  He was following in the footsteps (quite literally) of the preeminent foraminifera expert Joseph A. Cushman who, in 1944, had published A Paleocene Foraminiferal Fauna From the Coal Bluff Marl Member of the Naheola Formation of Alabama (Cushman, 1944).  (A foraminifera, or foram, is a unicellular organism, a protist, that, in most species, secretes a calcium carbonate shell that it adds to as it grows.)

Taxonomy is the science of arranging or classifying.  The two-part scientific names (binomen), drawn from Greek and Latin, that species bear are the work of taxonomists.  The first part is the specimen’s genus (italicized and capitalized); the second is the species name (italicized and lower case).  (Taxonomy has been treated previously in this blog, such as in this post,)  When done well, taxonomy reflects the careful application of encyclopedic knowledge of the taxon being analyzed, attention to detail, and consideration of a taxon’s evolutionary relationship to other taxa.  As paleontologist Donald R. Prothero has described it:
[T]axonomy is not just naming species, because species and higher taxa reflect evolution.  Taxonomists do much more than label dusty jars in a museum.  They are interested in comparing different species and deciding how they are related and ultimately in deciphering their evolution history. . . .  In short, they look at the total pattern of natural diversity and try to understand how it came to be.  (Prothero, 1998, p. 43.)
My favorite characterization of the science (though I’m not sure exactly what it means) is:  “Taxonomy is an art and we are all painters.”  (McCartney and Harwood, 1992, p. 819.)  To be honest, it frequently seems that it’s the same canvas being painted from different vantage points at different times by different painters.  A maze, indeed.

Given a small sample of material from the Alabama stream site, I was foolhardy enough to begin picking through it, crosschecking my finds against Munsey’s and Cushman’s papers.  It was an effort that drew me immediately into a taxonomic muddle that, from the outset, mirrored the Colossal Cave Adventure.  And, frankly, I don’t think my getting lost was always my fault (as the saga that follows might show).

An example of the very first type of ostracode that I tried to identify is pictured below.  Shown is a right valve – exterior (left image) and interior (right).  (As will become clear, I think it important to note that I have found several specimens like this one.)


Here is my description of this kind of ostracode (some of these aspects are visible in the pictures, some are not due to the limits of my microscope and photographic equipment).  It has the following attributes (with a few of the preferred taxonomic adjectives in parentheses and italicized):
  • a spiny (spinose) surface;
  • a rounded anterior and a pointed (acuminate) posterior with a narrowed (compressed) upper half;
  • perhaps two rows of small spines that follow and accentuate the anterior marginal rim;
  • large, blunt spines marking the lower half of the posterior marginal rim;
  • dorsal and ventral margins lined with spines;
  • several nodes (perhaps tight clusters of spines), mostly on the central portion of the shell, with the most prominent one about a third back from the anterior end;
  • a net-like (reticulate) pattern on the surface of the shell where not obscured by the spines;
  • internal ventral and dorsal margins that are mostly straight; and
  • hinges that involve balls (right valve) and sockets (left valves) straddling a straight bar.
This description was a productive exercise because it helped me wend my way through some of the taxonomic maze.

Here then are the highlights (or something) of my adventure with this species.  Described below are the major guideposts that I found; as is clear, this was a trip with some time travel elements.  The heading for each section below provides the year of publication of a relevant paper and the species name assigned in that paper which I thought applied to my specimens.  (I consulted other publications, but the ones cited are those that moved me along - to where was not always clear.)

1953 – Cythereis reticulodacyi

Based on photographs in Munsey’s 1953 paper (Plate 4, figures 1 and 16), I tentatively identified my specimens of this ostracode as Cythereis reticulodacyi Swain 1948.  I had only the pictures in this paper to go on because Munsey provided no description.  The images do resemble the specimens I have.  He noted that it’s “common” in the Coal Bluff Marl, and that adult specimens have a “heavily spinose surface.”

1948 – Cythereis reticulodacyi

Munsey’s main contribution to my effort was offering a path to follow; he cited the publication in which this species was first described by paleontologist Frederick M. Swain (Swain, 1948).

