PRESERVED AMMONITE SIPHUNCLE STRUCTURES FROM THE LOWER MOWRY FORMATION, WARM SPRINGS RANCH, HOT SPRINGS COUNTY, WYOMING
WDC-Ms-101 and WDC-Ms-102 were collected as flattened, compressed specimens with imperfectly preserved phragmacones. The nodes on the shell surface may have caused a clustering of at least two specimens during deposition. Shell material was missing via the dissolution of argonitic shell. Limited internal and external molds of ornamentation can be observed in the fine-grained matrix, though suture lines cannot be noted. Preservation of individual septum distance is not recorded. The crop and soft body was preserved internally and the aptychi were noted external to the body chamber.
As a taphofacies, the Lower Mowry Shales of Central Natrona County and the Southern Bighorn Basin consists of plated and laminated fissle shale formed from offshore mud and the fabric is rarely destroyed by bioturbation. Specimens were collected in organic sparse marlstones with iron-stone surfaces and record a lack of scavenging and major diagenetic changes post-deposition. Slow deposition compaction in fine-grained shale caused the ejection of the aptychi which were deposited with a symmetrical hinge surface exposed.
The chemical dissolution of the shell and compaction with retention of detail of internal structure warrants a more detailed description. The organic carbon content of the Lower Mowry Shale was at its lowest with direction of thinning beds. Damages to the siphuncle surfaces and disjointing are not noted. Depositional environments associated with the sea varied, but the result of similar pathways to preservation suggests more material would be found in detailed collection. Also, trace chemical analysis of preserved content may aid studies of the siphuncle of recent nautilids.