Thursday, October 11, 2012

Lingering Around and Shaking Things Up

An October week’s retreat to my summer cottage on the North Fork of Long Island, New York, shifted a small portion of my paleontological world.

It began when I went biking in the middle of the week and became convinced that, with the falling temperatures of the previous night, had come an invasion of body snatchers.  An invasion from Mr. Roger’s Planet.  Everyone I passed – jogger, biker, walker – old, young, middling – greeted me with a robust “Hello, neighbor!”  Well, more like “Hey,” “Hi,” “Hello,” “Morning.”  What had happened to the summer’s many pointed rejections of my overtures?  Had something transformed these aggressively isolated Homo sapiens into a community of greeters (and possibly huggers)?  I wondered if the coming of fall, and particularly the first cold snap, had signaled subliminally to the locals that anyone out and about now was one of them.  The summer invaders had, at long last, all gone home.  But little did they know that I was a lingerer, someone left over from those summer days.

I was still contemplating whether I had been enjoying a wave of overt friendliness under false pretenses when I parked my bike in the sand and went in search of shells.  I walked along one of the beaches that line the bays that separate the North and South Forks of this eastern end of the island.  The twin forks are shown in the map below, separated by various bays, Great Peconic, Little Peconic, Gardiners Bay, etc.

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The expected shells appeared, jingle shells, slipper shells, clams and a sprinkling of oyster drills.  Then I spotted a chalky white cusp of a shell, worn, with a scar.  (Pictured below.  The 3/4" line applies to the image of the interior of the shell and measures the distance from the posterior end to the anterior.)

I had seen this kind of shell before, just never here, and not of this time.  It’s a somewhat common shape among the fossil shells I've collected at the Calvert Cliffs from the Miocene Epoch (23.0 to 5.3 million years ago) – an ark shell – from the Arcidae family of shells.  A recent post features two fossil specimens of Anadara staminea, a much larger, more steeply arched, extinct member of this family.  [Later edit:  I should clarify.  I do know that there are many extant species in this family and many beaches where these shells are common.  On this particular beach, this shell appeared out of place.]

But this is Long Island.  Long Island, in my world, is among the “not-fossil” places, the product of glacier action toward the end of the Pleistocene Epoch (which ran from 2.6 million years ago to 11,700 years ago) that shaped the island and essentially plowed over rock formations that might yield fossils.  But I also would acknowledge that, despite my having dropped it into this category, the island is not totally bereft of fossils, though most of those very, very few that might be found are likely to be invaders from Connecticut, carried over by the glaciers.

I pocketed the shell and, back in the relative warmth of my cottage, consulted two volumes I had at hand:  Harald A. Rehder’s National Audubon Society Field Guide to North American Seashells (1981), and R. Tucker Abbott’s How to Know the American Marine Shells (1961, an aging paperback shedding its pages).

Shell guides are blunt instruments, generally ruling out candidates and only on occasion leaving me with just one species standing.  With these two guides, I rejected a number of ark shell candidates, relying mostly on such obvious physical attributes as the overall shape of the shell, placement and structure of hinge, and features of the umbo.  (The umbo is the beak-like projection that arches over the hinge.  Donald J. Borror, in his Dictionary of Word Roots and Combining Forms (1960), defines the Latin umbo as “a projecting knob; a shield.”  Curiously, the plural is umbones.)

Rehder steered me to the Ponderous Ark (Noetia ponderosa), but I hesitated to come to rest there because of two sources of serious doubt in the description – the size and the range.  My shell was less than an inch long; Rehder said N. ponderosa came in between 1 ½ to 2 ¾ inches.  The distributional range was described as from “Virginia to Florida and Texas.”  To be sure, neither size and range is necessarily dispositive for an identification.  Shell size is tricky given that it is age specific, and ranges defined in 1981 may have shifted in the warm decades since.

