Saturday, January 12, 2013

Strange Small Eruptions

Horatio         In what particular thought to work I know not;
     But in the gross and scope of my opinion,
     This bodes some strange eruption to our state.
~ Hamlet, Act I, Scene I

I haven’t thought through the precise implications of my fascination with the world of microfossils, but in general I sense that it “bodes some strange eruption to our state.”  Perhaps not tragedy, but certainly change in behavior. 

Case in point, the fossil shell pictured below.  It prompted new behavior and led to some small eruptions (though nothing really earthshaking).

In a recent post, I initially identified this shell as coming from an Ecphora tricostata.  I have reason to think something’s rotten in my analysis, given the location where it was found (Scientists’ Cliffs), the formation I think it came from (Calvert), and the likely age of the material (Middle Miocene – some 16 to 12 million years old).  The definitive study of the stratigraphy of this area posits that E. meganae, not E. tricostata, is found in these portions of the Calvert Formation, but my fossil favors the latter more than the former.  (Lauck W. Ward and George W. Andrews, Stratigraphy of the Calvert, Choptank, and St. Marys Formations (Miocene) in The Chesapeake Bay Area, Maryland and Virginia, Virginia Museum of Natural History, 2008.)

It’s symptomatic of my current obsession with the micro that, rather than continue to puzzle over the identification of this Ecphora, I am now consumed by the gray clay matrix that surrounded and filled the shell.  Some three months ago, when I prepared the fossil, I ignored some expert advice and removed the matrix from what, unfortunately, turned out to be a rather fragile shell.  In the process, the internal, central pillar (columella) broke free.  A shame.

But, in a move that I recognized at the time as somewhat quixotic and strange, I bagged the matrix I removed from the shell, slapped a label on the bag, and set it aside.

A couple of weeks ago, I came across the bag and thought, “What the hell, let’s see what’s in here.”

I wont go through the tedious details of my amateurish handling of these few grams of Miocene material, except to say that it involved one percent solutions of Calgon (in water), lots of soaking, lots of sieving, but no baking.  And still it’s plagued with lumps.  (The comedy of errors that is my preparation of material for microfossil searching is described in painful detail in a previous post.  I’ve added some scientific sieves to my equipment, providing a patina of the professional to my inherently sophomoric efforts.)

There was something of the miraculous when, under the microscope, from the gray miasma of this matrix, intricate microfossils made their first appearance, ghosts of minute Miocene fauna.  Amazing what now gets my heart racing.

From within that fractured Ecphora came a flood of foraminifera shells, accompanied by a much  sparser scattering of ostracode shells.  Remarkably, despite several hours of picking, I haven’t exhausted this sample yet.  At this juncture, the critical limit to my time before the microscope is what my hunched over shoulders can bear.

Though I am retrieving foraminifera shells in many different and complex shapes, it’s a couple of shells from the ostracodes, those miniscule crustaceans, that have captured my imagination for the moment.  These two fossils are ornate, exploding with blunt and pointed spines.  Among the many spines are some that, upon closer examination, form three curved rows marking portions of the length of each shell.

These two specimens are both about 0.8 mm long.  Ostracodes have right and left valves hinged at the dorsal edge (top edge of each specimen in the photograph).  The top specimen shown is a right valve, the bottom one is a left valve (the valves are certainly from two individuals).

I’ve identified this Miocene ostracode as Actinocythereis exanthemata (Ulrich and Bassler, 1904).  My primary source for this identification is Richard M. Forester’s paper titled A Systematic Revision of the Ostracode Species Described by Ulrich and Bassler and by Malkin from the Chesapeake Group in Maryland and Virginia (U.S. Geological Survey, Geological Survey Professional Paper 1128, 1980, p. 11 and plate 3, figures 7 and 8).

In 1904, E.O. Ulrich and R.S. Bassler, wrote the initial description of this species for the Maryland Geological Survey’s Miocene: Text (Volume I, 1904, p. 117-118).  They contributed the Ostracoda discussion to the Systematic Paleontology portion of the book.  Amid their lengthy characterization of this fossil, which they named Cythere exanthemata, is a pithy phrase that nicely captures the essence of their description – this ostracode shell has an “extremely nodose and spiny carapace.”  (p. 117)

Nodose challenged my vocabulary, but it’s a great word.  According to the New Oxford American Dictionary, it's an adjective meaning “having or characterized by hard or tight lumps; knotty.”  The neat noun form is nodosity.  Savor that word.  Makes sense that nodule has the same Latin root.