Swain, working with Eocene material from a well drilled in the Eastern Shore of Maryland, had identified a new species of ostracode (p. 202).  His description largely jibed with mine.  We agreed on the general shape and ornamentation of the anterior and posterior margins, though, among other differences, he asserted that the anterior had three (not two) rows of fine spines and the posterior had two.  We were in relative agreement regarding the surface, which he described as “reticulately ornamented with a pattern of ridges, at the junction of which there are blunt spinelike projects.”  He also noted that there was a grouping of nodes.

Rather disturbing for me was his admission when he described the hinge elements of this species, that he could only write about the configuration of the hinge of the left valve because he had found just one specimen, a single left valve.

I suppose it was naive to find it stunning that, from just one shell, Swain had identified a new species.  Perhaps I should have just shrugged my shoulders because such an action is not unprecedented in taxonomy.  Still, I think it would have made more sense for him to have considered ascribing it to a genus (the one he presumably felt was evident) without taking the next step to apply a new species name (clearly, he didn’t think his sole specimen belonged to any previously identified species).

There were more distressing twists in the trail at this juncture.  When I looked at the two photographs of this specimen, which the paper identified as figures 13 and 14 of Plate XIII, it became apparent quickly that those figures in that plate weren't what he described.  It turned out that the plate with these photographs had been misnumbered; Plates XIII and XIV were transposed.  After resolving that issue, the disappointments continued to multiply because the images (in the digital version that I had available to me) were somewhat light-struck and out-of-focus.  Also, his sole specimen bore only a somewhat limited resemblance to my specimens.  His photographs (exterior and interior views) are shown below.  (Hopefully not erroneously, I have treated Swain's paper, which appeared in a publication from the State of Maryland, as a public document, carrying no copyright restrictions on my use of these images.)


Where was I at this juncture in this adventure?  Was my specimen actually C. reticulodacyi?  No, apparently not.

1957 – Trachyleberis ? spinosissima

In 1957, geologist and paleontologist Willem Aaldert van den Bold identified some Paleocene ostracode specimens as Trachyleberis ? spinosissima (van den Bold, 1957, p. 9.)  (The question mark in his identification indicated that he was uncertain about the genus of these ostracodes.)  Subsumed under this new identification are Cythereis reticulodacyi.  (His citation to this species reads: “? Cythereis reticulodacyi Swain” which is puzzling since Swain didn’t use a question mark.)  Significantly, van den Bold also indicated that he applied the T. ? spinosissima name to the specimens that Munsey had identified as C. reticulodacyi.

Losing the Cythereis genus name wasn’t too surprising because, over the years, that genus had become a “dumping ground” for myriad genera and their species (Puri, 1956, p. 274).  At least as early as the mid-1950s, taxonomists were at work, trying to clarify matters by reassigning many of the species  to genera in the Trachlyeberidinae subfamily.

But I was uncomfortable at stopping the journey here.  Not only was van den Bold uncertain about the genus, his specimens were so abraded that his drawings of T. ? spinosissima lacked spines, one of the defining attribute of my specimens.  I delved deeper into the taxonomic history of Trachyleberis spinosissima to see if it might reveal whether or not this was actually the species I had in hand.

1889 – Cythereis spinosissima and Cythereis spiniferrima

The taxonomic history for T. spinosissima stretches back to the late 19th century when T. Rupert Jones and C. Davies Sherborn first identified Cythereis spinosissima.

I cannot find a copy of the publication in which that description appeared, but, the authors later renamed C. spinosissima as C. spiniferrima in an 1889 publication which I do have.  (Jones and Sherborn, 1889, p. 34-35.)    (According to the 1889 publication, the name originally given was C. spinossissma, not C. spinosissima as it’s spelled in van den Bold and everywhere else I’ve seen it.  I’m not sure who’s in error.)

Turns out Jones and Sherborn worked with just two specimens (sigh) – a right and left valve – from the Eocene London Clay.  Still their description dovetails fairly nicely with mine as does the drawing they provided.  Pictured below is the woodcut of the right valve.  I have turned this image so it’s oriented horizontally.


I wasn’t comfortable leaving van den Bold with the last word, given his uncertainty, so I ventured forth once more, this time in search of Trachyleberis spinosissima to see if van den Bold's doubt had been resolved later.

