Dunkirk Diatoms

Observations on SEM Images by Mary Ann Tiffany

 

These observations annotate the collection of SEM images made by Mary Ann Tiffany while she was at the Center for Inland Waters and Department of Biology of San Diego State University. To view an image, click on a numbered hyperlink in a paragraph. The image will open in a separate browser tab or window.

 

The entire collection may be browsed here. An alphabetical species list with links to the images is here. The collection in compressed form may be downloaded for local use. When expanded, the directory contains three web pages (HTML files) which can be opened separately.

 

Observations by Dick Carter

Completed in 2011

 

The diatoms of Dunkirk, like those found nearby at Nottingham, are among the oldest found in Miocene sediments of the American east coast (lat 38.709470 lon -76.694950, Google Maps) and fall within East Coast Diatom Zone 1 of Andrews. They date from the middle Burdigalian Stage, Early Miocene, with an estimated absolute age of 19.1 Ma to 18.9 Ma (Andrews 1988).

 

There are 481 individual images, in 96 rows of five images each, plus one image in a 97th row. I will refer to these by row and column number, in that order. The images and these observations are free to use as you see fit. I have placed hyperlinks to the individual images at the beginning of each entry.

 

I have offered identifications to genus and species when I was able to do so. Unfortunately, in many cases, this has not been possible. Images made with the SEM are rather different from what we are used to seeing with the light microscope, and "switching gears" between the two image types can be difficult.  One may not recognize an SEM view of a diatom that one knows well from LM observations.  The main difference lies in the total lack of transparency in SEM images; structures that are obvious in the transparent LM image are simply missing in SEM images. The lack of transparency also has advantages, however; details of surface structure that do not appear in LM images because of their transparency become very obvious under the SEM. One need only compare the SEM images of Coscinodiscus bulliens Schmidt, contained in this set, with LM images to see this dramatic difference!

 

I must say that these are among the best SEM images of diatoms that I have seen, and I have learned a great deal already by studying them and preparing these notes. I have offered quite a few observations on details of the ultrastructure seen in many of the images, in the hope that these will prove informative.

 

This file is certainly not definitive or final, and additions, corrections, suggestions, improvements, and criticisms will be warmly welcomed!

 

1.1 And we are off -- to a not very auspicious start.  The first five images are of a single diatom valve, which I am unable to recognize to either genus or species.  The relatively high, vertical mantle, set off from the valve face by a conspicuous rim, is reminiscent of the genus Endictya, but I strongly doubt that it belongs in that genus. The central elevation of the valve face, and the hyaline rays that continue onto the mantle, are not typical features of that genus, certainly. The valve wall structure is not entirely clear but appears not to be loculate.   The pores look like they are cribrate, and I see no trace of rimoportulae in this external view. (They are generally easier to see in internal view.)  1.2 A view at higher magnification, showing details of the cribra (most broken, but some intact) and the mantle rim.  1.3 A full-face view, suggesting a depressed center at the summit of the central elevation.  1.4 Further details of the cribra, and a clear indication of how the sectors defined by the hyaline rays widen outward; they are uniseriate near the center, becoming triseriate at the mantle rim.  (They continue to widen down the mantle to the valve margin.)  1.5 This view shows the center of the valve face, seemingly with no distinguishing characters.  Later images in the set, discussed below, show the valve interior, where it is clear that the hyaline rays are fairly thick ribs of silica.

 

2.1 The first four images in this row show the interior view of a very interesting diatom, Flexibiddulphia semicirculare (Brightwell) Simonsen.  The valve is roughly semicircular, as the specific epithet implies, with a concave basal margin.  2.2 In the opposite valve mantle, at the right end, we see a conspicuous pseudocellus of fine porelli.  (Intriguing -- ocelli and pseudocelli are normally found on the valve face, or on a "horn" that arises from the valve face.)  2.3 Inside the convex margin of the valve are four typical rimoportulae, slightly raised, with slit-like openings that are laterally thickened -- hence the alternative name "labiate process".  2.4 The four rimoportulae at higher magnification.

 

The next three images show the internal view of a valve of Lithodesmium ehrenbergii (Grunow) Forti.  2.5 and 3.1 show slightly different views of the full valve interior.  3.2 A higher magnification view of the center of the valve, showing the central rimoportula.  (The cracks are post-deposition.)  For an external view of this species in LM, see my photo album on the forum.

 

3.3 Here we enter the strange world of the resting spore, forms that several complex genera of centric diatoms, especially the very speciose genus Chaetoceros, adopt under conditions of environmental stress.  Since it is usually an open question as to which resting spore is associated with which "normal" diatom species, a number of different "form-genera" have been created to house them within the taxonomic system.  Such form-genera as Dicladia, Goniothecium, Periptera, Omphalotheca, and Xanthiopyxis are not at all uncommon in marine deposits, and many "species" have been described.  This image shows a resting spore that might be placed in either Dicladia or Periptera, depending upon one's concept of those genera.  It almost certainly belongs in Chaetoceros -- but which "normal" species it represents is not clear.  3.4 A different view of the same specimen.  Note that it is heterovalvar: the epitheca has a relatively deep mantle, while the hypotheca has virtually none, appearing to be a plate with a central elevation.  Of interest is the curving suture that descends from the margin of the larger valve, tracing an arc across the mantle, and seemingly ending at a small, tubular pore.  There is a ring of large, sharp spines around the margin of each (ovoid?) valve.  3.5 This form is quite similar, but probably represents a distinct species.  Here we see long spines on the elevated central areas of each valve, as well as around the margin; in addition, the valve outline is angulate.

 

4.1 begins a sequence of three images depicting the interior of a valve of a species of

Aulacodiscus.  This first image shows the internal opening of a complex rimoportula.  In species of Aulacodiscus, the internal rimoportula opening is a strongly curved slit, largely covered by a silica "flap" that extends across the tube.  The external opening is also quite complex, and highly species-specific; there is often a rather lengthy tube, frequently with expanded ends.  There are other species, however, in which the external opening is a simple round pore at the summit of an enlarged, thickened dome.  4.2 The full view of the interior of this broken valve reveals four rimoportulae, at the ends of a "cross".  There are no apparent hyaline rays between the rimoportulae and the center of the valve face.  4.3 Another interior view of a rimoportula.

