The November 2025 Paleontology Study Group Meeting was held on November 15th, 2025 via Zoom. John Catalani presented “Gonioceras: A Most Unusual Cephalopod”.

The diversity of nautiloid shell shapes in the Upper Ordovician of central Laurentia is remarkable. However, one shape is often missing from lists of shell shapes and that is the so-called “flat-fish” form of Gonioceras. This program begins with a discussion of the stratigraphy, paleogeography, and areal distribution of Ordovician rocks in the mid-west including the Mohawkian Sea that dominated the interior of Laurentia during the Late Ordovician. Following this introduction, the origin of cephalopods and the amazing success of the nautiloids through time is presented. The fauna of the Mohawkian Sea is often referred to as the “Gonioceras  Fauna” due to the uniqueness and relative abundance of Gonioceras specimens. Many specimens, mostly self-collected, are presented to illustrate various morphological features of  Gonioceras leading to a discussion of the life habits of this unique cephalopod. Most of these specimens are of Gonioceras occidentale  from the Forreston Bed of the Grand Detour Member (Platteville Formation) although additional Gonioceras specimens from various localities are presented.