1965 – Trachyleberis spinosissima

In a search for more recent taxonomic treatments of Trachyleberis spinosissima, I found just one (I’ve ignored passing mentions of this species).  In 1965, William Kenneth Pooser described specimens he identified as T. spinosissima as follows:
Characterized by a strongly rimmed anterior margin bearing double row of short stout spines, strongly compressed and triangular posterior, and coarsely reticulate carapace with numerous spines arising from junctions of the reticulations.  (Pooser, 1965, p. 55)
Ah, a concise description, and mostly in sync with mine.  The external view of a right valve that Pooser published (shown below) matches my specimens rather well.


(This image, used with permission, is from The University of Kansas Paleontological Contributions, Article 8, Biostratigraphy of Cenozoic Ostracoda from South Carolina, William Kenneth Pooser © 1965, The University of Kansas Paleontological Institute.)

And, thankfully, in Pooser's work van den Bold's T. ? spinosissima is subsumed under T. spinosissima as are Jones and Sherborn's Cythereis species names (C. spinosissima and C. spiniferra).

At this stage, somewhat exhausted, I have settled down in the Trachyleberis spinosissima chamber of this cave, where I remain (at least for the moment).

Closing

In closing, I must mention the wonderful YouTube video series The Brain Scoop at The Field Museum hosted by Emily Graslie which had an episode (March 24, 2016) devoted to taxonomy.  In it, Graslie asked several taxonomic experts how one might classify an array of different kinds of candies.  The discussion is informative, despite some of the distractions that candy brought to it.  On a serious note, regarding the science of taxonomy, The Field Museum’s Oliver Rieppel, curator of fossil reptiles, asked,
Is it us who brings order to the world or is the world coming to us in an ordered way?  And probably it’s the first way around.
And sometimes the order someone tries to bring may leave us lost in a maze of twisty little passages.


Sources

Joseph A. Cushman, A Paleocene Foraminiferal Fauna From  the Coal Bluff Marl Member of the Naheola Formation of Alabama, Contributions from the Cushman Laboratory for Foraminiferal Research, Volume 20, Part 2, No. 255, June, 1944.

T. Rupert Jones and C. Davies Sherborn, A Supplementary Monograph of the Tertiary Entomostraca of England, Printed for The Palaeontographical Society, 1889.

Kevin McCartney and David M. Harwood, Silicoflagellates From Leg 120 on The Kerguelen Plateau, Southeast Indian Ocean, in S.W. Wise, Jr., et al., Proceedings of the Ocean Drilling Program, Scientific Results, Volume 120, 1992.

Gordon C. Munsey, Jr., A Paleocene Ostracode Fauna From the Coal Bluff Marl Member of the Naheola Formation of Alabama, Journal of Paleontology, Volume 27, Number 1, January, 1953.

William Kenneth Pooser, Biostratigraphy of Cenozoic Ostracoda From South Carolina, Arthropoda, The University of Kansas, Paleontological Contributions Article 8, January 15, 1965.

Donald R. Prothero, Bringing Fossils to Life:  An Introduction to Paleobiology, 1998.

Harbans S. Puri, Two New Tertiary Ostracode Genera From Florida, Journal of Paleontology, Volume 30, No. 2, March 1956.

Frederick M. Swain, Ostracoda From the Hammond Well, in Cretaceous and Tertiary Subsurface Geology, State of Maryland Board of Natural Resources, 1948.

Willem Aaldert van den Bold, Ostracoda From the Paleocene of Trinidad, Micropaleontology, Volume 3, No. 1, January, 1957.