 

4.4 Another view of the resting spore shown in images 3.3 and 3.4.

 

4.5 Stephanogonia actinoptychus (Ehrenberg) Grunow in Van Heurck.  Note the ovoid outline and fine pores on the valve face.

 

5.1 Pseudopyxilla cf. americana (Ehrenberg) Forti.  Most illustrations show a more gently tapered shoulder, and a longer terminal spine.

 

5.2 The next four images are a mystery to me, and I hope someone will recognize this form.  It is a centric diatom with a raised ring concentric to the center; arising from this ring are long, thin tubes.  (In later images it will become clear that these are the external tubes of rimoportulae. )  This specimen has three tubes, but most have four.  5.3 Higher magnification, showing the sharp summit of the raised ring of silica.  5.4 Higher magnification, showing a narrow rim at the top of the mantle.  Areolae are irregular in shape, with a foramen at the bottom and cribrum at the top (mostly broken away).  5.5 The full-face view.  Strew slides from Dunkirk and Nottingham frequently have examples of the ring and tubes broken away from the valve face.  I have also seen this diatom from the Kirkwood Formation of New Jersey.

 

6.1 The images in this row, with the exception of 6.3 (to which we shall return), again show a specimen of Aulacodiscus, looking at the valve interior.  Probable identification: Aulacodiscus sollitianus Norman.  This first image is a highly magnified view of the areolae, with a cribrum at the bottom of each.  (Since this is an interior view, the cribra are clearly exterior, at the outer valve surface.)  6.2 Here is a rimoportula, with the strongly curving slit and silica "flap" typical of the genus.  Note the hyaline ray leading from the rimoportula to the center of the valve face.  6.4 Another view of a different rimoportula.  6.5 A more highly magnified view of the areolae, again showing the foramina at the bottom and the cribra at the valve surface.  7.1 Row 7 continues the images of this specimen.  Here we are looking at the specimen edge-on: the interior shows a hyaline ray proceeding inward from a rimoportula, while the exterior shows the long, exterior tube of the rimoportula.  This tube has a flared, thickened end, and a circle of stout spines around its base.  7.2 A similar view at less magnification.  There are three rimoportulae remaining in the broken valve, but we can see six hyaline rays; hence, there were originally six rimoportulae.  The center of the valve face is a hyaline circle.  (At left, a valve of Stephanopyxis; at right, a valve of Coscinodiscus.)  7.3 A closer view of the valve center.  7.4 Another rimoportula.  7.5 Another view of the full interior.  8.1 The last view of this specimen, another image of a rimoportula.

 

6.3, 8.2, 8.3 This is another species of resting spore, with the heterovalvar structure we saw earlier.  As of now, I have no name for this one.

 

8.4 Another specimen of Stephanogonia actinoptychus (Ehrenberg) Grunow in Van Heurck.  The areolae have an outer cribrum.

 

8.5 Araniscus lewisianus (Greville) Komura.  A synonym is Coscinodiscus lewisianus Greville.  The parallel rows of areolae make this species easy to recognize.  It is more abundant in deeper waters and is thus important in correlating the east coast diatom zones with deep-sea cores.

 

9.1, 9.2 Two internal views of a specimen of Diploneis.

 

9.3 An interior view of a valve of Xanthiopyxis.  It is ovoid, and apparently covered with sharp spines.  Since these resting forms are of apparently little value in biostratigraphy, species descriptions and images are hard to come by for the east coast Miocene, where most publications on diatoms have been produced by paleontologists.

 

9.4 The interior of a valve with a loculate wall, probably a Stephanopyxis.  There is an interior velum that looks different from the cribra we have seen previously, and the areolae are large and hexagonal -- both traits of Stephanopyxis.

 

9.5 This begins another series of images of Flexibiddulphia semicirculare (Brightwell) Simonsen.  

This interior view shows three rimoportulae, and the external pseudocelli at the corners.  10.1 A higher magnification of a pseudocellus.  10.2 A view from a slightly different angle.  10.3 A closer look at the three rimoportulae.  10.4 The valve interior seen end-on.  10.5 Another view. 

11.1, 11.2, 11.3, 11.4 Additional views of the same valve.

 

11.5 Another species of "thorny" Xanthiopyxis.  This one is more elongate.

 

12.1 This Xanthiopyxis appears to be an external view of the species seen in 9.3.

 

12.2 This is a silicoflagellate, Naviculopsis quadrata (Ehrenberg) Ling, an important index fossil.  It occurs with diatoms from the lower part of Andrews' East Coast Diatom Zone 1.

 

12.3, 12.4 Two views of a specimen of Araniscus lewisianus (Greville) Komura.  Cf. images 8.5,

13.2.

 

12.5 Another example of a rimoportula of Aulacodiscus.  In this species the rimoportula is heart-shaped in interior view, and the silica "flap" is tangential to the margin, rather than normal to it, as in 6.2.

 

13.1 No comment.

 

13.2 Another view of Araniscus lewisianus (Greville) Komura.

 

13.3, 13.4 Two more views of the species of resting spore seen in image 3.5.

 

13.5 to 16.4 are additional images of Aulacodiscus.  Probable identification: Aulacodiscus sollitianus Norman.  13.5 shows the external rim of the valve in cross-section.  14.1 is an internal view of a rimoportula.  In this specimen the long, ovoid "slit" is filled with silica, and the interior opening is reduced to a round area.  Note the external rim again.  14.2 is the same rimoportula at higher magnification.  14.3 is another view of the same rimoportula.  14.4 is the same, at higher magnification again.  14.5 An external view, showing the external tube of the rimoportula, with the ring of spines around the basal area.  Note the outer rim again.  15.1 The same specimen at less magnification.  Although there are hyaline rays on the interior, there is no sign of them in this external view.  15.2-16.4 Further images of the exterior of this valve.

 

16.5 begins a series of four images of a slender species of Gyrosigma.  17.1 and 17.2 show the valve apices.  17.3 reveals the proximal raphe endings, which turn in the same direction.  The areolae are ordered in parallel rows, at right angles to each other, as is characteristic for the genus.  It is clearly not the same species that Andrews and Abbot illustrate from the Hawthorn Formation of Thomas County, Georgia.  Hustedt described species with similar valve outlines from Beaufort, NC, but I have not seen that publication.

 

17.4 is another elongate, spiny specimen of Xanthiopyxis.

