Identification needed for Austin Chalk Ammonite

[Heteromorph]

  In March of this year I found a heteromorphic ammonite that has had me curious ever since. So yesterday I finally sent an email about it to a local ammonite expert, Ron Morin, who is associated with the Dallas Paleontological Society. I had a correspondence with him in May of this year as it related to him identifying my Phlycticrioceras trinodosum heteromorphic ammonite which I recently added to 'Collections'. That's when I first talked to him. Then at the Dallas Paleontological Society's Fossil Mania event in October, I was talking to Roger Farish about my unidentified ammonite. He recommended that I contact him again for identification.  

 

 

  Here is the email and the pictures that I sent him yesterday. I will post an update to this thread when he responds, which from my experience might be weeks. I have edited it to remove any slightly sensitive information like my name and more specific location information (I'm paranoid), as well as to fix any grammatical errors and to add relevant reference designations in between the < and > symbols:

 

 

 "Hello! I am Heteromorph, the one who contacted you to identify my Phlycticrioceras trinodosum specimen in May of this year, and I was wondering if you could help me identify another                                      heteromorphic ammonite from the Upper Coniacian stage of the Austin Chalk.

 

 

  This specimen was found on March 23 of this year in a creek in Ellis county. It is, in fact, within half a mile of where I found the last specimen that I sent to you for identification. The stratigraphy of this area is the Atco member of the Austin Chalk, Prionocycloceras gabrielense zone. My problem is that even though it resembles P. trinodosum, there are differences that would make me reluctant to indenify it as such.

 

 

  To date, I have not found one like it. It is similar to P. trinodosum in that the whorl section is compressed, it has ventral tubercles, and it has an open planispiral shape. But it also has 3 key differences that make me think it is either a different species or it is very pathological. I list these below.

 

 

  First and foremost, the main difference is the lack of any ventrolateral tubercles, which are one of the defining characteristics of P. trinodosum. On both the specimen itself and its negative, it appears to be free of any ventrolateral tubercles. The only tubercles that I can see are the ventral tubercles which are something that P. trinodosum has as well. 

 

 

  Second, the ribs are shaped differently than P. trinodosum. While P. trinodosum has rectiradiate ribs, this specimen has ribs which are rectiradiate until about half way up from the umbilicus, at which point it bends. Due to the fragmentary nature of this specimen, I have a hard time determining whether it bends abapically or adapically. 

 

 

  Third, the ribs are more costate on this specimen than any of the twelve P. trinodosum specimens that I have found in the Austin Chalk. It has a rib index of 7, while the most costate specimen that I have found and know for sure is a P. trinodosum specimen only has a rib index of 5. While this is not unheard of for this species, with specimens of this species having rib indexes of up to 8 (Emerson et al. 1994), yet from my experience it is apparently very unusual for this part of the Austin Chalk. 

 

 

  The closest thing that I have seen to my specimen is illustrated on Plate 11, fig 2 of Young, 1963 (as P. sp. cfr. douvillei), the similarity being the fact that they both have rib indexes of 7. After that, though, the similarity ends in that P. sp cfr. P. douvillei still has ventrolateral tubercles and rectiradiate ribs.

 

 

  I also found a very small P. trinodosum negative in the same creek just a few feet away. It has ventrolateral tubercles and a rib index of 4. The ribs are rectiradiate. A photo of it is not attached here. 

 

 

  My specimen is 87mm long including its negative and has a whorl height of 34½mm. The oval whorl section is compressed like P. trinodosum. It is shown first in the attached photo DSCN5355.

 

 

  Aside from the specimen in question, for reference I have also attached photos of two P. trinodosum specimens that I have found. They are both from within 5 miles of the creek site, so they are on roughly the same stratigraphic level. 

 

 

  What I am calling P1 is shown first in the attached photo DSCN5281 <F13> in comparison with the specimen in question. P1's negative is shown first in the attached photo DSCN5394 <22>. The positive is 69mm long when both pieces of it are measured together but 53mm when just measuring the largest piece. It has a whorl height of 31mm and a whorl breadth of 9mm. Rib index of 4. It was found within a quarter of a mile of the creek site. Because it is has just a slightly shorter whorl section to the specimen in question it is a good comparison piece. 

 

 

  The specimen which I am calling P2 is shown in the attached photo DSCN5361 <F27>. It is only a negative but I am attaching a picture of it here because it is the specimen that I referenced earlier with a rib index of 5. It is 23mm long and has a whorl height of 15mm. It was found about 4-5 miles to the south-west of the creek site.

 

  

  For reference, here is a post I made about the P. trinodosum specimen that I sent you a picture of in May.

  

 

  I thank you very much for your help in advance.

 

  Sincerely, 

 

  Heteromorph"

 

 

  I have given an alphanumerical designation to each picture for ease of reference. I guess it is probably kinda silly to have so many pictures that this is necessary. If this is stupid, than I extent my apologies to the Mods. I will patiently receive correction. 