Tuesday, January 31, 2017

I'll Take an Order of Nature Writing, and Don't Hold the Science



Bernd Heinrich
One Wild Bird at a Time:  Portraits of Individual Lives
2016

Richard Fortey
The Wood for the Trees:  One Man’s Long View of Nature
2016


In nature writing, the role of science has often been a source of tension, overt or sublimated.  Perhaps it was particularly an issue in the 19th century as professionals increasingly squeezed out amateurs in the various sciences, but it’s a thread that remains alive in this literature.  At times, science emerges as a villain in the piece, seen as the province of a lab coat-wearing cadre focusing an impersonal and moral-less eye on nature, deconstructing away its beauty and deeper meaning.  Henry David Thoreau (1817-1862), one of our foremost nature writers, made clear that it was living in nature, not studying it, that engendered the knowledge he sought.  The academic world, he wrote in Walden, is inhabited by professors in whose proximity “any thing is professed and practised but the art of life; – to survey the world through a telescope or microscope, and never with his natural eye; . . . .”  (Walden, Concord Library edition with an introduction by Bill McKibben copyrighted 1997)

One of my literary heroes, Walt Whitman (1819 – 1892), wrote passionately and perceptively about nature, and he, too, struck the same note.  In the poem When I Heard the Learn’d Astronomer, he recounted sitting through a lecture on astronomy, replete with proofs, figures, charts, and diagrams, until, “tired and sick,” he . . .
wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.
(Walt Whitman, Leaves of Grass, The “Death-Bed” Edition, Modern Library edition, 1993)
Later, naturalist John Burroughs (1837 – 1921), in his essay The Gospel of Nature, excoriated academic “nature-study” as an assault which “is likely to rub the bloom off Nature. . . . I myself have never made a dead set at studying Nature with notebook and fieldglass in hand.  I have rather visited with her.”  (The Gospel of Nature, from The Norton Book of Nature Writing, edited by Robert Finch and John Elder, 1990.)

It’s somewhat unfair to identify this tension and its anti-science stance, and not acknowledge how it has been reconciled by many of our nature writers.  Finch and Elder write in their introduction to The Norton Book of Nature Writing, “Even if nature writers have often resisted the model of impersonal and specialized science, though, it is also important to note how many of them have been influenced by its concepts, from genetics to molecular biology, from plate tectonics to quantum physics, from population ecology to cognitive theory.”  Evolution, above all of these, has most influenced nature writers.

Burroughs, in the very same essay where he asserted that science could rob nature of its freshness and splendor, acknowledged the importance of what science had to offer and, in the following passage, reached what I consider to be the happy and, perhaps, the best resolution of this tension:
I know it is one thing to go forth as a nature-lover, and quite another to go forth in a spirit of cold, calculating, exact science.  I call myself a nature-lover and not a scientific naturalist.  All that science has to tell me is welcome, is, indeed eagerly sought for.  I must know as well as feel.  (Gospel of Nature, emphasis added.)
I must know as well as feel captures the essence of what I, personally, look for in nature writing.  Nature writing, firmly grounded in science, can be personal and passionate.  I am particularly drawn to works in the genre penned by practicing scientists who immerse themselves in nature.  Last year saw the publication of two examples of the best that recent nature writing has to offer:  biologist Bernd Heinrich’s One Wild Bird at a Time:  Portraits of Individual Lives, and paleontologist Richard Fortey’s The Wood For The Trees:  One Man’s Long View of Nature.  Neither of these authors has a romanticized view of nature, nor are they clinical in their approach.  Rather, they offer a heady blend of solid science enriched with an appreciation of the wonder they find in nature.

At the outset of his new book, Heinrich, author of a host of superb, popular books principally about the flora and fauna in his neck of the woods in Maine and Vermont, addresses a challenging aspect of scientism.  Randomness and individual idiosyncrasies are anathema to purely scientific research, but “both of these are important parts of life, not peripheral to it and the goal of biology is to understand life in nature.  In this book I hope to celebrate individuals as they presented themselves during my encounters with them in the wild.”  This extends so far as to giving names to several of the birds whose stories he tells, such as Slick, the singing starling, and Pipsqueak, the struggling baby flicker.  Still, in no way is science set aside.  Indeed, the essays on specific individual birds or groups of birds are more often than not engaging accounts of his efforts to answer scientific questions about their behavior.

His essays typically open with a description of some behavior by a bird or group of birds that prompts one of those most scientific of biological questions:  Why?  In search of an answer, Heinrich observes, takes notes, hypothesizes, frequently experiments, tests, and, only sometimes, reaches a conclusion.  The reader accompanies the biologist each step of the way, a most genial journey.  Most essays offer some sense of what the bird behavior described might suggest for us humans.