 

17.5 begins a series of six images of the same Aulacodiscus species we saw in 12.5.  All are internal views of the same valve.  18.5shows four hyaline rays leading from a hyaline central area to the rimoportulae.  There is no indication of what the external tubes look like.

 

19.1 begins a series of three images of a species of Lyrella.  The emarginate central area looks most like  Lyrella spectabilis (Gregory) D. Mann in Round et al.

 

19.4 shows an elongate, panduriform valve of a Xanthiopyxis species.

 

19.5 is another valve of Stephanogonia actinoptychus (Ehrenberg) Grunow in Van Heurck.

 

20.1 begins another series of three images of Lithodesmium ehrenbergii (Grunow) Forti.

 

20.4 and 20.5 show slightly different views of what appears to be an ebridean, or perhaps a monopilarian radiolarian, a different type of siliceous microfossil.  Possibly a species of Semantebria Frenguelli.

 

21.1 This starts a sequence of four images of Pseudauliscus radiatus (Bailey) Schmidt.  This genus resembles Auliscus, but the surface structure is quite different.  Additionally, Pseudauliscus has a pair of marginal rimoportulae at the ends of an axis that is roughly normal to the ocellar axis.  This first image shows the large ocelli, with rows of fine porelli inside, partially broken away.  The external tubes of the rimoportulae are small and ovoid; they appear here as white ovoid pores, not much different in size from ordinary areolae.  This is probably why they do not appear in the drawings of earlier workers, e.g., Schmidt.  A LM photograph in Andrews 1986 shows them clearly, if you know where to look and use a hand-lens!  21.2 An ocellus at higher magnification.  21.3 and 21.4 show more magnified views of the rimoportulae.

 

21.5 This is the same species we saw in row 1, but an interior view of the valve.  The hyaline rays of the exterior view are here seen to be deep but thin ribs of silica.  22.1 A more highly magnified view.  The valve wall does not really seem to be loculate in this view, either.

 

22.2 Triceratium condecorum Ehrenberg.  The depressed central area shows up quite well in the light microscope.  The presence of pseudocelli rather than true ocelli indicate that this species should be formally moved to a different genus.

 

22.3 and 22.4 Two views of a valve interior, genus and species unclear.  The areolae are remarkably regular, with external cribra and large internal foramina.  Note the ring of small rimoportulae just inside the valve margin.

 

22.5 This begins a series of three images of Coscinodiscus bulliens Schmidt.  This first image shows the central area, with fine areolae surrounded by much larger ones.  The rough, irregular ring of areolae at mid-radius, and the very complex areolar structure, are what distinguish this species.  23.1 Note the remarkably complex structure of the areolae.  These details do not appear in the LM, of course, but the areolae do have a peculiar "bubbly" look to them.  (I have placed a LM photo of a specimen from Dunkirk in my photo album on the forum.)  23.2 Shows the full valve face, with the characteristic irregular ring of larger areolae.

 

23.3 A chain of frustules of a Paralia species, most likely the common P. sulcata (Ehrenberg) Cleve.

 

23.4 Another image of Naviculopsis quadrata (Ehrenberg) Ling.

 

23.5 Another Coscinodiscus with complex areolar structure, species unknown.  24.1 A full valve view of the same specimen.

 

24.2 An exterior view of a valve of Flexibiddulphia semicirculare (Brightwell) Simonsen.  No trace of pseudocelli on the valve face -- they're on the mantle!  Two external openings of rimoportulae are visible.  24.3 A more highly magnified view of the areolar cribra.  24.4 A straight-on view of the external valve face. 24.5 An end-on view of the valve exterior, with the pseudocelli visible on the mantle.  25.1 to 26.3 Further views of the same specimen.  26.4 and 26.5 Two interior views of a valve of the same species.

 

27.1 An interior view of a valve of Stephanogonia.

 

27.2 A specimen of Chaetoceros pliocenum Brun, originally described from the fossil deposits of Sendai, Japan.  

 

27.3 Lyrella lyra (Ehrenberg) Karajeva.  The valve apices are usually more attenuated than we see here, ranging right up to rostrate.  The gentle outward flare of the hyaline areas at the apices is fairly distinctive, though.

 

27.4 Here's the mystery diatom from 5.2 again, this time an interior view of the valve.  It rests on the thin external tubes of the rimoportulae; the latter are not raised on the interior, but are in depressions.  27.5 A better view of the internal openings of the rimoportulae.  It also looks as though there is an axis of bilateral symmetry, judging from the radiating pores on the interior valve face.  Perhaps my imagination?  28.1 A more highly magnified view of the internal opening of a rimoportula in its depression.

 

28.2, 28.3, and 28.4.  Three views of another species of Lyrella.  The scattered areolae within the largely hyaline areas suggest that this is L. praetexta (Ehrenberg) D. Mann.

 

28.5 through 29.5  Further views of Flexibiddulphia semicirculare (Brightwell) Simonsen.

 

30.1 and 30.2 Two views of a handsome Triceratium, species unknown at this point.  It looks a bit like T. spinosum Bailey, but the margin seems too narrow.  30.3 A clear view of the ocellus at the tip of a horn.

 

30.4 Another exterior view of the mystery diatom from row 1.

 

30.5, 31.1, and 31.2 Three additional images of the interior of a valve of the earlier-seen Gyrosigma species, 16.5 et seq.

 

31.3 Another specimen of Triceratium condecorum Ehrenberg.  A specimen of Actinoptychus heliopelta Grunow in Van Heurck below.

 

31.4 Another interior view of the mystery diatom from row 1.

 

31.5 This is a specimen of Triceratium kainii Schultze, originally described from the Miocene

Kirkwood Formation of New Jersey.  The absence of an ocellus indicates that it belongs in a different genus.  32.1 Although the areolae at the end of the arm average a bit smaller than those elsewhere, they do not seem to constitute a pseudocellus.   32.2 Another view of the full valve face.  32.3 Another view of the end of an arm.

 

32.4 A different species of Stephanogonia, not S. actinoptychus, which we saw earlier.  It lacks the many pores on the valve face characteristic of that species, and the many denticles around the valve margin also look different.  I have no specific name for it.

