Tuesday, July 8, 2014

Ark Shells, Wrack Lines, and "Ecological Infidelity"


One July morning during a lull between the storms, I walked the wrack lines on the beach at Flanders Bay (NY).  These lines mark where the tides deposit their jumbled cargo of sea weed, shells, egg cases, broken crab carapaces and claws, threads of sea grass, and bits of plastic and other debris.



Wrack, according to the Oxford English Dictionary (OED), has several definitions, many clearly kin to modern English’s wreck.  The wrack, as used in wrack line, is one of those and describes the “marine vegetation, seaweed or the like, cast ashore by the waves or growing on the tidal shore.”  These lines typically mark the latest high tide and often the highest tide of recent days.  (They are also sometimes called strand lines.  The strand is “the land bordering a sea, lake, or river; in a more restricted sense, that part of the shore that lies between the tide-marks . . . .”  (OED))

In this post, I venture into a paleontological issue involving wrack lines only because ark shells (arks are a type of mollusc) are showing up in some numbers on my beach this year, some caught up in the clutter of organic (and inorganic) material of wrack lines.  Below, a left shell from a Transverse Ark (Anadara transversa) appears in a wrack line.  (I have some confidence in this identification and believe that all of the arks turning up are Transverse Arks.)



I have a fondness for ark shells, which, upon Atlantic Coast beaches, can be fossils, churned up from offshore Pleistocene deposits (a post from last year delved into this).  In the past, they haven't appeared with much frequency on my beach.  Those of recent vintage are now here in sufficient numbers that I can find at least one example whenever I walk in search of shells.  Among them are articulated shells and specimens that are clearly immature.  (The large shell in the picture below is 1 inch long.


Why the change in frequency?  Perhaps it’s due to the effects of some change in the offshore location of a living community of the bivalve, a change in currents or wave action, a recent, serious storm, or something else altogether.

The research on the distribution of shells along shorelines explores the impact of a variety of factors on where shells are found, what kinds, how many, etc.  And, in that literature, I was pleased to find there is a bit focused specifically on ark shells and wrack lines.  It asks – How does one read the wrack line?  What does it tell us about the living bivalve community?  And, interestingly enough:  What’s the wrack-line message for interpreting the ecology of the past?

In 1987, Robert W. Frey penned a study titled Distribution of Ark Shells (Bivalvia:  Anadara), Cabretta Island Beach, Georgia (Southeastern Geology, Volume 27, pages 155-163, 1987).  Frey, a well known ichnologist (ichnology is the study of trace fossils), was one of a number of scientists who have conducted detailed analyses of the ecology of a small segment of the Georgia coastline – including its barrier islands, beaches, sounds, estuaries, and marshlands.  The work is currently being carried on at the Georgia Coast Ecosystems Long Term Ecological Research site under a program administered at the University of Georgia.

Frey analyzed factors influencing the distribution of the shells of two species – the Incongruous Ark (Anadara brasiliana) and the Blood Ark (A. ovalis) – in the wrack lines on Cabretta Island Beach, considering such factors as tide, current, wind, slope of the beach, predation damage (boreholes), and the site of the living population.  (The common names of these arks are interesting:  The Incongruous Ark is so named because its left valve is a bit larger than its right; the Blood Ark is one of the few molluscs to have hemoglobin, so its blood is red.)

Frey's findings show that, following the death of these bivalves in their offshore community, their shells were not randomly cast ashore and along the beach.  Rather, for example, the longshore drift and wave action disproportionately moved shells toward the northern end of the beach, with left valves of A. brasiliana and right valves of A. ovalis being more likely to be forced north.

Significantly, he suggested his analysis might inform a larger paleontological issue.  He posited that what he’d found in his Cabretta Island Beach study should be able to assist with the “taphonomic interpretations of ancient shell accumulations . . . .”  (Taphonomy is the study of the processes through which organic remains come to be fossilized, if indeed they do become fossils.)  I read Frey as asserting that, understanding the ways in which the shells come to be gathered and distributed upon present day beaches will inform scientists’ interpretation of the taphonomy of fossil assemblages and the relationship of those fossil groupings to living communities of the past.

A subsequent study of A. brasiliana on St. Catherines Island, Georgia, by paleontologists Harold B. Rollins and Ronald R. West, more directly grappled with the paleontological issue Frey raised.  (Taphonomic Constraints on Event Horizons:  Short-Term Time Averaging of Anadara brasiliana Valves, St. Catherines Island, Georgia, Chapter 2 in Paleontological Events:  Stratigraphic, Ecological, and Evolutionary Implications, edited by Carlton Elliott Brett and Gordon C. Baird, 1997.)

They considered whether the A. brasiliana valves deposited in the wrack lines on the beach they studied reflected the living community from which they came.  The contents of the wrack lines, they discovered, differed in important ways from the offshore, living community that offered up the empty ark shells.

But, significantly, on this beach, the ark shells found in the wrack lines were almost all articulated (that is, right and left valves remained connected) and most of the empty shells were deposited in a position typically maintained by the living animal.  This led the researchers to engage in a thought experiment.  What, they asked, would paleontologists make of such a wrack-line based assemblage of ark shells and other organic remains if it were to have been fossilized in place.  They concluded that it would “likely be viewed as a life assemblage and subjected to paleontological analyses to the point of ecological reconstruction.”  (p. 48)    As a result, they concluded that paleontologists coming upon a fossilized assemblage from such a wrack line might well misconstrue the paleoecology of the area because of the wrack-line assemblage’s “ecological infidelity” to the living community of the past.

To be honest, I initially found the thought experiment itself a bit misleading because it took me a moment to realize that Rollins and West were not arguing that, in a paleontological context, wrack lines have to be read carefully.  Actually, as noted below, wrack lines apparently are unlikely to be around to trouble paleontologists.  Rather, this analysis of ark shells and wrack lines offered a broader cautionary message for paleontologists as they consider any fossil assemblage:  factors that affect the taphonomy of an assemblage of organisms “can be incredibly subtle, requiring careful and complete taphonomic assessment prior to reconstruction of ecological interactions.”  (p. 53)  (From my position as an amateur in all of this, I have to ask:  Is this a truism?)

As for wrack lines of the ancient past, at the end of his paper, Frey wrote, “. . . wrack-line shell accumulations stand little chance for direct preservation in the fossil record . . . .”  (p. 161-162)  Rollins and West endorsed this conclusion, noting that the assemblage of A. brasiliana they studied “would probably be highly modified prior to final burial.”

So, I suppose, given the etymological link between wrack and wreck, not much of this should have been surprising.
 
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