Here are the drawings of C. exanthemata that Ulrich and Bassler included among the plates in the second volume of the work on the Maryland Miocene.

 They acknowledged some concern about precisely where C. exanthemata should come to rest taxonomically, but noted they were at work on a monograph which, “it is hoped, may result in a more natural and serviceable classification of the fossil species than the one now in use.”  I don’t think they ever followed through on this.  The taxonomic history of A. exanthemata prepared by Forester in 1980 includes no subsequent publication of theirs.

Now, of the 1904 duo who tackled the Maryland Miocene ostracodes, Ulrich had the reputation of being a splitter – seeing different species where others didn’t.  I’m not certain about Bassler in this regard.  (The career paths of the two men intrigue me and will be the subject of a future post.)

As a post-doctoral fellow at the U.S. Geological Survey, Forester reanalyzed the taxonomic decisions regarding these ostracodes made by Ulrich and Bassler in 1904 and several D.S. Malkin had made in a 1953 article (Biostratigraphic Study of Miocene Ostracoda of New Jersey, Maryland, and Virginia, Journal of Paleontology).  Forester concluded that Ulrich and Bassler were guilty of both splitting and lumping.  In some instances, they identified males, females, and juveniles of the same species as distinct species.  In others, the array of specimens they offered as examples of a single species should have been considered different species under current practice.  So, he redid the species.

Unfortunately, even after Forester’s work, the identification of A. exanthemata is, in my eyes, still rather obscure.  In his treatment of A. exanthemata, Forester drew a fine distinction among specimens that previous researchers had considered of the same species.  He concluded that figure 4 in the Ulrich and Bassler’s drawings of C. exanthemata (see above) was actually A. marylandica.  (Ulrich and Bassler noted that figure 4 was a drawing of the largest specimen of the purported species that they'd found.)  Forester acknowledged that “Actinocythereis exanthemata is most frequently confused with the lower Miocene to Holocene species A. marylandica (Howe and Hough, 1935),” and identified similar instances of this confusion by other researchers in the decades since 1904.  But, he asserted, A. marylandica is “larger and more robust” and has a “slightly different” spine arrangement.

That’s not much help for me, actually.  The size distinction isn’t useful without more specimens to work with and, it’s not really certain that any differences in the configuration of the spines are evidence of much.  In a later study of South Carolina ostracodes, Thomas M. Cronin commented that “[t]he degree of variability in the spines precludes separation [of these two species] on the basis of this characteristic.”  (Evolution of Neogene and Quaternary marine Ostracode, United States Atlantic Coastal Plain:  Evolution and speciation in Ostracoda, IV, appearing in Studies Related to the Charleston, South Carolina Earthquake of 1886 – Neogene and Quaternary Lithostratigraphy and Biostratigraphy, U.S. Geological Survey Professional Paper 1367, 1990, p. c-35.)

[Later edit:  I dropped a paragraph from the original post which suggested that the two species may actually be males and females of the same species.  I believe I misread a study by Frederick M. Swain (Some Upper Miocene and Pliocene(?) Ostracoda of Atlantic Coaster Region for Use in Hydrogeologic Studies, U.S. Geological Survey Professional Paper 821, 1974, p. 30.).]

Nevertheless, as far as I can unpack Forester’s description of A. exanthemata, I think my two specimens fit.  And, in this instance, I’m going to follow the National Football League approach (instant replay overrides the ruling on the field only in the face of indisputable evidence) and stay with A. exanthemata.

Of course, I also have a nonscientific motivation, I love that species name – exanthemata.  Its Greek root, exanthema, means “an eruption.”

[Much later and decidedly sheepish edit:  I'm really beginning to think that these two ostracode specimens may be from Henryhowella evax, not Actinocythereis at all.  H. evax is apparently more ovate than A. exanthemata.  Further, the spines on A. exanthemata appear more pronounced than those on the specimens above.  If this rethinking of mine is correct, it leaves me looking more than a bit foolish for having rambled on at length in this post about a couple of misidentified fossil ostracode shells.  My apologies.]

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