Search the Community

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