Thursday, August 9, 2012

A Summer Punctuated in Small Ways by the Unexpected

I was reminded of the role of the unexpected when the Curiosity Rover touched down on Mars a couple of days ago.  John P. Grotzinger, the project scientist, commenting on one of the earliest pictures sent back to Earth, noted that the foreground contained a gravel field.  He asked, “The question is, where does this gravel come from? It is the first of what will be many scientific questions to come from our new home on Mars.”  (Kenneth Chang, Curiosity Rover Lands Safely on Mars,  New York Times, August 6, 2012.)  Were this gravel field expected, I have to assume Grotzinger would have had an explanation at hand.

The unexpected catches you without an explanation, only questions.  The search for answers ensues.

For me, this has been a summer punctuated in small ways by the unexpected.

Take, for instance, the Common Periwinkle.  In this scene are a tumble of large rocks and concrete blocks that lie half buried in the sand at the edge of Flanders Bay on the North Fork of Long Island, New York.  Many of the rocks and blocks disappear when the tide is in, and slowly reemerge as the tide retreats.  There’s a mystery here.

The flat surfaces of all these chunks of rock and concrete seem to offer a particularly attractive habitat for barnacles and the cramped nocks and crannies formed when the rocks and blocks abut hold special interest for gastropods.  Stationary colonies of Northern Rock Barnacles (Balanus balanoides) mostly dominate, and are sometimes so crowded the individual barnacles grow tall not squat, creating row after narrow row (a marine farmer’s field).  Gatherings of Common Periwinkles (Littorina littorea) often occupy the edges of the barnacle colonies, and the sides of the stones that are shielded from the sun, as well as the shadowy crevices amid the stones.  Atlantic Slipper Shells (Crepidula fornicata) are sprinkled throughout, sometimes layered upon themselves or the periwinkles, and, through it all, a solitary Atlantic Oyster Drill (Urosalpinx cinerea) or a Thick-lipped Drill (Eupleura caudata) will be slowly wending its way, sometimes crawling atop the periwinkles or slipper shells (I wonder if it’s cruising for a meal).  Barnacles, periwinkles, and a lone Atlantic oyster drill appear in the picture below.

There’s nothing unexpected about this fauna occupying the intertidal or littoral zone of the shoreline.  Indeed, the Common Periwinkles’ scientific name – Littorina littorea – is a Latin redundancy that signals the appropriateness of it being here – littor means “the seashore.”  What is unexpected to me is that this living fauna is not reflected fully in the death record one can read in the empty shells washed up on the sandy shores.  There only the drills seem to appear in just the right proportion.  Empty slipper shells abound, with only a very occasional cluster of hollow barnacles tossed in.  The latter is certainly not represented here as it is on the rocks.

But the most unexpected aspect of the shells on the shore is the absence of empty periwinkle shells.  Over several weeks of walking this beach, I have found just one, small, damaged example.

Why?  Are periwinkle shells particularly fragile, unable, when empty, to withstand the clash of wave and rock?  I doubt that.  The shells seem robust and have to be given that the animal can live from five to ten years.

Is it that perhaps they are a favorite of hermit crabs and so get occupied before they can be tumbled onto the sand?  Periwinkles are, in fact, among the shells favored by the hidden but ever present multitudes of Long-Clawed Hermit Crabs (Pagurus longicarpus); so says A Field Guide to the Atlantic Seashore (Kenneth L. Gosner, 1978).  But are there enough of these crabs looking to upgrade their housing at any one time to occupy all of the empty shells?

Or are periwinkle shells collected in particular by that population of early risers whose footprints invariably dot the shoreline when I finally make it down?  If so, they’re uncommonly thorough.

This is the first summer I’ve paid any attention to the periwinkle population and so this unexpected disconnect between intertidal zone and beach is new to me and not just a little disconcerting.  Something is happening in the space of merely a few feet that keeps the periwinkle shells from this beach.  It merits some study.

Then there’s the scene further inland, near my summer cottage, where the woods of oaks, maples, locusts, and sumacs offer shade to an undergrowth of English ivy and poison ivy.  There the Eastern Box Turtle (Terrapene carolina carolina) was always an expected denizen and often a visitor to my backyard.  But over the years the sightings became fewer and fewer until for the last several years they have been exceedingly rare.  The once expected is now the unexpected.

When it reappears, unexpectedly, the box turtle is the focus of a celebration.  Twice this summer I have come upon a box turtle hell-bent (well, as hell-bent as turtles can be) on crossing a road or a path to the safety of an expanse of ivy amid the trees.  Perhaps the same one, I’m uncertain.  But I take comfort that, even if just one, it was healthy and unscarred.

Is this one a sign of a revived colony of turtles, a harbinger of good times ahead?  Or is this the “Lonesome George” of its species in these woods?