 

32.5 Another specimen of Chaetoceros pliocenum Brun, this time in girdle view.

 

33.1 Another view of the Stephanogonia species seen in 32.4.

 

33.2 and 33.3 Two views of Odontella weissflogii (Grunow in Van Heurck) Grunow.  Although the valve outline and apparent "ocellar axis" do make this look like an Odontella, there is no apical elevation, and no ocellus.  The ridges defining an ovoid central area are "pure Odontella", though.  Seriously discordant characters!  The four spines (some specimens have three) are again the external tubes of rimoportulae.  Indeed: the four long tubes, and the silica ridge defining a central area make this look a lot like the mystery diatom in 5.2!  This one is ovoid and angulate; that one is perfectly round.  Could they belong to the same genus?  Or are they perhaps resting spores of Odontella species?  Probably just whimsy on my part.........

 

33.4 An interior view of a Diploneis species, not the same as that seen in row 9.  I am unable to place it to species, as I know essentially nothing about how the species differ in the internal view.

 

33.5 No comment.

 

34.1 The first of three specimens of Lithodesmium undulatum Ehrenberg, all seen in interior view.  Although this first specimen looks a bit different, I think all three represent the same species.  This specimen more clearly shows the corner zones, and there is a clear central rimoportula.  (The long external spine that arises from this portule serves to link the valves into chains.)  34.2 and 34.3show no obvious trace of the central rimoportula.

 

34.4 begins a series of three images of a four-poled Biddulphia or Trigonium species.  Although it is obviously very distinctive, I have not yet been able to place it.  The areolae are coarse, there is a flattened center, and there are pseudocelli at each pole.  34.5 The full-face view.

 

35.1 A more highly magnified view of a pseudocellus, mostly broken away.

 

35.2 Here is yet another species of Aulacodiscus , seen in exterior view.  This first of nine images shows the external rimoportula opening, in this case a thickened dome-like structure with a large central pore.  Note also the thickened rim at the valve margin.  35.3 A more highly magnified view of the marginal rim.   35.4 A highly magnified view of the rimoportula.  35.5 An oblique view of the entire valve.  Each rimoportula opening sits atop an elevated portion of the valve, and the areolae are relatively coarse.  36.1 Another oblique view.  36.2 A more detailed view of the areolae at the valve center.  36.3 Another view of the same rimoportula.  36.4 A straight-on view of the full valve face.  36.5 Essentially the same view.  The areolae lose their alignment at the valve center, and become disorganized.

 

37.1 Begins a series of six images of the exterior of a broken valve, showing fine spines and a loculate wall-structure, with a thin, elevated rim.  There is an exterior velum and interior foramina.  This is the same species of Aulacodiscus that we saw in 6.1-6.2, 6.4-8.1, and 13.516.4.  Probable identification: Aulacodiscus sollitianus Norman.  37.2 Closer to the center of the valve the fine spines become pointed nodules of silica.  Note the complex, cribrate structure of the velum.  37.3 Essentially the same view as 37.1.  37.4 A highly magnified view of the complex, cribrate velum.  37.5 The long exterior tube of this rimoportula has broken off.  38.1 Another view of the wall structure of the valve.

 

38.2 Probably the same Coscinodiscus species seen in 24.1.

 

38.3 Here we begin a series of nine images of the exterior of a valve of Craspedodiscus coscinodiscus (Ehrenberg) Ehrenberg.  (The series is interrupted by two images of a Stephanogonia.)  This first image shows the central area of the valve face, with a small hyaline area and a rosette of slightly enlarged areolae.  These show a common feature of such rosettes, visible also in Coscinodiscus species, even in LM images: the foramina at the bottom of the locules are displaced toward the margin of the valve.  This displacement is better seen in 38.4. 39.1 A view of the valve margin -- I'm guessing that the small pores are external openings of rimoportulae.  39.2 A slightly more highly magnified view of the same marginal structure.  39.3 Another view of the center of the valve face.  39.4 An oblique view of the entire valve, showing the characteristic depressed center with finer areolae.  40.1and 40.2show details of the structure of the areolae, with the foramina at the bottom of the locules.  40.3 A straight-on, full-face view of the valve exterior.

 

38.5 and 39.5 Two views of the same Stephanogonia species we saw in 32.4.

 

40.4 Another series of six exterior images of the Aulacodiscus we saw in 35.2-36.5, interrupted by one image of a probable Stephanopyxis.  In this first image we again see the external form of a rimoportula.  40.5 An oblique view of the entire valve.  41.1 The valve margin and rimoportula opening.  41.2 Another oblique view of the entire valve.  41.4 Another view of the margin with a rimoportula.  41.5 A straight-on view of the valve face.

 

41.3 An interior view of what appears to be one of the dome-shaped species of Stephanopyxis, possibly S. turris (Greville) Ralfs in Pritchard.  Note the flange-like valve margin, flared outward.

 

42.0 - 42.3  No comment.

 

42.4 Another view of Stephanogonia actinoptychus (Ehrenberg) Grunow in Van Heurck.

 

42.5 - 43.1  No comment.

 

43.2 and 43.3 Two views of Pseudopyxilla cf. americana (Ehrenberg) Forti.

 

43.4 An ovoid Xanthiopyxis.

 

43.5 A lenticular Xanthiopyxis.

 

44.1 This begins a sequence of five images of Aulacodiscus rogersii (Ehrenberg) Schmidt.  The valve is in external view, and the coarse surface sculpturing is very apparent.  Observe the ridged margin and strong angle between the valve face and mantle; an external tube of a rimoportula has been broken off.  44.2-44.5  These images show the same view, but at continuously increasing magnification.

 

45.1 A complete frustule of Stephanonycites coronus (Ehrenberg) Komura.

 

45.2 Commencing a series of five images of Craspedodiscus coscinodiscus (Ehrenberg) Ehrenberg.  This first image shows the smaller areolae at the valve margin; the ring of smaller round pores may be the external openings of rimoportulae.  45.3 The rosette of enlarged areolae at the center of the valve face, again showing foramina that are displaced toward the valve margin.  45.4 An oblique view of the entire valve, showing the finer areolae of the central region and the center rosette.  45.5 Looking straight down on the center rosette, again showing the displaced foramina.  46.1 A full-face view of the entire valve.

