Architectonica (I Think) From Lake Nacogdoches, Texas

[Earl]

Hello all,

I think this fossil is a staircase shell, probably a member of the genus Architectonica. Here's a few photographs of the top and sides:

I found it last month in the Weches Formation (Eocene) in East Texas, in the greensand deposits along the shore of Lake Nacogdoches. Unfortunately, it wasn't intact. I collected as many pieces as I could and have done some repair on the top. (The shiny blurry parts in the photographs are cyanoacrylate glue; it was in rather sad shape, regrettably.)

As you can see in the photo with the metric ruler, the diameter of the top is close to 3.5 cm. This seems large, given my current research, for Architectonica.

I have looked at three references so far:

1. Finsley, Charles. A Field Guide to Fossils of Texas.

2. Emerson, John and Barbara. Middle Eocene Claiborne Group Invertebrate Fossils from Stone City Bluff, Burleson County, Texas.

3. Garvie, Christopher. The Molluscan Macrofauna of the Reklaw Formation, Marquez Member (Eocene: Lower Claibornian), in Texas.

Here are some my candidates for species, based on reading in the above references:

Architectonica scrobiculata

The Emersons say that A. scrobiculata is the most common Architectonica found at Whiskey Bridge, which is also Eocene; so that made me give it a close look. Finsley describes it as being "ornamented with spiral beaded lines" and if you look closely at the photograph in the Emerson reference on page 53, you can probably see those beads. But my specimen doesn't have any beads on the spiral lines. So I think that eliminates A. scrobiculata.

Architectonica alveata

On page 109 of his book, Chris Garvey mentions A. alveata being found in the Weches formation. Finsley describes A. alveata as having a "low, flat spiraled cone. Outer edge usually sharp-angled." Those characteristics probably fit my specimen; the other details about the umbilicus I can't verify. The Emersons don't mention A. alveata.

Architectonica (Stellaxis) reklawensis

This is a new species that Garvie describes on page 109. And when I look at his Plate 11, figures 1 and 2, I see the most similarity to my specimen. Again, much of the textual description includes basal characteristics, which I don't have on my specimen. But A. reklawensis seems a possibility to me. However, I'm bothered a bit about size. Garvie examined 17 specimens of A. reklawensis for his description, with a maximum diameter of 28.6 mm for the group. My specimen is nearly another centimeter or so greater in diameter.

Most other species in Architectonica that are described in the Emerson book have beaded spiral lines on top, so I have eliminated them from consideration.

Anyway, this is my current state of research. A. alveata and A. reklawensis seem possibilities. The large size of my specimen seems uncharacteristic for the descriptions that I have read of Architectonica; maybe I haven't yet touched on the reference that includes something of that size. But I also understand that I may not be able to make much headway in species identification, since I only have the top of the shell. It seems many of the specific characters of Architectonica are found on its base: the umbilicus, for example. So I'm out of luck in that regard.

But I thought I would show this fossil to you all for your always useful help and comments. I have the feeling that I may be missing something obvious.

Much thanks,

Earl

[Auspex]

This is a very information-packed post! I may be a vert-head, but your synthesis of the references (and your reasoning process) sucked me in, and I have learned a few things. Thank you!

Now, go back and find the bottom

[piranha]

Although Gastropoda are typically bottom rung for me you have managed to elevate this particular fossil among the elite with great photos and a splendid presentation. Another fine example of the great diversity of interest here at TFF and why it is such a great forum. After checking Index Fossils of North America there are two other species of Architectonica that have a smooth appearance similar to your fantastic specimen; Architectonica psilaxis & Architectonica nobilis. I cannot say with any certainty one way or the other although I was eager to assist after reading your inviting and informative post.

Thanks !!

[erose]

So all the more reason to go back next year (or whenever) and get better specimens. Also you state that your specimen is another centimeter bigger than Garvie's measured specimens. I see only a few millimeters difference.

[Earl]

Erich,

You might be right when you see only a few millimeters greater diameter than Chris Garvie's stated 28.6 mm maximum diameter for his group of Architectonica (Stellaxis) reklawensis. I have been assuming that there is a missing piece of outer whorl in my specimen, which would put my specimen at closer to 3.5 cm or greater. But I might be wrong about the missing whorl. It could be that the aperture occurs where I was imagining a continuation of the whorls.

You all are absolutely right about my needing to go back to Lake Nac. Last month's field trip with the Paleontological Society of Austin was my first (and so far, only) visit there. I definitely want to return and, as Auspex says, "find the bottom"!

Earl

[jkfoam]

Earl,

I think you have come to the correct conclusion that it will be very difficult to identify your specimen as to A. alveata or A reklawensis with out the bollom half of the specimen.

I believe you were correct to eliminate A. scrobiculata The A. scrobiculata is concave (Slightly between the whorl suture lines). While both A. alveata and A. reklawensis are convex between the whorl suture lines.

Also, don't make too much of the fact that your specimen is larger than any reported by C.L. Garvie. I have collected numerous A. scrobiculata from one Cook Mountain Formation location and one is clearly twice the diameter of the next largest specimen.

All things considered I think I would tentatively identify your specimen as A. alveata because Garvie did not report finding A. alveata in all his field work in the Reklaw. And several investigators have reported A. alveata in the Weches Formation. See K.V.W. Palmer, Bulletins of American Paleontology, Vol 7, 1937.

JKFoam

[Earl]

Thanks to all for the helpful comments and references for further research. The sharing of knowledge is the great thing about this forum. (I've got some more reading to do, thanks to piranha and jkfoam.)

Architectonica seems a lovely genus. I look forward to finding more specimens and learning more details about them.

Hopefully the next time I post about one, it will be whole and intact!

Regards,

Earl

[jkfoam]

Earl,

For Texas and gulf coast gastropods there are two primary reference books, First is "The Claibornian Scaphopoda, Gastropoda,and Dibranchiate Cephalopoda of the Southern United States", Bulletin of American Paleontology, Vol 7, No. 32, 1937 by K.V.W. Palmer. The second is "Turrid Illustrations", Paleontographica Americana, Vol 2, 1937, by G.D. Harris (Deals with Turrid snails of the Claiborne) Parentheses are mine. Fortunately both of these publications are available on line. There is a third reference that is invaluble in ID'ing Claiborne mollusks and that is "Catalogue of the Paleocene and Eocene Mollusca of the Southern and Eastern United States" Part 1 and Part 2, Bullentins of American Paleontology, Vol 48, 1965, By K.V.W. Palmer and D.C. Brann. This third reference gives the latest name changes, at least up to 1965 but it is best for facilitating the use of the other two references on-line by providing page numbers to refer to for each specie. I really dislike using e-books! I think the first two references are way out of print but the last reference is still available for purchase. Incidently, if anyone knows where I can buy a copy of K.V.W. Palmers 1937 Bull. of Am. Paleo. publication I'd be willing to pay several hundred dollars to secure a copy. I downloaded an on-line copy but it is a real pain e-leafing through 90 e-plates.

Earl, keep posting pics of your finds even if they are not complete or intact. I enjoy them. Good luck with your research.

JKFoam

[Earl]

Hey Jim,

Thanks for the recommended references. I found all of them on the web. That online Biodiversity Heritage Library is a wonderful resource. I think I have a lot of reading ahead!

Regards,

Earl