Very Odd Texas Baculites from Austin Chalk Formation

[JamieLynn]

So I am 90 percent convinced the site is Austin Chalk Formation. I have found a couple of fossils that lead me to that conclusion, a gastropod and a nautiloid specifically. That being said, I cannot quite figure out what these bacultes are. From the HGMS book the Boehmoceras arculus seems to be the closest but it says it is a curved shaft, which these most certainly are not.  But the strong nodes on these are really not falling under any other possibility.  Any insights? 

3.5 inches

 

 

 

2 inches

 

 

1.5 inches

 

this is the only one that you can really see the suture patterns. The nodes don't show up very well but they are there

 

 

[Jared C]

Not sure on the ID either, but I think I’ve seen these strongly “noded” baculites in the Dessau member.

 

if this is the site you suspected was Ozan, know the pflugerville member is what underlies the Ozan and that it’s absolutely freaking rife with nautiloids. Tons of nautiloids and common occurrences of Pachydiscus paulsoni mark the pflugerville for me

[JamieLynn]

Interesting, my friend who knows baculites said she thought it looked Santonian and thats the Pflugerville.....however, these fossils are what have me leaning toward Austin Chalk - a Neithea casteeli, which I am 95 % confident on ID, a gastropod xeonmorphia (100% positive) and a Trigonia thoracica, all of which are Austin Chalk.  So i'm still confused.....

[Jared C]

I think your confusion might come from the classification of the members of the Austin chalk.

 

The "Austin group" and "Austin chalk formation" are used mostly interchangeably. In it's type area, it's comprised of multiple members that are different than its members up north.

Ex: the blossom sands fm is a member of the Austin chalk found in North Texas, but not in central Texas. 

 

In central Texas, the members of the austin chalk, from oldest to youngest, are as follows:

-Atco

-Vinson

-Jonah

-Dessau

- Burdit

- Pflugerville

 

33 minutes ago, JamieLynn said:

my friend who knows baculites said she thought it looked Santonian and thats the Pflugerville.....however, these fossils are what have me leaning toward Austin Chalk

With that clarified, we see that the Pflugerville is simply the uppermost member of the Austin Chalk. 

 

In regards to age: Durham 1957 pushed that the Atco is Coniacian while the rest of the members are Santonian. But, Young & Woodruff Jr. 1985 disagreed and established the timeline below:

 

The Atco and lower half of the Vinson are Coniacian

The upper Vinson is lower Santonian

The Jonah + the lower 1/4 to 1/3 of the Dessau are upper Santonian

The upper 3/4 to 2/3 of the Dessau is lower Campanian

The Burdit and Pflugerville are successively younger lower Campanian

 

Hope that clarifies it

 

[JamieLynn]

Gonna have to wrap my head around that......hahhaha! Thanks Jared! I appreciate the info! 

[JamieLynn]

 

From a paper that is unfortunately $40 

Upper Cretaceous (upper Santonian) Boehmoceras fauna from the Gulf Coast region of the United States

Abstract: 

An upper Santonian fauna dominated by the curved baculitid Boehmoceras Riedel, 1931, occurs as phosphatic moulds in the Tombigbee Sand Member of the Eutaw Formation in Mississippi and Alabama, and also is reworked into the base of the Mooreville Chalk; a similar assemblage is present in the middle part of the Pen Formation of the Big Bend area, Texas. The assemblage is: Pseudoschloenbachia (Pseudoschloenbachia) mexicana (Renz, 1936), Placenticeras syrtale (Morton, 1834), Reginaites leei (Reeside, 1927a), Reginaites exilis sp. nov., Texanites (Texanites) sp. juv. cf. gallicus Collignon, 1948, Texanites (Plesiotexanites) shiloensis Young, 1963, Texanitinae incertae sedis, Hyphantoceras (?) amapondense (van Hoepen, 1921).Glyptoxoceras spp., Boehmoceras arculus (Morton, 1834) and Scaphites (Scaphites) leei Reeside, 1927a, form I. The assemblage is referred to the uppermost Santonian Texanites (Plesiotexanites) shiloensis zone of the Gulf Coast sequence. Common occurrence of Scaphites (S.) leei I and Reginaites leei suggest correlation with the Upper Santonian Desmoscaphites erdmanni zone of the U.S. Western Interior; common occurrence of Boehmoceras arculus (= B. loescheri of European authors) suggests correlation with the Marsupites–granulata zone at the top of the North German Santonian. These data strengthen intercontinental correlation of the Santonian–Campanian boundary sequence. They also provide a biostratigraphic link to a numerical age of 84.2±0.9 Ma from a bentonite at a slightly lower level in the Tombigbee Sand Member of the Eutaw Formation in Mississippi.

[piranha]

JamieLynn: The paper is incoming via PM!

[JamieLynn]

@Jared C  Can you take a look and see if you think this is a Pachydiscus paulsoni? I was thinking it was a Eupachydiscus jimenez which HGMS book says is in the Dessau.  It looks so very similar to the P paulsoni but I think I see bifurcated ribs. I know it's pretty waterworn so it may not be discernable. This is from the same site.

 

 

 

 

[Jared C]

On 4/5/2024 at 8:59 AM, JamieLynn said:

@Jared C  Can you take a look and see if you think this is a Pachydiscus paulsoni? I was thinking it was a Eupachydiscus jimenez which HGMS book says is in the Dessau.  It looks so very similar to the P paulsoni but I think I see bifurcated ribs. I know it's pretty waterworn so it may not be discernable. This is from the same site.

 

 

 

 

The P. paulsoni I’ve found in the pflugerville look very similar but don’t have ribs- though I believe ribs are absolutely possible in Pachydiscus. Perhaps @historianmichael would have something to say on this ammonite.