The range of avian characters that inhabit this volume is broad.  Some are tragic like the red-breasted nuthatch pair that successfully makes a “house” in which to raise a family only to be unable to overcome a lack of food, or Pipsqueak, the runt of a brood of baby flickers that probably did not make it.  Others are inspiring, such as the male eastern phoebe that loses his mate but, through persistent effort, manages to finally attract another mate; it is a romantic drama that plays out over three years.  Slick, the starling, offers comic relief, even as he reveals the beautiful singing and mimicking abilities of this much vilified species.  Alone, apparently, starlings are delightful; in their usual hoards, obnoxious.

One of my favorite essays in this volume, titled Blue Jays in Touch, offers much of what attracts me to Heinrich’s nature writing.  It’s an exemplar of applying the scientific method in a personal setting.

What blue jays mean by their vocalizations becomes an open question for Heinrich when, while seated on a branch up in a pine tree several years ago (clearly he goes anywhere and everywhere in his woods), he observes a blue jay pair interacting as they build a nest. They make “soft calls” clearly directed solely to each other; the nature of these specific subdued vocalizations is evident.  But, ponders Heinrich, what of the so dramatically different blue jay screams that loudly crackle through woods and suburban neighborhoods?  These screaming calls are assumed to be a form of scolding or warning, though Heinrich admits he doesn’t know that for a fact.  “Having no idea what most of the blue jays’ screams meant but suspecting they must mean something beyond attracting other jays to mob an owl or a hawk, I tried to get clues by looking systematically as occasions presented themselves.”

Over several years, as he takes notes during walks in his woods or elsewhere, he finds that blue jays are typically not in flight when they scream and that the vast majority of these calls are made by solitary birds.  This latter finding challenges the common understanding of these calls – they are not primarily a mob scene phenomenon.  His experimenting includes monitoring the response from other blue jays to the screams from a pair of blue jay parents he provokes (at first, accidentally) when he comes upon its nest and subsequently when he touches the babies.  (No harm is done to the babies as Heinrich reports it’s a myth that birds abandon their babies if they’re touched.)

Ultimately, he concludes that most of those brash screams from solitary blue jays are not typically a warning, but quite possibly a statement to unseen jays in the far surrounding area:  “Here I am.  How are you?”  In vocalizing this way, the caller may mostly be announcing his or her presence, making contact with perhaps little to say.  Heinrich muses,
After realizing the likely nonspecific but nevertheless meaningful social nature of the blue jay screams, I thought that now I might be less hesitant to call, e-mail, or send a letter or a hello, even when, as almost always, I had nothing important to say.
In 2011, paleontologist Richard Fortey and his spouse Jackie bought Grim’s Dyke Wood, “four acres of ancient beech-and-bluebell woodland, buried deeply inside a greater stretch of stately trees.”  This “greater stretch of stately trees” is the Lambridge Wood which lies southeast of Oxford, England, in the Chiltern Hills, a high chalk escarpment.  Fortey, an expert on trilobites and author of numerous popular science books, including the superb Trilobite:  Eyewitness to Evolution (2000), an engrossing foray into the world of those extinct arthropods, sees the acquisition of Grim’s Dyke Wood as an opportunity.  “I spent years handling fossils of extinct animals; now, the inner naturalist needed to touch living animals and plants.”  The Wood for the Trees emerges from that impulse.

Though these woods do merit the label ancient, they are not wild, untamed remnants of woodland dating back many, many millennia.  Rather, Fortey’s wood and the larger Lambridge Wood are actually the managed product of a longstanding give-and-take involving economic, social, and political forces using or, at times, neglecting these woods (for much of the latter we may give thanks).  “I cannot say exactly when the wildwood was erased, never to return.  We do not even know exactly what the wildwood looked like.”  Fortey traces the roots of Grim’s Dyke Wood in something like its current guise to Anglo-Saxon times, asserting that it’s older than the poem Beowulf (though that’s not actually too much help since we don’t know when this epic poem was actually composed nor when the earliest extant handwritten copy was drafted).  Suffice to say, the woods are very old.