 

46.2 Six more images of the valve exterior of Aulacodiscus rogersii (Ehrenberg) Schmidt.  This first photo shows an intact exterior tube of a rimoportula, relatively long, with a flared end.  46.3 Again we see the coarse surface structure, formed by a network of silica ridges.  The ridges delineate large "cells" which contain groups of areolae; the latter have an outer cribrum, often broken away.  46.4 An oblique view of the entire valve, with one intact external tube of a rimoportula, the remaining three being broken off.  The center of the valve face is slightly concave, and separated from the outer part of the valve face by a well-defined ridge.  This concavity can be seen in the light microscope, although it is most often described as "flat" in the literature.  It is this central feature that distinguishes the species from the very similar Aulacodiscus argus (Ehrenberg) Schmidt, in which the valve is gently dome-shaped across the center, being convex rather than concave.  There seems to be an age difference between the two species, A. rogersii being the common form of the Early Miocene, while A. argus appears in the seas of the modern world.  46.5 Another view of the external tube of the rimoportula. 

47.1-47.2  Views of the surface sculpture at higher magnification.

 

47.3 Here begins a series of external images of two different valves of Biddulphia suborbicularis Grunow in Van Heurck.  This is not a true Biddulphia, as the presence of ocelli at the ends of the apical projections mark it as a member of Odontella.  (In this first image the delicate pore fields of the ocelli are broken away.)  The valve is almost circular, but with a slight flattening of the apices, where a small angulation of the margin appears.  The tubular apical projections arise above the flattened apices, and are of slightly different sizes, as is often the case in the Triceratiaceae.  The central concavity of the valve face does not appear in Schmidt's drawings of this species, which led me to confuse it with Biddulphia radialis Andrews.  This latter species, described from the later Miocene Choptank Formation of Richmond, Virginia, has this central concavity, but with a more rhombic valve outline and no apical flattening.  Thin external tubes of the two rimoportulae are very prominent, appearing to either side of the ocellar axis in about the middle of the valve length.  47.4 An oblique view of a second valve; the pore fields in the ocelli are clearly visible.  48.1 Higher magnification, showing the ocellar porelli.  48.2 A fullface view of the entire valve; the apical flattening is easy to see.  48.3 The broken base of a rimoportula, in a small hyaline area.  48.4 A very clear view of an ocellus.

 

47.5 This appears to be a view of the central raphe endings of a Pleurosigma.

 

48.5 This is another interesting silicoflagellate, Mesocena quadrangula Ehrenberg.

 

49.1 A frustule of Stephanopyxis lineata (Ehrenberg) Forti.

 

49.2 Dossetia hyalina Andrews, probably another resting spore.

 

49.3 A highly magnified interior view of a relatively flat centric diatom, possibly one of the many Coscinodiscus species.  It has a very low mantle, and radiating striae of very regularly spaced areolae.  49.4 A face-on view of the full valve interior; note how the areolae diminish in size as they approach the center of the valve face, where there is a hyaline area in a slight concavity.  49.5-50.1  Two additional views at high magnification.  50.2 and 50.3 are interior views of a different specimen.

 

50.4 A highly magnified view of the exterior margin of Craspedodiscus elegans Ehrenberg.  50.5 An oblique view of the entire valve.  51.1 A face-on view of the full valve.  This species is very easy to recognize at the light microscope, as its appearance is very distinctive.  It is also among the more beautiful diatoms, in my opinion; the high marginal rim has areolae that look like the shiny scales of a snakeskin belt, or hatband.

 

51.2-51.4  No comment.

 

51.5 A perfect specimen of Aulacodiscus rogersii (Ehrenberg) Schmidt with three rimoportulae.

 

52.1 The apex of a valve of Pleurosigma affine var. marylandica Grunow in Cleve & Grunow.  The type locality of this variety is Nottingham, MD, also in East Coast Diatom Zone 1.  52.2 A view of the central raphe endings.  52.3 A view of the entire valve fragment.  These exhibit strongly metallic diffraction colors, blue and green, under the light microscope, particularly with dark field illumination.  52.4 A more highly magnified view of the terminal raphe ending, showing the interior helictoglossa.

 

52.5 An exterior view of a valve of the variable Biddulphia tridens (Ehrenberg) Ehrenberg.  Note the pseudocelli at the valve apices, and the irregular circle of rimoportulae at the center of the middle lobe.  53.1 A more highly magnified view of the middle lobe, showing the rimoportulae.  I have no idea why there are silica crescents around the areolae.

 

53.2 and 53.3 Probably a form of Pseudopyxilla americana (Ehrenberg) Forti, without the apical spine.

 

53.4 Stephanogonia actinoptychus (Ehrenberg) Grunow in Van Heurck, this valve a little more oval than most.

 

53.5 A lenticular (?) species of Xanthiopyxis.

 

54.1 A specimen of Aulacodiscus rogersii (Ehrenberg) Schmidt with four rimoportulae, all with exterior tubes broken off.

 

54.2 An excellent specimen of Stephanopyxis lineata (Ehrenberg) Forti.

 

54.3 One of the more cylindrical species of Stephanopyxis.  The ring of central rimoportulae have the external tubes broken away; these tubes serve to link frustules together into chains.

 

54.4 An oblique view of Odontella weissflogii (Grunow in Van Heurck) Grunow.  See 33.2 and 33.3 for comments.  54.5 A face-on view of the same valve.  55.1 A more highly magnified view of the external tubes of the two center-most rimoportulae, both broken.

 

55.2 A very interesting valve margin, genus and species unknown.

 

55.3 A specimen of Actinocyclus octonarius Ehrenberg.  The striae are center-fasciculate, and there is a rimoportula at the outer end of each interfascicular stria.  The pseudonodulus is at 4 o'clock, right where the valve face turns downward into the mantle.

 

55.4 This might be another species of Actinocyclus, judging from the finer areolae on the valve mantles, but I see no rimoportulae and no clear indication of either fasciculation or a central annulus.  So, I'm probably wrong.

 

55.5 Probably a species of Coscinodiscus, with no clear mantle area.  There is a ring of small external openings of rimoportulae.

 

56.1 Another species of Coscinodiscus, this one with a complex velum over the valve face.  The velum does not appear on the mantle; again, there is a ring of small external openings of rimoportulae.

 

56.2 - 56.5  Three views of Mastogonia crux Ehrenberg.  It is not clear to me why this genus deserves to be separated from Stephanogonia.

 

57.1 Actinoptychus heliopelta Grunow in Van Heurck.  The raised sectors have three rimoportulae each, and the depressed sectors have two.

