Internal Features of the Embryonic Shells of Late Carboniferous Goniatitina

Abstract

Discovery of specimens of Glaphyrites (Goniatitina: Gastriocerataceae) without any matrix Discovery of specimens of Glaphyrites (Goniatitina: Gastriocerataceae) without any matrix inside the shell permits a three-dimensional view of the internal features. The specimens were found in phosphatic nodules from the Upper Carboniferous (Missourian Stage = lower Stephanian) of Oklahoma. Although the original shell microstructure is not preserved, the internal features are well exposed. The caecum is elliptical in outline in the median plane (= the plane of symmetry) and is attached to the inside surface of the initial chamber by a short, curved prosiphon. There is a single, elongated muscle scar on the inside surface of the initial chamber just above the flange. A ridge occurs along the line of attachment of the proseptum on its adapical side. The distance between the proseptum and the second septum in the median plane is much smaller than that between the second and third septa. The proseptum has a short septal neck that is projected both adorally and adapically. The second septum is achoanitic or possibly has a short retrochoanitic neck. The siphuncle is generally ventral throughout ontogeny, although it extends straight across the chamber in a few chambers in the first whorl. The ammonitella does not show any micro-ornamentation but lirae appear immediately adorai of the primary constriction. The presence of such features as the muscle scar on the inside surface of the initial chamber just above the flange and the ridge along the line of attachment of the proseptum on its adapical side are similar to features observed in some Mesozoic ammonoids (e.g., in the Ancyloceratina) and may indicate similarities in embryonic development.