The Wood for the Trees offers a generic year in the life of Grim’s Dyke Wood beginning in “exuberant” April with a flood of bluebells on the floor of this beech dominated wood.  Across the year in the course of the book, as the flora and fauna of the wood respond to the changing seasons, Fortey skillfully interweaves the histories of the economic, social, and political forces that have affected his wood over the past many centuries.  On occasion, he turns to ancient time and draws out the geological history of the area, allowing it take its proper place undergirding (literally) the entire history of the wood.  Through the course of the year, Fortey introduces the reader to the wood’s flora and fauna, including its wild flowers, its avian population, its insects, its mammals, its reptiles (only toads make an appearance), its fungi, and, of course, its trees.  He describes many of the species that inhabit his wood in rich detail, rounding out their portraits with accounts of their life cycles.

Grim’s Dyke Wood is, to my mind, surprisingly fecund.  Many of Fortey’s colleagues from the Natural History Museum, London, descend at times on the wood, each pursuing his or her scientific specialty.  They hunt through every nook and cranny; at times, they even perch high in cherry-pickers to gather specimens from the leafy canopy.  These forays help reveal just how diverse the ecology of this four-acre parcel is, particularly with regard to its fauna.  So, we learn, for example, that there are 6 different bat species using the wood, along with 3 species of deer, over 100 species of beetles, 16 crane fly species, at least 30 species of spiders, over 150 moth species, and more than 300 species of fungi.  And all of this is in a woodland that humans have managed for many centuries with myriad economic and social objectives, such as the harvesting of certain types of trees (hence the paucity of oaks), supporting pheasant-shooting by setting aside some beech woods, and creating visually appealing landscapes.  As Fortey notes, keeping human hands off Grim’s Dyke Wood may not be a good thing.  Indeed, “’Crop rotation’ and selection of trees for felling in the sustainable way that happened over past centuries is better for biological richness.”

In one of the few references in his book to climate change, Fortey acknowledges that rising temperatures may spell the doom of his beech woods and the rest of those of the Chiltern Hills.  It’s actually quite surprising how little attention he pays to this threat to our environment.  Heinrich, as far as I can recollect, pays no attention to it at all in his.

Though it's probably one of the lesser examples of the connections that Fortey makes among the flora and fauna of his wood and the myriad forces that have shaped it, I was quite taken by his tale of the near absence of snails in this wood.  Slug species abound in Grim’s Dyke Wood, but only eight very small, thin-shelled snail species live here.  The question for Fortey is:  Why?

The answer emerges from his consideration of the geological foundation of his wood.  Although the Chiltern Hills are underlain by white chalk (limestone) formed from the precipitation of countless calcium carbonate shells in an ancient sea, Grim’s Dyke Wood on its hilltop, in contrast, sits principally on an ancient lens of hard flint.  The flint here is, itself, the product of a biogenic deposit, owing its existence to that ancient sea and the abundance of sponges that lived in it.  The accumulated sponges’ silica-based spicules (internal struts) over time were transformed into this flint, far harder than the white chalk.

Relevance of this geology to the scarcity of snails?  Snail shells, as Fortey notes, are composed of calcium carbonate and, sadly, his wood offers very little in its soil for its denizens to absorb and put to use.  Slugs, mollusks that have jettisoned their shells through evolution, find the lime-poor soil to be no issue.  To test his hypothesis, Fortey visits a nearby area with a “lovely chalky landscape” and large snail shells are everywhere as are myriad plants that thrive in chalky soil.
The mystery of the rarity of shells in our wood is really no enigma:  lime is just one of the hidden controls on what lives where.  Grim’s Dyke Wood simply does not have enough of it to make big snails.
A small story, almost an aside, but a fascinating excursion.  The Wood for the Trees offers many.

In these two books, practicing scientists write with understanding about, and genuine love for, the natural world.  In shaping their stories, these writers offer a tapestry of details that, sometimes, only a scientist can assemble and explain.  Fortey observes:
Some contemporary nature writing is rich in the details of the author sympathising in some fuzzy way with the totality of nature and the interconnectedness of things, but engagement with the nitty-gritty details of living animals and plants is not on the literary agenda.  I prefer the eloquence of detail.  I believe that all organisms are as interesting as human beings, and certainly no less important than the observer.

 
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