 

57.2 - 57.5  Another specimen of Craspedodiscus elegans Ehrenberg.  As is often the case, the complex velum is largely broken away from the summit of the outer ridge.

 

58.1 - 59.3  Another specimen of Aulacodiscus rogersii (Ehrenberg) Schmidt with four rimoportulae.

 

59.4 A valve of Asterolampra marylandica Ehrenberg.  Since the electron beam of the SEM cannot penetrate the valve, the suture lines between the bases of the hollow rays are invisible.  This is why the image looks so different from what we see in a light microscope.  Note that the external openings of rimoportulae are visible in the "mouths" of the hollow rays. 

 

59.5 An interior view of the head pole of Sceptroneis caduceus Ehrenberg.  The apical pore field, which secretes mucilage to attach the frustules to substrates and to one another, is clearly visible.  The interior opening of a transverse rimoportula is also visible at the proximal margin of the pore field.  60.1 The entire valve.  60.2 The foot pole of the valve, showing the other apical pore field.

 

60.3 - 60.5  Another specimen of a Coscinodiscus with a complex velum on the valve face.

 

61.1 - 61.2 Two views of a specimen of Goniothecium, in girdle view.  It is difficult to find acceptable illustrations or descriptions of species in this common form-genus.  Once again, I am reminded of my favorite quote from the great diatomist Albert Mann: "It would be well for diatomists to either forego the luxury of illustrations or make them near enough like the objects in nature to be capable of recognition."  Later photos in this set are pretty clearly Goniothecium odontella Ehrenberg, judging from the elevated rims of the concentric ridges on the valve face, so this one is probably the same.

 

61.3 Another specimen of Biddulphia suborbicularis Grunow in Van Heurck.

 

61.4 The first of three images of what I believe to be Thalassionema nitzschioides (Grunow) Mereschkowsky, based on the single row of marginal areolae.  Unfortunately, the best taxonomic characters are visible in external view, while here we are looking at the inside of the valve.  61.5 I would call this the head pole, as it is slightly broader than the other.  There is a fairly prominent elongate areole at the very tip of the valve: Round et al. (1990) consider this to be a rimoportula.  62.1 The foot pole.  This asymmetry is well-known in Thalassionema, and the foot pole seems always to be less highly developed.  The terminal areole seems reduced when compared with that of the head pole.

 

62.2 and 62.3 The head pole and foot pole of another specimen of Sceptroneis caduceus Ehrenberg.  Note the apical pore fields.

 

62.4 Another silicoflagellate, one of the cannopilid species in the genus Distephanus.

 

62.5 Another specimen of the Stephanopyxis species seen in 54.3.

 

63.1 This looks like a valve of Stephanonycites coronus (Ehrenberg) Komura, in face-on view.

 

63.2 This begins a series of four images of Goniothecium odontella Ehrenberg.  Here we have a frustule in girdle view.  63.3 A high magnification view of the interlocking projections that link the valves together.  63.4 A frustule that is tipped, so we can see both the girdle and valve face.  The raised rims on the curved ridges of the valve face distinguish this species from G. rogersii Ehrenberg, which is said to lack them.  Both species occur in the east coast Miocene deposits.

 

63.5 Here the valve face is seen in profile; the raised rims of the ridges are quite prominent.

 

64.1 A nice specimen of a Stephanopyxis species, possibly S. turris (Greville) Ralfs in Pritchard.

 

64.2 A chain of interlocking frustules of a Paralia species, probably the ubiquitous P. sulcata (Ehrenberg) Cleve.

 

64.3 Begins a sequence of three images of a Coscinodiscus or Actinocyclus species.  (The small structure at 7 o'clock on the margin of the valve face could be a pseudonodulus.)  The valve face has a complex velum, and there is a ring of rimoportulae on the upper mantle.  There are two apparent girdle bands, one wide and split, the other narrow and ligulate.  64.4 A little better exposure of the same view.  64.5 The complete frustule in oblique view.

 

65.1 The head pole of Sceptroneis caduceus Ehrenberg.  65.2 The complete frustule.

 

65.3 Another valve of Goniothecium odontella Ehrenberg, lying on its mantle.

 

65.4 The interior view of a valve of Actinoptychus heliopelta Grunow in Van Heurck, with two highly magnified openings of rimoportulae.  65.5 The entire valve interior.  Note the small hyaline areas at the corners of the depressed sectors.  The large central hyaline area is quite thick, and is often the last part of the diatom to remain in a corrosive environment.  If you find four- or five-pointed silica "stars" in Early Miocene deposits, then the message is "Actinoptychus heliopelta was here!"  (Irrelevant Aside: A graffito seen in the men's room of a small rural tavern in the Black Hills of South Dakota said "Heisenberg may have been here.")

 

66.1 Here begins a series of seven images of the exterior of Aulacodiscus rogersii (Ehrenberg) Schmidt.  There are four rimoportulae, all with broken tubes.

 

67.3 The interior of a broken valve of Craspedodiscus elegans Ehrenberg.  67.4-69.1  Further images of this valve.

 

69.2 Another image of Triceratium condecorum Ehrenberg.

 

69.3 Another eight-sectored specimen of Actinoptychus heliopelta Grunow in Van Heurck.

 

69.4 A perfect specimen of Stephanopyxis diadema Ehrenberg.  The type locality for this species is Hollis Cliff, VA.  On Plate 123, fig. 2, of the Schmidt Atlas we find the very similar Stephanopyxis weissflogii Schmidt, listed as "ohne Fundort".  I suspect that this is a junior synonym.

 

69.5 Begins a series of eleven images of a very perfectly preserved specimen of Craspedodiscus elegans Ehrenberg, seen in exterior view.  The beautiful cribra in the areolar openings on the outer "ring" of the valve show uncommonly perfect preservation!

 

72.1 Another specimen of Aulacodiscus rogersii (Ehrenberg) Schmidt, this one with five rimoportulae, all with broken tubes.  The broken area on the right side of the valve reveals some inner structure.

 

72.2 A valve face and wide girdle band of Goniothecium odontella Ehrenberg.  The fine pores aligned in rows, seen on the valve face, are another characteristic trait that distinguishes this species.  72.3 The interior view of a valve of the same species; notice again the aligned pores.