 

 

  Thank you to everyone in advance.

 

 

F1

F2

 

 

F3

F4

[Heteromorph]

F5

F6

F7

F8

[Heteromorph]

F9

F10

F11

F12

[Heteromorph]

F13

F14

F15

F16

F17

F18

[Heteromorph]

F19

F20

F21

F22

F23

[Heteromorph]

F24

[Heteromorph]

F25

F26

F27

 

 

  That's it.

[Heteromorph]

  Upon recommendation from @DPS Ammonite, I forwarded my email to Dr. W. J. Kennedy at Oxford to help in the identification process. He has not written me back yet, but when he does I will post that reply here. 

 

 

  Here is my correspondence with Ron Morin about my ammonite so far. Like my last email that I posted here, I have edited these to remove any slightly sensitive information like my name, as well as to fix any grammatical errors, add hyperlinks, and to add relevant reference designations in between the < and > symbols:

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Ron Morin			November 18 (4 days ago)
to me

Heteromorph,

 

P1 seems to be Phlycticrioceras trinodosum, but since it is a fragment, it was a bit more of a challenge to identify than your specimen of P. trinodosum from last May. It took a while to find an illustration of a Phlycticrioceras comparable in size to P1, but Scott and Cobban (1964, pl. 3.3) have one (see attached pdf). Their specimen has a higher rib index because it is probably a microconch or gracile form, so it is not exactly the same as yours. However, Kennedy and Cobban (1991, pl. 9) illustrate two morphotypes of P. trinodosum: one of these has a lower rib index like P1, the other is similar to the form from Scott and Cobban (1964, pl. 3.3). Apparently, Phlycticrioceras trinodosum is a dimorphic species and P1 is a macroconch or robust form.

 

P2 is not identifiable as you suggested.

 

Ron

 

References

 

Kennedy, W.J. and Cobban, W.A., 1991. Coniacian ammonite faunas from the United States Western Interior: Palaeontological Association (London) Special Papers in Palaeontology 45, 96 pp. (not available on-line but by interlibrary loan)

 

Scott, G.R. and Cobban, W.A., 1964. Stratigraphy of the Niobrara Formation at Pueblo, Colorado: U.S. Geological Survey Professional Paper 454-L: 30 pp. (available on-line here)

 

P1 ammonite_P. trinodosum.pdf

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Heteromorph       November 18 (4 days ago) to Ron

  Thank you very much! Do you know of any of Phlycticrioceras trinodosum specimens that don't have tubercles like the specimen first shown in DSCN5355 <F1>, the first photo? Is that also a Phlycticrioceras trinodosum specimen? 

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Ron Morin			11:49 AM (2 hours ago)
to me

Heteromorph,

 

By definition, Phlycticrioceras trinodosum has ventrolateral and siphonal tubercles, and none are visible on the ammonite in photo DSCN5355 <F1> or its close-up in DSCN5319 <F8>.  However, there seem to be ventrolateral swellings on this specimen. Similar swellings are seen on the Phlycticrioceras trinodosum in photo DSCN5398 <F15>, as well as in other photos of the ammonite you called P1, BUT P1 clearly has tubercles. It is within the realm of possibility that there may have been tubercles on the exposed portion of the DSCN5355 ammonite but they were removed by weathering. All of this is, of course, extreme conjecture; but if DSCN5355 <F1> is P. trinodosum, it would be a gracile form or microconch because it has the high rib index and the type of coiling seen in the individual illustrated by Scott and Cobban (1964, pl. 3.3).

 

Ron

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Heteromorph			1:17 PM (1 hour ago)
to Ron

 

Thank you very much!

 

  It's odd. If this is a P. trinodosum specimen with its ventrolateral tubercles weathered off, I would not expect to see the typical yellow on the outside of the specimen. While some of the yellow has flaked off, it appears that most of it is still on the specimen, including the ventrolateral swellings, which indicates to me that those areas have not really had much weathering, if any at all. I see that on all the other P. trinodosum specimens that I have, the ventrolateral tubercles are very prominent and would take a good amount of weathering to remove.

 

I will forward to you the response from Dr. Kennedy when he writes me back. But in the mean time, Happy Thanksgiving!

 

Sincerely,

 

Heteromorph

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[Heteromorph]

When Keith Minor came to photograph some of my ammonites and check out a few sites with me in late December I showed him this specimen. He was intrigued by it but ended up telling me that it is hard to make a final determination about it and that it is probably the gracile form of Phlycticrioceras trinodosum, perhaps pathological or weathered. I am still not sure what to think since the ventrolateral tubercles are obviously absent and only ventrolateral swellings are present. I don’t think that it is from weathering, so I am leaning towards it being pathological or some other species.

 

Perhaps one day I will have a more definitive answer. For now I am content to label it cfr. Phlycticrioceras trinodosum.