Wednesday, January 30, 2019

Interpreting Fossil Tableaus Frozen in Time

No, this post isn’t about the polar vortex.

I’ve come to the conclusion that one of paleontology’s core missions is the reconstruction of ancient communities of plants and animals.  At the heart of this effort is the challenge of deciphering and understanding what the fossil record says about the biological communities of organisms that lived and interacted in specific places in deep time.

One key to meeting this challenge is the field of taphonomy, first defined by paleontologist J.A. Efremov as:  “. . . the study of the transition (in all its details) of animal remains from the biosphere into the lithosphere . . . .”  In essence, analysis of how organic remains become rock.  (Taphonomy:  New Branch of Paleontology, Pan-American Geologist, September 1940.)  The utility of this field of study is how it enables us to determine, as paleontologist Anna K. Behrensmeyer and her colleagues have written, “how faithfully biological history has been recorded” in the fossil record.  (Taphonomy and Paleobiology, Paleobiology, January 2000.)

Not surprisingly, it turns out we must approach the fossil record with great care because what we see in the fossils from any specific site may not provide a reliable picture of the biological reality at that time in that place.

The particular interest that prompts this post is quite narrow and, as it often true of this blog, idiosyncratic.  I am wrestling with a few of the basic questions that might be asked of fossils that offer the remains of organisms affixed to each other, in other words, specimens and moments frozen in a fossil tableau.  Two such are pictured below.

The first picture shows the exterior and interior of a shell from the scallop Chesapecten nefrens.  This shell measures roughly 4.5 inches vertically and was found at the Scientists’ Cliffs portion (Choptank Formation) of the Calvert Cliffs on the western shore of the Chesapeake Bay.  Roughly 12 to 16 million years old, the scallop sports the remains of a number of barnacles, including remnants of the basal edges of several barnacle shells.  I won’t hazard an opinion on the species (one or more) of barnacle that encrusts this scallop.

The second photo shows two views of a 3.25 inch tall shell of the gastropod Ecphora gardnerae germonae.  An amazing gift from a friend, this specimen comes from the St. Mary’s Formation, further south along the Calvert Cliffs than the scallop, and is approximately 8 to 10 million years old.  The basal edges of barnacle shells abound on this Ecphora specimen.  Again, I’ve made no effort to identify the species of encrusting barnacle.

And here’s my rather simplistic set of questions.  For fossil specimens like those shown above, what can we say about the interaction of the different species that, through fossilization, are fixed in this kind of direct relationship with each other?  Did these individual specimens live at the exact same time in the same place?  Were the hosts alive when their “guests” took up residence?

My assumption has been that, in most cases, the best we can do is draw from modern analogs of the interaction of similar kinds of organisms and only offer educated suppositions about what these tableaus from deep time are telling us.  But I should have realized that brilliant people find brilliant ways to tease out greater meaning than the rest of us are resigned to.

An article that only recently came my way offers a kind of direct response to my assumption.  The renowned evolutionary biologist Geerat Vermeij has coauthored a fascinating piece on a specific species of Miocene barnacle, Chesaconcavus chesapeakensis, and its interactions with the gastropod Conradconfusus parilis and the scallop Chesapecten santamaria.  (Vermeij and Sara K. Ruch, Barnacles, Their Molluscan Hosts, and Comparative Ecology in the St. Mary’s Formation (Late Miocene) of Maryland, USA, Journal of Paleontology, Volume 92, Number 2, 2018.)  These species lived during the Tortonian age of the Miocene epoch, between 7 and 12 million years ago.

Working with a collection of these fossil species, Vermeij and Ruch concluded that representatives of this barnacle species came to be anchored on a high percentage of the fossil specimens of C. parilis and C. santamaria they had for analysis, and did so only when those gastropods and scallops were alive.  No question in their minds that these barnacles settled on the snail and mollusc shells when the hosts were definitely going about their daily business.

How can these scientists know that these hosts were alive, carrying living barnacles on their shells?

Their logic is simple and, to me, persuasive.  Consider, first, the barnacles and gastropods.  Vermeij and Ruch found that, of the gastropod specimens in their study collection that sported barnacles, none had barnacles encroaching on their hosts' apertures.  Had the barnacles done so, they would have interfered markedly with their hosts’ ability to survive.  No barnacles were found to have taken up residence on the interior of the gastropod shells.  Further, and, to my mind, quite telling, the shape of many of the barnacles was curved toward the growing edge of the gastropod shells signaling that they were growing even as their hosts, themselves, grew.

As for the barnacles on the scallops, they, too, refrained from overlapping the outer edges of the shells, so did not impede the normal opening and closing of the articulated valves.  Also, in no case, did barnacles inhabit the interiors.

Accepting the logic of this analysis, one obvious takeaway is that clearly the species under study lived at precisely the same time and in the same place.  That is, they were members of the same community, interacting as members of biological communities do.  Quite beautiful I think.

But I hesitate to take too much inspiration from the work of Vermeij and Ruch because that might mean I’d have to consider approaching the specimens depicted in the tableaus presented earlier with their analysis in mind.  My fossils are, after all, single samples, not part of a broader representative collection of Chesapecten or Ecphora that might be subjected to the methodology these scientists brought to bear.  Further, there are many basic pieces of information I’m missing (e.g., what species of barnacles are these?).  I’m smart enough to know my limits and am content just to enjoy the Vermeij and Ruch tour de force.

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