 

72.4 Three more views of the exterior of a valve of Coscinodiscus bulliens Schmidt.  73.1 An oblique view of the entire valve, showing the irregular ring of greatly enlarged areolae.

 

73.2 Another valve of what may be Stephanopyxis turris (Greville) Ralfs in Pritchard.  The velum occluding each areole seems to be perforated by thin slits, rather than round pores.  This is true for other members of the genus as well.

 

73.3 A specimen of Stephanonycites coronus (Ehrenberg) Komura, in oblique view, showing two rings of spines: a dense inner ring, and a more diffuse outer ring.

 

73.4 Mastogonia crux Ehrenberg.

 

73.5 A perfect specimen of Triceratium condecorum Ehrenberg.  Note the small pseudocellus.

 

74.1 A more highly magnified view of the pseudocellus.

 

74.2 Possibly Stephanopyxis turris (Greville) Ralfs in Pritchard, lying on its very deep mantle.  Mantle depth varies considerably in this species, and there are numerous varieties based primarily on this one (probably insignificant) character.

 

74.3 A valve of an Actinocyclus species, possibly A. octonarius Ehrenberg.  There is a clear central annulus, fasciculate striae, a pseudonodulus at 7 o'clock, and a ring of rimoportulae on the mantle.  74.4 A highly magnified view of the mantle and margin of the valve face, with two rimoportulae.

 

74.5 Another specimen of Stephanopyxis diadema Ehrenberg.  75.1-75.2  More highly magnified views of the spines.  75.3 A highly magnified view of the surface sculpture, with the base of a broken spine -- probably the external tube of a rimoportula.

 

75.4 A view of a Coscinodiscus (?) species with a very complex, cribrate velum occluding the areolae.

 

75.5 The first of three views of a chain of Paralia sulcata (Ehrenberg) Cleve.

 

76.3 An exterior oblique view of a six-sectored valve of Actinoptychus heliopelta Grunow in Van Heurck.  The strong "striations" on the marginal band are very clear; this has certainly helped my understanding of what we see under the light microscope.

 

76.4 An oblique view of a frustule of an Actinocyclus species.  The relatively flat valve face has areolae with complex vela, while the mantle is relatively bare-looking, save for the usual ring of rimoportulae.  There is a pseudonodulus at 7 o'clock, right at the point where the valve face turns downward into the mantle.  The girdle bands are interesting: there seem to be three of them visible, a thin, ligulate one, a slightly wider split one, and a broad one.  None show surface sculpture of any kind.  76.5 A more highly magnified view of the same specimen.  77.1 Another view, from a slightly different angle.  77.2 A closer look at the interlocking girdle bands.  77.3 Another oblique view of the entire valve.

 

77.4 and 77.5  Two views of Mastogonia crux Ehrenberg.  The type locality for this species was the "Bermuda Tripel", now thought to be a deposit in the Dunkirk area.  78.1 A closer look at the center of the valve face.

 

78.2 An oblique view of a linking valve of the ubiquitous Paralia sulcata (Ehrenberg) Cleve. These valves are very strong, and are usually quite well preserved.  The separation valves, as is normal in the Paraliaceae, look a bit different, having a different pattern in the center of the valve face.  This species has a very long geological record, ranging from the Cretaceous to the Recent.  It is one of the commonest neritic diatoms.

 

78.3 The first of three images of another diatom with a very complex velum, this time with the velum covering both the valve face and the mantle.  I see no evidence of rimoportulae in this view.  78.4 An oblique view of the entire valve, which appears to have a central annulus.  I see no sign that the striae are fasciculate.  78.5 A highly magnified view of the cribrate vela occluding the areolae.  This is probably a species of Coscinodiscus.

 

79.1 An excellent specimen of Stephanopyxis lineata (Ehrenberg) Forti.  The "lineatus" organization is very clear: the areolae are arranged in quincunx, with each areole being a member of three different striae that cross one another at 120° angles.  Many other Stephanopyxis and Pyxidicula species exhibit this type of organization, as does Thalassiosira leptopus (Grunow) Hasle & Fryxell and Thalassiosiropsis wittianus (Pantocsek) Hasle & Syvertsen.  Except for being centric diatoms, none of these three taxa are closely related, and the "lineatus" organization is presumably the product of convergent evolution.

 

79.2 Another specimen of Stephanopyxis diadema Ehrenberg.  Here the "lineatus" organization has been disturbed by an apparent "growth flaw" near the center of the valve face, a not too infrequent occurrence.  79.3 At higher magnification, showing the surface sculpture.  The hollow spines are the external tubes of rimoportulae.

 

79.4 Another eight-sectored valve of Actinoptychus heliopelta Grunow in Van Heurck.  The raised sectors have one rimoportula; three of the depressed sectors have two rimoportulae, while the fourth has three.

 

79.5 The first of four images of Triceratium robustum Greville, the type locality for which is Calvert County, Maryland.  The flattened central elevation and large pseudocelli at the ends of the arms are obvious characters visible in the light microscope.  The absence of true ocelli indicates that this taxon does not belong in Triceratium sensu stricto.  (Note the small linked valves of a Stephanopyxis (?) species to the right.)  80.1 A closer look at one of the pseudocelli. 80.2 The whole valve in face-on view, as it typically appears under the light microscope.  80.3 A more highly magnified view of the flattened central area; the elongate slit is probably the opening of a rimoportula.

 

80.4 This begins a sequence of ten images of the same Aulacodiscus species we saw in 6.1-6.2,

6.4-8.1, 13.5-16.4, and 37.1-38.1.  Probable identification: Aulacodiscus sollitianus Norman.  Some of these show nice examples of the loculate wall.

 

82.4 Shows the internal central raphe endings of a Pleurosigma, presumably Pleurosigma affine var. marylandica Grunow in Cleve & Grunow.

 

82.5, 83.1, and 83.3 Three images of Pseudopyxilla cf. americana (Ehrenberg) Forti.  This name may actually represent a swarm of closely similar species -- unless it is just extremely variable.

 

83.2 An internal view of one valve of a resting spore.  It probably belongs in either of the formgenera Dossetia or Xanthiopyxis.