Why did their numbers dwindle in the past decades?  Indeed, did they in fact decline?  Has the encroachment in this area of houses and particularly lawns on the woods and the fragmentation of surrounding land harmed these reptiles, some of whom, at certain seasons, roam (though roaming to these turtles apparently seldom takes them more than 125 yards from where they were born)?  I have to believe the apparent decline in past years is real.  It certainly is to the State of New York which has labeled the species of “special concern.”  Even where a comparatively large swath of land (613 acres) is preserved and attention paid to its wild flora and fauna, like the William Floyd Estate (though part of the Fire Island National Seashore, the estate is on the “mainland” of Long Island), the survival of the these turtles is not guaranteed.  The landed sides of the estate have over the years become surrounded by residential buildings.  A recent National Park Service report assessing the status of amphibians and reptiles on the estate concluded:
Although box turtles currently appear to be common and relatively well protected at WFE [William Floyd Estate], they reside within a habitat island surrounded by dense residential development, essentially creating an isolated population. The adjoining landscape likely functions as a population “sink” and box turtles within WFE’s boundaries face a number of potential stressors, including genetic isolation, illegal collection, and introduction of disease via release of other box turtles by visitors. Given these stressors, and data collected during this survey that suggest recruitment of young into the population has declined and the estimated population size is not that large . . ., the future viability of this population should not be taken for granted.  (R. P. Cook, et al., Inventory of Amphibians and Reptiles at the William Floyd Estate, Fire Island National Seashore, National Park Service, 2010, p. 46)
As an aside (in a post of mostly asides), the box turtle colony on the William Floyd Estate is of special interest to herpetologists because of the detailed data on it that date back to the early 1900s.  John Treadwell Nichols (1883 – 1958), curator in the department of ichthyology at the American Museum of Natural History, married into the Floyd family and kept records of his wildlife sightings at its estate from 1904 until his death.  (He should not be confused with John Treadwell Nichols, author of The Milagro Beanfield War.) Turtles were of particular interest to Nichols.  When he found a box turtle, he carved his initials, a date, and a specimen number into its carapace.  (I wince at that.  Too much empathy, I suppose.)  Anyway, turtles he marked have continued to show up in inventories of the population on the estate.  In 2002, one was found that Nichols had “tagged” in 1921 when he estimated it to be about 20 years old.  That made it over 100 years old in 2002.  The records of this colony depict the impact of changes in the environment for over a century – from pesticide applications in the past (particularly DDT in the 1950s and 1960s) to loss of habitat outside the estate’s boundaries.  (An interesting article on efforts to save the estate’s turtle population appeared in the New York Times in 1988 – Malcolm W. Browne, A Push to Save L.I. Turtle Colony From Man’s Intrusion, June 20, 1988.)

And then there was the unexpected appearance of a fossil in this part of the island.  Well, that’s misleading and it may be a cheat to include this last incident in this post’s account of the unexpected.  It's a bit unusual in that regard.

My spouse whose interest in natural history is minimal but whose attention to antique stores and thrift stores knows no bounds is the one to whom credit for this fossil find must be made.  In a consignment store out here on fossil-barren Long Island, she spotted, hanging on the wall, a framed piece of limestone from the Green River Formation in Wyoming.  Slashing diagonally across that rock are the fossil remains of a Knightia fish, a herring relative.  The fossil is about 4 ½ inches long.

How unexpected, I thought when I first saw it.  I am never brought in to see something like this in one of her stores.  I bought it, of course.

For me, this find in an obscure consignment store was truly unexpected.  Frankly, I cared not about who had owned it and why it had been consigned to the shop.  Instead, the unexpected triggered questions about the Green River Formation which, according to the typed label on the back was part of the provenance of this fossil.  I have a couple of other very small fish fossils from the formation but had never been prompted to learn about it.  Ah, the power of the unexpected, I suppose.

The Green River Formation is renowned for its incredibly well preserved specimens, and particularly for its fish.  According to one guide to the formation, it “has yielded the richest, most diverse sample of early Tertiary freshwater aquatic communities in the world.”  (John R. Nudds and Paul A. Selden, Fossil Ecosystems of North America:  A Guide to the Sites and Their Extraordinary Biotas, Chapter 11 – Green River Formation, 2008, p. 186)  The formation, created by the sediment collected in a series of three great, fresh water lakes in parts of what are now Wyoming, Utah, and Colorado, dates from the early through late Eocene (roughly 55 to 38 million years ago).  If I can trust the label on the back of the slab of rock containing the fish fossil, this one is from Wyoming and apparently from the so-called “18-inch layer” of the Butte Member, the source of the “best-preserved” fish fossils in the formation, according to Nudds and Selden.

Chapter 11 (Green River Formation) in Nudds and Selden's book is a great introduction to the formation.  There are myriad other resources one can consult, including a very nice, very short piece written by Tom Caggiano for the Fossil News.

But, should it have been unexpected that a fossil fish from the Green River Formation would show up at least once in my wife’s wanderings through the dusty recesses of ramshackle antique stores and dingy thrift stores on Long Island?

Actually, not.  It was probably inevitable.  Nudds and Selden cite a report that over a million fossil fish specimens have been collected from the formation in the last quarter century alone.  As for Knightia in particular, Caggiano observes that it is “probably one of the most common fossils in the world.”  And, so, the unexpected has morphed into the expected.

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