But when I read the concluding sentence of Rehder’s description, it was suddenly immaterial whether my shell was a Ponderous Ark or not.  There, in black and white, were words that nudged the continents of my paleontological world in a slightly new direction:
Fossil shells of this species are sometimes found on beaches as far north as southern Massachusetts; washed out from fossil beds, they are testimony that the waters in that area were once warmer.  (p. 672)
Fossils in the wash at the beaches in Massachusetts?  Perhaps even Long Island as well?  Was that possible?  I grabbed Abbott’s book and thumbed to his description of N. ponderosa (a page or two came loose as I did):
Fossil specimens are occasionally found on Nantucket, Massachusetts beaches.  (p. 133)
Not as encouraging, though still acknowledging a fossil connection.

So, what is the possibility of finding fossil shells on Long Island beaches?  I googled “Noetia ponderosa” and “fossil,” and found naturalist Susan J. Hewitt.  She took the lead in reshaping my paleontological reality, writing that the chances of collecting fossil shells on some Long Island beaches are actually quite good.
On some of the exposed sand beaches of the outer Atlantic coasts of New York and New Jersey, the beach drift can contain numerous fossil shells of bivalve and gastropod mollusks. . . . On the beaches that I am familiar with, on days when there is a lot of beach drift, there are usually lots of fossils present.  Sometimes fossil shells are nearly as common as fresh shells, and always they are mixed in with the fresh shells higgledy-piggledy in the drift lines.  (Fossils on the Beach, American Paleontologist, Summer, 2008, p. 12)
Simply amazing.  She plows through the narrow confines of species and locations that Abbott and Rehder had erected for the phenomenon.  In one spellbinding article, Hewitt released me from my paleontological exile on Long Island.  No longer do my summer journeys here have to cut me off from hunting fossils in the surf.

What’s the story behind these fossils?  They date as far back as the late stages of the Pleistocene Epoch, not very old compared to what I’m used to, but still old enough.  Importantly, these are fossil specimens of species that are mostly still with us today, though, such as in the case of the N. ponderosa, not necessarily living in the same geographic range in which they used to live.  More than 12,000 years ago, the Ponderous Ark’s range extended into these waters.

Hewitt cites an article by Thomas C. Gustavson (Paleotemperature Analysis of the Marine Pleistocene of Long Island, New York, and Nantucket Island, Massachusetts, Geological Society of America Bulletin, January, 1976) which shows that these various kinds of fossil shells are Pleistocene in origin, coming from species that lived in this area during a warmer interglacial period.

I haven’t read this piece by Gustavson (unfortunately, my local college connection wasn’t up to penetrating the paywall that the Geological Society of America puts up around its articles).  But, elsewhere, Gustavson has written about fossil finds on the eastern end of Long Island that reveal a fauna characteristic of warmer waters, waters of a temperature closer to those prevailing today from Virginia southward.  (A Warm-Water Pleistocene Fauna From the Gardiners Clay of Eastern Long Island, The Journal of Paleontology, May 1972.  This also hides behind a paywall, but it’s JSTOR and accessible to me.)

Though the specific material he worked with – Gardiners Clay – was found on the South Fork of Long Island, Gustavson mentions other research identifying fossiliferous Pleistocene sediments, including Gardiners Clay and Jacobs Sand, on Robins Island and Gardiners Island.  Both of these small islands are positioned very nicely in the middle of the waters that separate the twin forks of Long Island.  Not too much of a stretch to think that fossil shells on my beaches might have eroded out from those islands.

Hewitt goes on to make my life more complicated by explaining that some extant species that currently live off the Atlantic beaches are likely to be represented in the wash by both fossil and non-fossil shells.  This makes it all the more important to figure out how to tell fossil from non-fossil versions of specimens from the same species.  For instance, it’s a given that fossils will not have the periostracum (an organic layer that covers the shells of many mollusks, it’s the often dark material that seems stuck to the exterior of some shells found on the beach), nor will there be any trace of the actual ligament that once joined two valves together.  Further, bright colors will be gone.  Hewitt observes that “the fossil shells tend to be oddly discolored:  many are various dull and unnatural-looking shades of gray, but they can also be off-white, tan, or faintly rust-colored.”  She emphasizes that a fossil shell will be “opaque and extremely dull-looking, even in the interior of the shell (p. 12).”

These are all attributes I’ve come to expect of the shells I find in places where nearly every shell is a fossil.  When the mixture of shells may contain a healthy offering of non-fossil shells, these indicators will not always do the job of separating out the fossils.  Further, shells that are just somewhat old, on the order of decades or even a century or two, are likely to share many of these fossil-like attributes.  This wont be easy.

