Large Ammonite

[Julian]

I came across this large ammonite while fossil hunting by lake Texoma. I found it on the shoreline of an artificial barrier that lead to some oil pumps. I suspect it's not native to the area and was brought in with construction material for the barrier itself. It appears to be shale stone. Other then that I can't guess what it could be. It's quite large though, with a diameter of 16 1/2 inches from the end chamber to the top. The ridges aren't too well defined and can only be seen under certain light conditions.

Sorry about the images, my digital camera isn't too good.

[Guest Nicholas]

Nice find, is that it as found? or was there some prep previously done?

[Guest bmorefossil]

I came across this large ammonite while fossil hunting by lake Texoma. I found it on the shoreline of an artificial barrier that lead to some oil pumps. I suspect it's not native to the area and was brought in with construction material for the barrier itself. It appears to be shale stone. Other then that I can't guess what it could be. It's quite large though, with a diameter of 16 1/2 inches from the end chamber to the top. The ridges aren't too well defined and can only be seen under certain light conditions.

Sorry about the images, my digital camera isn't too good.

wow very nice, its got a nice size to it

[Julian]

Nice find, is that it as found? or was there some prep previously done?

No, that's how I found it. I thought it unusual since most of the ammonites I saw were imbedded within the rock and this just happened to be sitting out on the shore.

It's an impressive size but I'm not sure there would be any kind of stand strong enough to hold it. It weighs 50 pounds.

[Guest Nicholas]

I thought it was odd too, very lucky for you though. Great find.

[Auspex]

That's a big boy; must have been Boss of the lagoon! Wonder how many trilobites it ate?

[LanceH]

That's an EoPachydiscus ammonite (internal cast) from the Duck Creek formation that crops out around Lake Texoma. They occur in the lower portion of the Duck Creek and are very common, the dominant fossil in fact. Pachydiscus is the genus but not sure the exact species, there are a few. Some are smooth, some are ribbed, some suddenly step up in diameter going around the shell.

I went on the Dallas Paleo field trip to Texoma and saw tons near the spillway but many were compressed. I actually find better ones in new construction near my home in Lake Worth, Texas.

[Julian]

Thanks a bunch and that's interesting to know. I found a lot of ammonite specimens as well as a fossilized clam along the artificial barriers/bridges, sadly most were broken fragments and partials. May I ask what you mean by "compressed?"

[LanceH]

Squished flatter.

I think the biggest one found in Texas is 6 feet in diameter.

Thanks a bunch and that's interesting to know. I found a lot of ammonite specimens as well as a fossilized clam along the artificial barriers/bridges, sadly most were broken fragments and partials. May I ask what you mean by "compressed?"

[Auspex]

...Wonder how many trilobites it ate?

Since he's Cretaceous, I guess that would be "none".

[Uncle Siphuncle]

Eopachydiscus marcianus as Lance said is an index fossil of the Duck Creek formation. I don't have references in front of me, but they may have ranged into the overlying Fort Worth formation as well, but I could be wrong. There is another species E. laevisculus (or similar spelling) I've read about in the Washita group as well, but I've never learned the diagnostic differences between the two. My biggest Eo is 19 inches and 90 LBS, but they do get bigger. I've taken others less than an inch in diameter, but they seem to average around 10-12 inches, some with wonderful sutures. I've taken them from Lake Texoma through Fort Worth, west of Waco, and down into central Texas. While most veteran collectors have limited out on these things I just can't seem to leave them when they are as nice as yours. I've used some as gifts, others as yard art, and of course the better ones are in the house and at the office.

[Mike Murphy]

Julian:

Lance and Dan are both correct in that the large ammonite is the genus Eopachydiscus. Eopachydiscus can be as large as 2 feet across; 14 inches in diameter would be an average adult while the largest officially documented was 24 inches in diameter by F.W. Cragin in 1893. The largest that I have found has been 19 inches, the same as Dan's. The smallest that I have found have been 0.25 inch pyritic micromorphs near Lake Texoma. Eopachydiscus is limited to the lower 3 to 4 feet of the Duck Creek Formation and its equivalence in Texas, Oklahoma and New Mexico (Tucumcari Formation). As a result, Eopachydiscus marcianus is the key zone fossil for the base of the Duck Creek and the Washitan group. When Eopachydiscus is found in situ, the person can be sure that they are near the base of the Duck Creek and the Washitan. In southern Oklahoma, the genus is found in the lower Caddo Formation, which is equivalent to the Duck Creek. Eopachydiscus is widespread in Texas and is also found in central texas (lower Georgetown Formation), the Llano Estacado (Duck Creek), northern hill country (Fort Lancaster Formation) and in trans-Pecos Texas (Boracho Formation, Levinson member). Where they are found, these ammonites can be quite abundant, especially on both the Texas and Oklahoma sides of Lake Texoma. The majority of the larger specimens, especially those found loose, however, will be fragmented partials. Craginites serratescens occurs in the same basal Duck Creek and is also used as a key zone fossil, but Craginites serratescens is much less common.

Originally, these large ammonites were considered to be the genus Desmoceras, but the suture pattern was not desmoceratid but was pachydiscid. As a result, the ammonites were moved from Desmoceras to Eopachydiscus in 1955. Although there were 3 species originally attributed to Eopachydiscus (Eopachydiscus brazoensis, Eopachydiscus laevicaniculatus and Eopachydiscus marcianus) based on the development of ribbing and the thickness of the cross section, they are all considered to be synonymous and combined into the same species, Eopachydiscus marcianus (Shumard, 1854). This was suspected and reported by W.S. Adkins in 1928 and later confirmed by Keith Young in 1979 and W.J. Kennedy et al in 1983. A well preserved adult specimen of Eopachydiscus will have all 3 rib characteristics present, with no ribs in the juvenile whorls, more closely spaced and stronger ribs medailly and widely spaced weaker ribs in the most mature whorls. The cross section will range from thicker and globose in the juvenile portions and compressed in the adult portions.