Discocyclina Marginata?

[danco]

Here is an agatized polished slab and I wonder if Discocyclina marginata is embedded?

It is from Indonesia (Sumatra?).

Thanks for giving me our opinion.

Edited May 29, 2015 by danco

[dshamilla]

There is a possibility that Discocyclina is embedded in the agate, but only close microscopic examination can provide a definite answer. If you haven't looked at the Sumatran feather agate site, see the following http://www.indoagate.com/feather.html.

[danco]

Thank you, Dshamilla.

[Diceros]

You guys should provide more data for the rest of the members, to be informative, and less like a slab dealer at a rock show.

First, what is Discocyclina marginata? It's a larger foraminiferan (a single-celled animal, like a shelled amoeba) from the early Tertiary, generally found in shallow marine water, where carbonate is available to build their large calcite shells, called tests. Discocyclina has a circular outline, and is fairly flat, except for the thicker central part. They're often found in large numbers.

So what are they doing in this slab of Sumatran feather agate? They were originally in limestone, but here the limestone's been silicified, and the marine fossils show up clearly against the matrix. The slabs are called feather agate, because what you see is hundreds of Discocyclina marginata tests, all lying flat on the sea floor, so when you section them perpendicularly to the flat tests, they look like the pinnules of a bird feather. When the sectioned tests are examined microscopically, the tiny chambers of the test can be seen

How does the limestone come to be silicified? Sumatra is a volcanically active area of the world, and at times, huge volumes of volcanic ash fall on the limestone. The ash is full of easily-soluable shards of amorphous opaline silica, so the water sucks the silica out of the ash, and deposits it on the limestone, which then converts to silicified limestone.

[danco]

Thank you Diceros. A most enlightening answer.

[Diceros]

No problemo.

I used to collect m. Eocene Discocyclina advena in NW Louisiana.

My Master's thesis was on modern forams, in the failed hope of getting an oil co. job as a "bug man".

There probably aren't many who can ID both fossil foraminiferan tests and rhino teeth to genus

(I helped curate the American Museum's fossil rhino collection in the 70's).

Three guesses as to which I prefer.

[danco]

To my limited knowledge (I am a very amateurish collector) forams must have been very small. That's why I raised the question.

[abyssunder]

Looking to comparable evidence to determine if the slab has Discocyclina marginata, a good reference might be STUDIES IN TERTIARY STRATIGRAPHY, CALIFORNIA COAST RANGES-LARGE FORAMINIFERS OF EOCENE AGE - Geological Survey Professional Paper 1213,page 46:

Family DISCOCYCLINIDAE

"The classification of Discocyclinidae is a complex problem for American species. The classifications are numerous and varied: Vaughan and Cole (1941),Bronnimann (1945),Cole (1948), and Caudri (1972). We accept Cole's classification for our work.
Discocyclina ( Discocyclina) marginata (Cushman, 1919)

Orthophragmina marginata Cushman, 1919, pl. 1,fig. 2: pl. 2, fig. 1.
Orthophragmina marginata Cushman, 1920, pl. IX,fig. 1-2.
Discocyclina (Discocyclina) marginata (Cushman).Cole and Gravell, 1952, pl. 93, fig. 1-9; pl. 94, fig.1-8; pl. 95, fig. 7-8.
Discocyclina (Discocyclina) marginata (Cushman).Cole and Applin, 1964, pl. 10, fig. 1-8.

Description.Lenticular species, inflated, diameter 10-14 mm, equatorial chambers higher than wide(h/w = 1.5) and very small. Thirty to forty rows of lateral chambers are very compressed and poorly visible.
Distribution. California, localities 71CB983C,71CB978B, and EB633 near San Jose; Caribbean region (Cuba, St. Bartholomew), Florida, Georgia.
Range. Middle Eocene.
Remarks. Previously described in California as D. californica (Schenck, 1929, pl. 28, fig. 5; pl. 29, fig. 1-3; pl. 30, fig. 2-3), according to Cole and Applin (1964)."

Looking at the document Bulletins of American Paleontology Vol.XLVII 1964/Bulletin no.212/PROBLEMS OF THE GEOGRAPHIC AND STRATIGRAPHIC DISTRIBUTION OF AMERICAN MIDDLE EOCENE LARGER FORAMINIFERA - by Cole and Applin,on Pl.10 we find the pictures of Discocyclina(Discocyclina)marginata.

If your forams are similar to what has been presented above then they could be Discocyclina marginata.

The documents are in my Library.

Edited May 30, 2015 by abyssunder

[Diceros]

Excellent answer abyssunder! Poor foraminifera haven't been getting their due on Fossil Forum.

Danco - Most forams are tiny (usually less than a mm), but there's a group called "larger forams" which are far larger (all are single-celled animals, some just like to built up their tests). These include the Permian fusulinids, the Eocene discocylinids, the Oligocene Lepidocyclinids (known in America since the 1830's), and some big modern forms. The largest foram I ever saw was a big lt. Oligocene lepidocyclinid (I think it was Lepidocyclina ungulata) from the Florida panhandle - it looked like a fifty-cent piece (sorry, you rarely see those anymore - let's say about 1.5 in. in diameter), with a wrinkled edge. Anybody out there have one, of which you could to submit a photo? Fossil collectors should know about the larger forams.

[danco]

Thank you, again. Everything is now clear to me.

[Natural Oneironaut]

If there is anybody wanted to examine this agate, I have some pieces in here that you can check.

[abyssunder]

14 hours ago, Natural Oneironaut said:

If there is anybody wanted to examine this agate, I have some pieces in here that you can check.

Nice specimen! Thank you for posting it here.