Arcadia Park/Britton Gastropod/Ammonite?

[Heteromorph]

On Sunday afternoon I went out with Keith Minor to a North Texas site exposing the lower Arcadia Park formation and (possibly?) the top of the Britton Formation (lower Turonian, upper Cenomanian respectively). The hunt almost never happened after various storm cells were menacing us and the high winds were thrusting cranes into sky scrapers and whipping up emphysema inducing dust clouds. Yet, even though everyone around us was getting Kansas blown at them, we were only exposed to the high winds and rain so cold and blown so hard that it felt almost like hail. But that lasted for only a few minutes, leaving the rest of the day to muck around in the Turonian while the winds blew most of the clouds away by hunt’s end.

 

The Kamp Ranch Limestone is exposed very nicely at the site, as well as meters of shale beds above and below it, making a short study of the successive stratification obligatory. 

 

FIG 1: The roughly 38 cm (15 inch) thick Kamp Ranch jutting out amongst  the soft shale and clay above and below it. The clouds foretell the showers to befall us.

 

(ID request incoming)

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FIG 2: The Kamp Ranch and the underlying clayey shale, the later exhibiting the telltale signs of water infiltration and subsequent mineral deposition there in by the white splotches. 

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FIG 3: Cross section of Kamp Ranch and overlying meters of Arcadia Park. Poorly drawn edited-in red bracket indicates Kamp Ranch. 

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FIG 4: More Kamp Ranch. Not a distinct unit from the Arcadia Park, just a member or facies of it.

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When last I was at the site in October I had only focused on the Kamp Ranch itself, finding a few tiny Ptychodus whipplei, Squalicorax, and a big Cretodus crassidens. This time I spent about on hour of the hunt on it, but with no such luck. The limestone itself proved mostly barren, apart from a few Collignonicerid ammonite impressions and some fish scales. My only vertebrate luck came from digging through the lose sandy layer directly atop the Kamp Ranch, finding a bunch of tiny Squalis, a tiny fish tooth, fish vert, Ptychodus, and Cretodus blade. 

 

Anyway, meanwhile my mother was walking around the site and came upon a lovey little Worthoceras on a chunk of fresh, blue shale. That immediately got Keith’s and my attention, and we started seeking out other chunks of the blue shale that were shattered about the site. The shale was dug up from a few meters below the Kamp Ranch for sewer stuff, probably being of the Watoniceras coloradoensis zone at the top of the Britton formation. 

 

FIG 5: Kamp Ranch at the top, blue shale with ammonites and Mytiloides inoceramids  at the base.

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FIG 6: Dug up blue shale mixed with Kamp Ranch.

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Now here comes the question. While Keith was splitting the helpfully bedded shale (which had the texture and feel of a soap bar, by the way), he came upon a planispiral shell that, when we saw it, reminded us both of the Pennsylvanian aged gastropods found tens of miles to the west of us. I am not very familiar with the gastropods of the Eagle Ford, but if this isn’t one then this is a very smooth ammonite. Possibly Placenticeras? Now it must be remembered that most of the ammonites here a crushed almost flat, so perhaps it was a normal cone shaped gastropod crushed flat to look like a plaispiral one. Ruler for scale. Both the positive and negative of the specimen are shown.

 

FIGS 7-8: Positive & negative of mystery mollusk.

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What say you?

 

More ammonite pictures tomorrow.

[Tidgy's Dad]

I say nice report.

And I think ammonite. 

[Heteromorph]

Thanks! I may very well be an ammonite, it just looked weird compared to the others.

 

Here are a few other ammonites and various bivalves.

 

FIGS 9-10. Various ammonites including Worthoceras, possibly Watinoceras coloradoensis, and a few others. And some bivalves.

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FIGS 11-12: Badly oxidized pyrite ammonite, Collignonicerid of some variety, possibly Prionocyclus? Positive & negative.

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Lastly, Bivalves.

 

 

The list of ammonites that we found in the blue shale is something like Baculites yokoyami, Worthoceras, W. coloradoensis, Allocrioceras, a strange fragment with two ventrolateral tubercles resembling Placenticeras, and probably Prionocyclus and Meticoceras.