Hewitt points to the barrier islands off Atlantic coast beaches of New York and New Jersey, those “ancient, sand bars,” as the source of the fossils.  “Under the topsoil, these [barrier] islands consist of sand deposits of various ages: some old, some very old, and some ancient (p. 13).”  As these barrier islands erode and re-form, they expose “shells from the older, much older, and ancient deposits” and the waves do the rest, unleashing invaders who have lingered from another time and are now shaking things up for me.

Though my collecting is along the beaches between the North and South Forks of Long Island, not on the Atlantic beaches of the South Fork, I think I can still count on ocean currents and wave action to spread fossils my way from those barriers islands.  And, as I noted earlier, the islands between the forks could also offer up their fossils to my beaches.

One final question.  Who is Susan Hewitt?

Well, I really like her vita.  She is a dedicated, published, amateur naturalist who has studied mollusks for many years, recently as a field associate in the American Museum of Natural History’s Division of Invertebrate Zoology, and as a volunteer in the Museum’s Division of Paleontology.  (See the volunteer page for the Division of Paleontology of the American Museum of Natural History.)

This citizen-scientist calls herself “a naturalist in the British tradition.”  All very 19th century.  Appropriately, she seems to be a lingerer from another time.


At this stage, I’m not sure of the identity of the shell that helped precipitate this post.  I still think it might be a fossil Ponderous Ark, albeit very much on the small side.  But it may also be a Transverse Ark (Anadara transversa) which has a present-day range covering Long Island and which is typically smaller than the Ponderous Ark.  If a Transverse, this worn veteran of the seas could still be a fossil, or just old.

Of course, there's never a final question.  After consulting several other shell guides for North America's Atlantic coast, all of which suggest fossil N. ponderosa shells may appear on beaches in some places to the north of its current range, I wonder why none of them suggest that fossil shells from other species might be in the drift lines on the beach.

[Later edit:  There's another question that merited consideration in this post.  How does Hewitt distinguish between merely old shells and fossil shells?  It's a matter of how old.  She writes, ". . . these shells are considered fossils only because they are so extremely old . . . . (p. 12)"  It's an accepted distinction.  As paleontologist Donald Prothero notes in Bringing Fossil to Life (1998), the label fossil is applied to "many shells (particularly those of Pleistocene age) [that] still have their original shell material unaltered . . . . (p. 6)"]


  1. I found an adorable little shell that looks just like the one in the photos, albeit with a light orange tinge to it, just this Sunday at Revere Beach in Revere, Massachusetts. Mine's a tad bit smaller, though.

  2. Tony's ark shell is probably Anadara transversa. Yours might be the same species. Either could be recent in age, or subfossil, or fossil.

    I am the Susan J. Hewitt mentioned in this blog post. I was glad to see that someone really enjoyed my article in American Paleontologist. :)


    1. Susan

      What a treat to receive a comment from you. Your wonderful article really did change my perspective on the shells that gather in the wash on my Long Island beaches.


    2. What caused the scar on the shell.

  3. That damage is caused by a bristle worm species that likes to burrow along the surface of shells.

    I am certainly not an annelid expert, but I don't think those worms live this far north; they seem to be warm-water critters.

    1. Susan: Are there resources that systematically describe the different kinds of damage one might find on mollusk shells and the sources (perpetrators) of that damage? Your link multiplied several-fold, I guess. Best, Tony.

  4. If such a resource existed, that would be really great! But right now I don't know of a source like that. Plus the species responsible would be somewhat different for each faunal zone in each part of the world.

    I have learned some of them bit by bit, like for example boring sponges, Clionidae which make thousands of tiny holes in an old empty shell. And serpulid worms, which live on the surface of shells after the shell is empty.

    And there are various bivalves that live inside the surface of very much larger shells while the mollusks are still alive.

    All kinds of things. It would make a cool theme for an exhibit at a shell show actually. But I don't do exhibits myself.

  5. The worm that makes these grooves (burrows) in shells is a spionid polychaete, probably a species in the genus Polydora.


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