Edited by Heteromorph
Heteromorph AmmoniteKingdom: Animalia
International Age: Coniacian (middle/late)
Austin Chalk Formation
Date Collected: 03/04/2017
Acquired by: Field Collection
This heteromorphic species is characterized by an open plain spiral shape with slightly rursiradiate ribs and 3 sets of tubercles, 2 sets of ventrolateral tubercles, and 1 set of ventral tubercles. The whorl section is compressed and does not have constrictions in United States specimens but does have constrictions in many European specimens. The distance between ribs is roughly the same as the width of a rib. As far as I know, there are only two species reported for this genus, with the other being Phlycticrioceras rude from the late Santonian of France (Kennedy 1995). P. trinodosum is the only species reported in Texas.
This species has two distinct forms, with the more commonly found robust form having a lower rib index and the less commonly found gracile form having a higher rib index, both being found at the same stratigraphic level. This possibly indicates very definite sexual dimorphism within this species, perhaps with the robust form being female and the gracile form being male. This particular specimen is a robust form with a rib index of roughly 3 1/2, but some gracile specimens of this species have been known to have a rib index of up to 8. (Emerson 1994). The highest rib index of a P. trinodosum specimen that I have found is 7 on a fragment of a very mature gracile specimen. That specimen (seen here) shows very weak ventrolateral tubercles, probably due to it being the gracile form since I have found multiple robust specimens of about the same maturity that have very clearly defined ventrolateral tubercles. P. trinodosum shows considerable variation in ornamentation between forms.
It was broken in two when it separated from the rock shown in the last photo, with its outer whorl being shown in the 4th and 5th photos. The outer whorl is 53mm long, and at the top where the whorl height is measurable, it is 16mm. You can see in the photos of the main part of the specimen, the impression of where its outer whorl once was. The complete specimen would be about 65-70mm in diameter if its outer whorl was still connected.
Mine shows a bit of pathology in some places, with two examples being the large gap in between two ribs shown in the 4th and 5th pictures, along with two ribs being very close to each other, which is shown in the 2nd picture.
Here are a few references, with the hyperlinked references being underlined. The first 4 references that I have hyperlinked are open access, while the 5th is not open access but can be obtained online without having to request the text from the authors. The 6th reference has to do with the species Phlycticrioceras rude, the only other species in this genus. I have added additional links to sources with information about this paper due to the fact that it is not open access and must be requested. The 7th and 8th references are not open access and are not hyperlinked because I cannot find any way to obtain them online. The last hyperlink is an open access stratigraphic reference for the Austin Chalk and has no information about this genus.
When applicable and needed, I have put the relevant pages for information, plates, and text figures at the end of references:
Ulrich Kaplan und William James Kennedy (1994). Ammoniten des westfälischen Coniac. Geologie und Paläontologie in Westfalen, Heft 31, 155 S. Pages 53, 54; Tafel 37, Figures 2-4, 9-15 on pages 142, 143; Tafel 43, Figure 3 on pages 154, 155.
Zdenek Vašíček (1990). Coniacian ammonites from Štíty in Moravia (Czechoslovakia). Sbornik geologickych ved, Paleontologie 32, Pages 163-195. Pages 177, 179; Plate VI with its explanation is on page 193.
Young, K. (1963). Upper Cretaceous Ammonites from the Gulf Coast of the United States. University of Texas, Publication 6304, 373 pp. Pages 45, iv, 39, 47, 371; P. sp. cfr. douvillei on pages 45, iv, 23, 26, 29, 371; Plate 4, figures 2, 3 on pages 150, 151; Plate 11, figure 2 on pages 168, 169; text figure 7 f, h on pages 156, 157.
W. J. Kennedy (1984). Systematic Paleontology and Stratigraphic Distribution of the Ammonite Faunas of the French Coniacian. Palaeontological Association, London, Special Papers in Palaeontology, No. 31. Pages 136, 137; Plate 32, figures 4, 11 on pages 140, 141; text figure 42E on pages 146, 147.
David L. Clark (1963). The Heteromorph Phlycticrioceras in the Texas Cretaceous. Journal of Paleontology, Vol. 37, No. 2, pp. 429-432.
W. J. Kennedy, M. Bilotte and P. Melchior (1995). Ammonite faunas, biostratigraphy and sequence stratigraphy of the Coniacian-Santonian of the Corbieres (NE Pyrenees). Additional links to information concerning this paper can be found here and with the species Phlycticrioceras rude, listed here.
Kennedy, W.J. and Cobban (1991). Coniacian Ammonite Faunas from the United States Western Interior. Palaeontological Association, London, Special Papers in Palaeontology, No. 45, 96pp.
Barbra L. Emerson, John H. Emerson, Rosemary E. Akers and Thomas J. Akers (1994). Texas Cretaceous Ammonites and Nautiloids. Paleontology Section, Houston Gem and Mineral Society, Texas Paleontology Series Publication No. 5, 438 pp. Pages 285, 286, 388, 422.
Ulrich Andrew S. Gale, William James Kennedy, Jackie A. Lees, Maria Rose Petrizzo and Ireneusz Walaszczyk (2007). An integrated study (inoceramid bivalves, ammonites, calcareous nannofossils, planktonic foraminifera, stable carbon isotopes) of the Ten Mile Creek section, Lancaster, Dallas County, north Texas, a candidate Global boundary Stratotype Section and Point for the base of the Santonian Stage. Acta Geologica Polonica, Vol. 57, No. 2, pp. 113-160.
The 1st, 2nd, 4th and 8th papers also contain information on the genus Tridenticeras which is found in the Austin Chalk alongside P. trinodosum, just in case anyone is interested in that genus as well.
A big thanks to DPS Ammonite. This is my first post to 'Collections' and he helped me get it all straight.
Edited by Heteromorph