 

83.4 An internal view of a valve of what is probably a Coscinodiscus species.  The perspective is a bit unusual -- watch out for an optical illusion of depth reversal!  We are looking straight down the inner wall of the mantle to the valve face below.  At the inner edge of the curved zone where the mantle meets the valve face there is an intramarginal ring of small, slit-shaped rimoportulae, slightly elevated.  83.5 A full-face view of the valve interior, showing radiate striae and a hyaline central area, with no rosette.  The valve face is clearly concentrically undulate.

 

84.1 Begins a series of five images of another good specimen of Craspedodiscus elegans

Ehrenberg.  This first image again shows the beautiful cribrate vela that occlude the outer opening of each areole.  84.2 Another view of the same.  84.3 Another view, showing that the areolae on the "floor" of the depressed toroidal ring do not possess the cribrate vela.  84.4 A view of the entire valve, face-on.  Note the smaller valve of a Stephanopyxis lying inside.  84.5 A highly magnified view of the center of the valve face, showing that the alveolae on the central "plateau" are again velate; there are tiny rings of silica teeth around the center-most areolae.

 

85.1 - 86.4  Nine images of the same Aulacodiscus species we saw in 80.4.  86.4 A good image of the surface sculpture, with cribrate vela occluding the areolae and fine siliceous papillae.  Probable identification: Aulacodiscus sollitianus Norman.  

 

86.5 Another image of Pseudopyxilla cf. americana (Ehrenberg) Forti, with more of the branching tip of the apical spine remaining.

 

87.1 Triceratium condecorum Ehrenberg.  The center is depressed; again, watch out for the optical illusion of depth reversal.

 

87.2 A panduriform species of the form-genus Xanthiopyxis.

 

87.3 A different species of Chaetoceros than the one we saw earlier in the series.

 

87.4 One valve of a globular (?) species of the form-genus Xanthiopyxis.

 

87.5 Pseudopyxilla cf. americana (Ehrenberg) Forti.

 

88.1 Another specimen of the resting spore we saw in image 3.5.

 

88.2  No comment.

 

88.3 This is a true Triceratium valve, seen in internal view.  Note the small rimoportulae near the base of each apical extension -- these are commonly seen in this genus.  And unlike what we have observed in many centric diatoms here, the cribrate vela occlude the interior openings of the loculi, not the exterior.  This is almost certainly the same species we saw in image 30.1, which showed the external view of the valve.

 

88.4 An elongate, ovoid species of the form-genus Xanthiopyxis.  The frustule is heterovalvar, as are many resting spores: the epitheca is quite spiny, while the hypotheca is not.

 

88.5 - 89.3  Four images of an excellent specimen of the mystery diatom of row 1, seen in external view.  This first image clearly shows the slightly domed, cribrate velum that occludes the outer opening of each loculus.

 

89.4 - 90.2  Four images of the same species, this time in internal view.

 

90.3 A frustule of the resting spore (?) Dossetia hyalina Andrews.  It is heterovalvar, the epitheca having a flanged, spinose margin, while the hypotheca is a simple, panduriform plate.  These latter valves have usually been placed in the form-genus Xanthiopyxis.

 

90.4  No comment.

 

90.5 - 91.3 Four more images of the intriguing mystery diatom seen in images 5.2 et seq.  No new details that I can see.

 

91.4 Begins a series of three images of a typically ragged specimen of Lithodesmium undulatum Ehrenberg, seen in interior view.

 

92.2 Begins a series of six images of a broken specimen of the same species of Aulacodiscus we saw in image 35.2 et seq.  No new details.

 

93.3 - 93.5 Three images of the same species of Aulacodiscus we saw in 17.5 et seq.  No new details.  93.5 shows the full valve interior, with what looks like a valve of Actinocyclus curvatulus Janisch lying inside of it.

 

94.1 Another specimen of an ebridean, possibly a Semantebria.

 

94.2 Here begins a sequence of four images of a different species of Aulacodiscus, seen in exterior view.  This first view shows the peculiar, cap-shaped exterior opening of a rimoportula, on a raised hyaline structure.  The opening itself is a complex narrow slit.  The central branch of this slit "wanders" in this image but is straight in others.  94.3 Another view of a rimoportula opening, this one broken and revealing some interior structure.  94.4 An oblique view of a complete valve of this species, with two loose girdle bands.  There is a central rosette of areolae, and indistinct "lanes" leading from the center to the four rimoportulae.

 

94.5 An apparent "stack" of valves of this species.  I do not understand how to interpret this image!  The lowest valve is seen in interior view; the internal rimoportulae are deeply recessed, and the "lanes" appear as hyaline rays leading to a hyaline central area.  95.5 and 96.2 Other specimens of this intriguing species.

 

95.1 Begins a series of four images of a convex diatom valve, seen in internal view.  There is a hyaline rim around the margin, and a ring of four rimoportulae at the center of the valve face, oriented radially.  This is Stellarima stellaris (Roper) Hasle & Sims.  These images look precisely like those in Round et al.,1990, p. 181, figs. d-f. 95.2 A closer look at the central rimoportulae. 95.3 The central rimoportulae in face-on view; note the small hyaline areas near the rimoportulae.  95.4 The entire valve interior in face-on view.

 

96.1, 96.3, 96.4, 96.5 No comments provided.

 

97.1 The deeply recessed rimoportulae, hyaline rays and central area, all make this look like an internal view of the same Aulacodiscus seen in 94.2 et seq.

 

Version History

 

The first version (1.5) was completed by Mary Ann Tiffany and Dick Carter in April 2011 and hosted on the website (now out of use) built by Bill Dailey. This version (1.6 completed February 2023) was edited by Bill Dailey and Rob Kimmich with assistance from David Walker of Micscape Magazine to make the observations and illustrations more available. All the collaborators of this effort are members of the online group, Diatom Forum.

 

References

 

Andrews, G.W., 1988, A revised marine diatom zonation for Miocene strata of the

southeastern United States: U.S. Geological Survey Professional Paper 1481,

29 p. https://pubs.er.usgs.gov/publication/pp1481

 

************************

 

Hosted on Microscopy-UK with the generous permission of Mary Ann Tiffany and Dick Carter.
Queries on the content should be sent to Rob Kimmich, email - rkimmich12 AT gmail DOT com.

 

Queries on the suite functionality on this website should be sent to the Micscape editor,

email - micscape AT ntlworld DOT com.

 

First published on the Microscopy-UK website in the February 2023 issue of Micscape magazine.

www.micscape.org